Now let's learn about what field-effect transistors are. Field-effect transistors are very common in both old circuitry and modern ones. Now devices with an insulated shutter are used to a greater extent, about the types field effect transistors and their features today we will talk. In the article, I will make comparisons with bipolar transistors, in separate places.

Definition

A field effect transistor is a semiconductor fully controllable switch controlled by an electric field. This is the main difference in terms of practice from bipolar transistors, which are controlled by current. The electric field is created by the voltage applied to the gate relative to the source. The polarity of the control voltage depends on the type of transistor channel. There is a good analogy here with electronic vacuum tubes.

Another name for field-effect transistors is unipolar. "UNO" means one. In field-effect transistors, depending on the type of channel, the current is carried out by only one type of carriers, holes or electrons. In bipolar transistors, the current was formed from two types of charge carriers - electrons and holes, regardless of the type of devices. Field-effect transistors can generally be divided into:

transistors with a control p-n-junction;

insulated gate transistors.

Both of them can be n-channel and p-channel, a positive control voltage must be applied to the gate of the first to open the key, and for the second - negative with respect to the source.

All types of field effect transistors have three outputs (sometimes 4, but rarely, I met only on Soviet ones and it was connected to the case).

1. Source (source of charge carriers, analogue of the emitter on a bipolar).

2. Drain (receiver of charge carriers from the source, analogue of the collector of a bipolar transistor).

3. Gate (control electrode, analogue of the grid on lamps and bases on bipolar transistors).

Transistor with control pn junction

The transistor consists of the following areas:

4. Shutter.

In the image you see a schematic structure of such a transistor, the leads are connected to the metallized sections of the gate, source and drain. In a specific circuit (this is a p-channel device), the gate is an n-layer, has less resistivity than the channel region (p-layer), and the p-n junction region is located more in the p-region for this reason.

a - n-type field effect transistor, b - p-type field effect transistor

To make it easier to remember, remember the designation of the diode, where the arrow points from the p-region to the n-region. Here also.

The first state is to apply an external voltage.

If a voltage is applied to such a transistor, plus to the drain and minus to the source, a large current will flow through it, it will be limited only by the channel resistance, external resistances and the internal resistance of the power source. An analogy can be drawn with a normally closed key. This current is called Isnach or the initial drain current at Uzi=0.

A field-effect transistor with a control p-n junction, without a control voltage applied to the gate, is as open as possible.

The voltage to the drain and source is applied in this way:

The main charge carriers are introduced through the source!

This means that if the transistor is p-channel, then the positive terminal of the power source is connected to the source, because. the main carriers are holes (positive charge carriers) - this is the so-called hole conductivity. If the n-channel transistor is connected to the source, the negative terminal of the power source, because in it, the main charge carriers are electrons (negative charge carriers).

The source is the source of the main charge carriers.

Here are the results of a simulation of such a situation. On the left is a p-channel, and on the right is an n-channel transistor.

Second state - apply voltage to the gate

When a positive voltage is applied to the gate relative to the source (Uzi) for the p-channel and negative for the n-channel, it is shifted in the opposite direction, the area of ​​the p-n-junction expands towards the channel. As a result, the channel width decreases, the current decreases. The gate voltage at which no current flows through the switch is called the cutoff voltage.

The cutoff voltage has been reached and the key is fully closed. The picture with the simulation results shows such a state for the p-channel (left) and n-channel (right) dongle. By the way on English language such a transistor is called a JFET.

The operating mode of the transistor when the voltage Uzi is either zero or reverse. Due to the reverse voltage, you can “cover the transistor”, it is used in class A amplifiers and other circuits where smooth regulation is needed.

The cutoff mode occurs when Uzi = Ucutoff for each transistor it is different, but in any case it is applied in the opposite direction.

Characteristics, VAC

The output characteristic is a graph that shows the dependence of the drain current on Usi (applied to the drain and source terminals), at various gate voltages.

It can be divided into three areas. At first (on the left side of the graph), we see the ohmic region - in this gap, the transistor behaves like a resistor, the current increases almost linearly, reaching a certain level, goes into the saturation region (in the center of the graph).

On the right side of the graph, we see that the current starts to grow again, this is the breakdown area, the transistor should not be here. The uppermost branch shown in the figure is the current at zero Uzi, we see that the current is the largest here.

The higher the Uzi voltage, the lower the drain current. Each of the branches differs by 0.5 volts at the gate. What we have confirmed by simulation.

The drain-gate characteristic is shown here, i.e. the dependence of the drain current on the gate voltage at the same drain-source voltage (in this example 10V), here the grid pitch is also 0.5V, we again see that the closer the Uzi voltage is to 0, the greater the drain current.

In bipolar transistors, there was such a parameter as the current transfer coefficient or gain, it was designated as B or H21e or Hfe. In the field, to display the ability to amplify the voltage, the steepness is used, denoted by the letter S

That is, the slope shows how many milliamps (or Amperes) the drain current grows with an increase in the gate-source voltage by the number of volts with a constant drain-source voltage. It can be calculated from the drain-gate characteristic, in the above example the slope is about 8 mA/V.

Switching schemes

Like bipolar transistors, there are three typical switching circuits:

1. With a common source (a). It is used most often, gives gain in current and power.

2. With a common shutter (b). Rarely used, low input impedance, no gain.

3. With a common drain (c). The voltage gain is close to 1, the input impedance is high, and the output is low. Another name is a source follower.

Features, advantages, disadvantages

The main advantage of the field effect transistor high input impedance. Input resistance is the ratio of current to gate-source voltage. The principle of operation lies in the control using an electric field, and it is formed when a voltage is applied. That is FETs are voltage controlled.

• practically does not consume control current, this is reduces loss of control, signal distortion, signal source current overload...

Average frequency FET performance is better than bipolar, this is due to the fact that less time is needed for the "resorption" of charge carriers in the regions of the bipolar transistor. Some modern bipolar transistors may even be superior to field-effect transistors, this is due to the use of more advanced technologies, a reduction in base width, and other things.

The low noise level of field-effect transistors is due to the absence of a charge injection process, as in bipolar ones.

Stability under temperature change.

Low power consumption in the conductive state - greater efficiency of your devices.

The simplest example of using high input impedance is in matching devices for connecting acoustic acoustic guitars with piezo pickups and electric guitars with electromagnetic pickups to line inputs with low input impedance.

A low input impedance can cause the input signal to drop, distorting its shape to varying degrees depending on the frequency of the signal. This means that you need to avoid this by introducing a cascade with a high input impedance. Here the simplest circuit such a device. Suitable for connecting electric guitars line input computer audio card. With it, the sound will become brighter, and the timbre richer.

The main disadvantage is that such transistors are afraid of static. You can take an element with electrified hands, and it will immediately fail, this is the consequence of controlling the key with the help of the field. It is recommended to work with them in dielectric gloves connected through a special bracelet to ground, with a low-voltage soldering iron with an insulated tip, and the transistor leads can be tied with wire to short them out during installation.

Modern devices are practically not afraid of this, since protective devices such as zener diodes can be built into them at the entrance, which work when the voltage is exceeded.

Sometimes for beginner radio amateurs, fears reach the point of absurdity, such as putting foil caps on your head. Everything described above, although it is mandatory, but not observing any conditions does not guarantee the failure of the device.

Insulated gate field effect transistors

This type of transistors is actively used as semiconductor controlled switches. Moreover, they work most often in the key mode (two positions “on” and “off”). They have several names:

1. MIS transistor (metal-dielectric-semiconductor).

2. MOSFET (metal-oxide-semiconductor).

3. MOSFET transistor (metal-oxide-semiconductor).

Remember - these are just variations of the same name. The dielectric, or oxide as it is also called, plays the role of an insulator for the gate. In the diagram below, the insulator is shown between the n-region near the gate and the gate as a white zone with dots. It is made from silicon dioxide.

Dielectric excludes electrical contact between the gate electrode and the substrate. Unlike a control p-n junction, it does not operate on the principle of junction expansion and channel overlap, but on the principle of changing the concentration of charge carriers in a semiconductor under the action of an external electric field. MOSFETs come in two types:

1. With built-in channel.

2. With induced channel

In the diagram you see a transistor with a built-in channel. From it you can already guess that the principle of its operation resembles a field-effect transistor with a control p-n junction, i.e. when the gate voltage is zero, current flows through the switch.

Two regions with a high content of impurity charge carriers (n+) with increased conductivity are created near the source and drain. A substrate is a P-type base (in this case).

Please note that the crystal (substrate) is connected to the source; on many conventional graphic symbols, it is drawn in this way. When the gate voltage increases, a transverse electric field appears in the channel, it repels charge carriers (electrons), and the channel closes when the threshold Uz is reached.

When a negative gate-source voltage is applied, the drain current drops, the transistor begins to close - this is called depletion mode.

When a positive voltage is applied to the gate-source, the reverse process occurs - the electrons are attracted, the current increases. This is the enrichment mode.

All of the above is true for MOSFETs with a built-in N-type channel. If a p-type channel changes all the words "electrons" to "holes", the voltage polarities are reversed.

According to the datasheet for this transistor, the gate-source threshold voltage is in the region of one volt, and its typical value is 1.2 V, let's check this.

The current is in microamps. If you increase the voltage a little more, it will disappear completely.

I chose a transistor at random, and I came across a fairly sensitive device. I'll try to change the polarity of the voltage so that the gate has a positive potential, check the enrichment mode.

At a gate voltage of 1V, the current increased four times compared to what it was at 0V (the first picture in this section). It follows that, unlike the previous type of transistors and bipolar transistors, without additional strapping, it can work both to increase the current and to decrease it. This statement is very rude, but in the first approximation it has the right to exist.

Everything here is almost the same as in a transistor with a control transition, with the exception of the presence of an enrichment mode in the output characteristic.

On the drain-gate characteristic, it is clearly seen that a negative voltage causes a depletion mode and closing of the key, and a positive voltage at the gate - enrichment and a greater opening of the key.

MOSFETs with an induced channel do not conduct current in the absence of voltage on the gate, or rather, there is current, but it is extremely small, because. this is the reverse current between the substrate and the heavily doped drain and source regions.

Field-effect transistor with an insulated gate and an induced channel is an analogue of a normally open key, no current flows.

In the presence of a gate-source voltage, because we consider the n-type induced channel, then the voltage is positive, under the action of the field, negative charge carriers are attracted to the gate region.

This is how a “corridor” appears for electrons from the source to the drain, thus a channel appears, the transistor opens, and current begins to flow through it. We have a p-type substrate, the main ones in it are positive charge carriers (holes), there are very few negative carriers, but under the action of the field they break away from their atoms, and their movement begins. Hence the lack of conduction in the absence of voltage.

The output characteristic exactly repeats the same for the previous ones, the only difference is that the voltages Uzi become positive.

The drain-gate characteristic shows the same thing, the differences are again in the gate voltages.

When considering the current-voltage characteristics, it is extremely important to carefully look at the values ​​\u200b\u200bprescribed along the axes.

A voltage of 12 V was applied to the key, and we have 0 on the gate. Current does not flow through the transistor.

This means that the transistor is completely open, if it were not there, the current in this circuit would be 12/10 = 1.2 A. Later, I studied how this transistor works, and found out that at 4 volts it starts to open.

By adding 0.1V each, I noticed that with every tenth volt, the current grows more and more, and by 4.6 Volts the transistor is almost completely open, the difference with the gate voltage of 20V in the drain current is only 41 mA, at 1.1 A it is nonsense.

This experiment reflects that the induced channel transistor turns on only when the threshold voltage is reached, which allows it to work perfectly as a switch in pulse circuits. Actually, the IRF740 is one of the most common.

The gate current measurements showed that the field-effect transistors actually consume almost no control current. At a voltage of 4.6 volts, the current was only 888 nA (nano!!!).

At a voltage of 20V, it was 3.55 μA (micro). For a bipolar transistor, it would be on the order of 10 mA, depending on the gain, which is tens of thousands of times greater than for a field transistor.

Not all keys open with such voltages, this is due to the design and features of the circuitry of the devices where they are used.

A discharged capacitance at the first moment of time requires a large charging current, and rare control devices (pwm controllers and microcontrollers) have strong outputs, so they use drivers for field gates, both in field-effect transistors and in (bipolar with an insulated gate). This is an amplifier that converts the input signal into an output of such a magnitude and current strength sufficient to turn the transistor on and off. The charge current is also limited by a resistor in series with the gate.

At the same time, some gates can also be controlled from the microcontroller port through a resistor (the same IRF740). We have touched on this topic.

They resemble field-effect transistors with a control gate, but differ in that on the UGO, as in the transistor itself, the gate is separated from the substrate, and the arrow in the center indicates the type of channel, but is directed from the substrate to the channel if it is an n-channel mosfet - towards the shutter and vice versa.

For keys with an induced channel:

It might look like this:

Pay attention to the English names of the pins, they are often indicated in datasheets and diagrams.

For keys with built-in channel:

Semiconductor devices, the operation of which is based on the modulation of the resistance of a semiconductor material by a transverse electric field, are called field-effect transistors. They have in the making electric current only one type of charge carriers (electrons or holes) are involved.

Field-effect transistors are of two types: with a control p-n junction and with a metal-dielectric-semiconductor structure (MIS transistors).

Rice. 2.37. Simplified structure of a field-effect transistor with a control (a); symbols of a transistor having an n-type channel (b) and a p-type channel (c); typical structures (d, e): transistor structure with increased speed (e)

A transistor with a control p-n junction (Fig. 2.37) is a plate (section) of a semiconductor material with a certain type of electrical conductivity, from the ends of which two conclusions are made - drain and source electrodes. An electrical junction (p-n junction or Schottky barrier) is made along the plate, from which a third conclusion is drawn - a shutter.

External voltages are applied so that an electric current flows between the drain and source electrodes, and the voltage applied to the gate biases the electrical junction in the opposite direction. The resistance of the region located under the electrical junction, which is called the channel, depends on the gate voltage. This is due to the fact that the dimensions of the transition increase with an increase in the reverse voltage applied to it, and an increase in the region depleted of charge carriers leads to an increase in electrical resistance channel.

Thus, the operation of a field-effect transistor with a control p-n junction is based on a change in the channel resistance due to a change in the size of the region depleted of the main charge carriers, which occurs under the action of a reverse voltage applied to the gate.

The electrode from which the main charge carriers begin to move in the channel is called the source, and the electrode to which the main charge carriers move is called the drain. A simplified structure of a field-effect transistor with a control p-n junction is shown in fig. 2.37 a. Conventions are given in fig. 2.37, b, c, and the structures of commercially produced field-effect transistors are shown in fig. 2.37, Mr. e.

If zones with p-type electrical conductivity are created in a semiconductor plate, for example, n-type, then when a voltage is applied to the p-n junction that biases it in the opposite direction, regions are formed that are depleted by the main charge carriers (Fig. 2.37, a). The resistance of the semiconductor between the source and drain electrodes increases as the current flows only through the narrow channel between the junctions. A change in the gate-source voltage leads to a change in the size of the space charge zone (dimensions), i.e., to a change in the channel resistance. The channel can be almost completely blocked and then the resistance between the source and the drain will be very high (a few - tens).

The voltage between gate and source at which the drain current reaches the specified low value, is called the cutoff voltage of the field effect transistor. Strictly speaking, at the cutoff voltage, the transistor should close completely, but the presence of leaks and the difficulty of measuring especially small currents make us consider the cutoff voltage to be the voltage at which the current reaches a certain small value. Therefore, in the technical specifications, the transistor is indicated at what drain current the measurement was made.

The width of the pn junction also depends on the current flowing through the channel. If, for example (Fig. 2.37, a), then the current flowing through the transistor will create a voltage drop along the length of the latter, which turns out to block the gate-channel transition.

Rice. 2.38. Output characteristics of a field-effect transistor with its control input characteristic (6) and transmission characteristic (stoke gate) (c): I - steep region; II - flat area, or saturation area; III - breakdown area

This leads to an increase in the width and, accordingly, to a decrease in the cross section and conductivity of the channel, and the width of the p-n junction increases as it approaches the drain region, where there will be the largest voltage drop caused by the current on the channel resistance. So, if we assume that the resistance of the transistor is determined only by the resistance of the channel, then at the edge of the p-n junction facing the source, the voltage will act, and at the edge facing the drain, the voltage will act. At low voltage values ​​and small, the transistor behaves like a linear resistance. An increase leads to an almost linear increase, and a decrease leads to a corresponding decrease. As the characteristic grows, it deviates more and more from the linear one, which is associated with the narrowing of the channel at the drain end. At a certain value of the current, the so-called saturation mode occurs (section II in Fig. 2.38, a), which is characterized by that. that as the current increases, the current changes slightly. This is because at a high voltage, the channel at the drain contracts into a narrow neck. A kind of dynamic equilibrium sets in, in which an increase and an increase in current cause a further narrowing of the channel and, accordingly, a decrease in current. As a result, the latter remains almost constant. The voltage at which saturation occurs is called the saturation voltage. It is, as can be seen from Fig. , changes as the voltage changes. Since the influence on the channel width at the drain output is almost the same, then

So, the cutoff voltage, determined at a small voltage, is numerically equal to the saturation voltage at , and the saturation voltage at a certain gate voltage is equal to the difference between the cutoff voltage and the gate-source voltage.

With a significant increase in the voltage of the drain end, a breakdown of the p-n junction is observed.

In the output characteristics of a field-effect transistor, two working areas, OA and OB, can be distinguished. The OA region is called the steep characteristic region, the AB region is called the flat or saturation region. In the steep region, the transistor can be used as an ohmic controlled resistance. In amplifying stages, the transistor operates on a flat section of the characteristic. Beyond point B, breakdown of the electrical transition occurs.

The input characteristic of a field-effect transistor with a control junction (Fig. 2.38, b) is the reverse branch of the volt-ampere characteristic of the junction. Although the gate current varies somewhat with voltage and reaches the greatest value subject to a short circuit of the source and drain terminals (gate leakage current) - in most cases it can be neglected. A change in voltage does not cause significant changes in the gate current, which is typical for a reverse current junction.

When operating in a flat region of the current-voltage characteristic, the drain current at a given voltage 11sh is determined from the expression

where is the initial drain current, under which the current at and the drain voltage exceeding the saturation voltage: .

Since the field-effect transistor is controlled by the gate voltage, the steepness of the characteristic is used to quantify the control action of the gate

The steepness of the characteristic reaches its maximum value at . To determine the value of S at any voltage, we differentiate the expression

For , expression (2.73) takes the form

Substituting (1.74) into expression (1.73), we obtain .

Thus, the slope of the characteristic of a field-effect transistor decreases with increasing voltage applied to its gate.

The initial value of the steepness of the characteristic can be determined by a graph-analytical method. To do this, we draw a tangent from a point to the drain-gate characteristic (Fig. 2.38. c). It will cut off a segment on the stress axis, and its slope will determine the value of .

The amplifying properties of field-effect transistors are characterized by the gain

which is related to the slope of the characteristic and the internal resistance by the equation , where is the differential internal resistance of the transistor.

Indeed, in general.

If with a simultaneous change in and , then whence

As with bipolar transistors, field-effect transistors distinguish between large and small signal modes. The large signal mode is most often calculated using the input and output characteristics of the transistor and the equivalent circuit of Fig. 2.39, a. To analyze the small signal regime, small-signal equivalent circuits are widely used in Figs. 2.39, b-g (transistor with a p-type channel). Since the resistances of closed junctions in silicon field-effect transistors are large (tens to hundreds of MΩ), in most cases they can be ignored. For practical calculations, the equivalent circuit in Fig. 1 is most convenient. 2.39, d, although it reflects the actual physical processes occurring in the transistors under consideration much worse. All gate capacitances in the circuit are replaced by one equivalent capacitance C ", which is charged through the average equivalent resistance .

Rice. 2.39. Simplified FET equivalent circuit with control p-n junction for direct current(a); small-signal equivalent circuits: complete (b), simplified (c), modified (d).

We can assume that it is equal to the static resistance in the steep region of characteristics - the resistance between the drain and source in the open state of the transistor at a given drain-source voltage, less than the saturation voltage. Gate resistance (ohmic) reflected equivalent resistance, which, due to its large value (tens-hundreds), can be ignored.

Typical values ​​of the parameters of silicon transistors included in the equivalent circuit: .

The capacitances of the field-effect transistor, as well as the final speed of charge carriers in the channel, determine its inertial properties. The inertia of the transistor in the first approximation is taken into account by introducing the operator slope of the characteristic

where is the limiting frequency, determined at the level of 0.7 of the static value of the slope of the characteristic.

When the temperature changes, the parameters and characteristics of field-effect transistors with a control change due to the influence of the following factors: changes in the reverse current of a closed p-n-junction; changes in the contact potential difference changes in the resistivity of the channel.

The reverse current at the closed one increases exponentially with increasing temperature. Roughly, it can be considered that it doubles with an increase in temperature by 6-8 C. If there is a large external resistance in the transistor gate circuit, then the voltage drop across it, caused by a changed current, can significantly change the gate voltage.

The contact potential difference decreases with an increase in temperature by approximately . With a constant gate voltage, this leads to an increase in the drain current. For transistors with low cutoff voltage, this effect is dominant and changes in drain current will be positive.

Since the temperature coefficient characterizing the change in the channel resistivity is positive, the drain current decreases with increasing temperature. This opens up the possibility of correctly choosing the position of the operating point of the transistor to mutually compensate for current changes caused by changes in the contact potential difference and channel resistivity. As a result, the drain current will be almost constant over a wide range of temperatures.

The operating point at which the change in flow rate with temperature change has minimum value, is called the thermostable point. Its approximate position can be found from the equation

From (2.78) it can be seen that with a significant steepness of the characteristic at a thermally stable point is small and a much lower gain can be obtained from the transistor than when working with a low voltage.

Rice. 2.40. The inclusion of a field effect transistor in circuits: a - with a common source; b - with a common drain

Modern field-effect transistors made on the basis of silicon are operable up to a temperature of 120-150 C. Their inclusion in the circuits of amplifying stages with a common source and a common drain is shown in fig. 2.40, a, b. Constant voltage provides a certain value of the channel resistance and a certain drain current. When an input amplified voltage is applied, the gate potential changes, and the drain and source currents, as well as the voltage drop across the resistor R, change accordingly.

The increment of the voltage drop across the resistor R at a large value is much greater than the increments of the input voltage. This amplifies the signal. Due to the low prevalence, switching on with a common shutter is not shown. When you change the type of electrical conductivity of the channel, only the polarity of the applied voltages and the direction of the currents change, including in equivalent circuits.

The main advantages of field-effect transistors with a control p-n junction over bipolar ones are high input resistance, low noise, ease of manufacture, and the absence of residual voltage in the open state between the source and drain of an open transistor.

MIS transistors can be of two types: transistors with built-in channels (the channel is created during manufacture) and transistors with induced channels (the channel appears under the action of a voltage applied to the control electrodes).

Transistors of the first type can operate both in the depletion mode of the channel with charge carriers, and in the enrichment mode. Transistors of the second type can only be used in enrichment mode. In MIS transistors, unlike transistors with a control p-n junction, the metal gate is isolated from the semiconductor by a dielectric layer and there is an additional output from the crystal on which the device is made (Fig. 2.41), called the substrate.

Rice. 2.41. Structures of the MIS transistor: a - planar transistor with an induced channel. b - planar transistor with built-in channel; , transistor - and .

Rice. 2.42. Distribution of charge carriers in the surface layer

The control voltage can be applied both between the gate and the substrate, and independently to the substrate and the gate. Under the influence of the resulting electric field, a -type channel appears near the surface of the semiconductor due to the repulsion of electrons from the surface into the depth of the semiconductor in a transistor with an induced channel. In a transistor with a built-in channel, the existing channel is expanded or narrowed. Changing the control voltage changes the channel width and, accordingly, the resistance and current of the transistor.

A significant advantage of MIS transistors is their high input resistance, reaching Ohm values ​​(for transistors with an Ohm control junction).

Let us consider in more detail the operation of an MIS transistor with an induced -channel. Let as source material The transistor uses silicon having a -type electrical conductivity. The role of the dielectric film is performed by silicon dioxide. In the absence of bias, the near-surface layer of a semiconductor is usually enriched with electrons (Fig. 2.42, a). This is due to the presence of positively charged ions in the dielectric film, which is a consequence of the previous oxidation of silicon and its photolithographic processing, as well as the presence of traps at the interface. Recall that traps are a set of energy levels located deep in the band gap, close to its middle.

When a negative voltage is applied to the gate, the electrons of the near-surface layer are repelled into the depth of the semiconductor, and the holes move towards the surface. The surface layer acquires hole electrical conductivity (Fig. 2.42, b). A thin inverse layer appears in it, connecting the drain to the source. This layer plays the role of a channel. If a voltage is applied between the source and the drain, then the holes, moving along the channel, create a drain current. By changing the gate voltage, it is possible to expand or narrow the channel and thereby increase or decrease the drain current.

The gate voltage at which the channel is induced is called the threshold voltage. Since the channel appears gradually, as the gate voltage increases, to eliminate ambiguity in its definition, a certain drain current value is usually set, above which it is considered that the gate potential has reached the threshold voltage .

As the distance from the semiconductor surface increases, the concentration of induced holes decreases. At a distance approximately equal to the channel thickness, the electrical conductivity becomes intrinsic. Then comes the section depleted of the main charge carriers (-transition). Thanks to him, the drain, source and channel are isolated from the substrate; - the junction is biased by the applied voltage in the opposite direction. Obviously, its width and channel width can be changed by applying additional voltage to the substrate relative to the drain and source electrodes of the transistor. Therefore, the drain current can be controlled not only by changing the gate voltage, but also by changing the substrate voltage. In this case, the control of the MOS transistor is similar to the control of a field-effect transistor with a control junction. To form a channel, a voltage greater than .

The thickness of the inverse layer is much less than the thickness of the depleted layer. If the latter is hundreds - thousands of nm, then the thickness of the induced channel is only 1-5 nm. In other words, the holes of the induced channel are "pressed" to the surface of the semiconductor; therefore, the structure and properties of the semiconductor-insulator interface play a very important role in MOS transistors.

The holes that form the channel enter it not only from the -type substrate, where there are few of them and they are generated relatively slowly, but also from the source and drain -type layers, where their concentration is practically unlimited, and the field strength near these electrodes is quite high.

In transistors with a built-in channel, the current in the drain circuit will also flow at zero gate voltage. To stop it, it is necessary to apply a positive voltage to the gate (in the case of a -type channel structure) equal to or more voltage cutoffs . In this case, holes from the inverse layer will be almost completely forced into the depth of the semiconductor and the channel will disappear. When a negative voltage is applied, the channel expands and the current increases. In this way. MOS transistors with built-in channels operate in both depletion and enrichment modes.

Rice. 2.43. The structure of the MIS transistor with a changed channel width during the flow of current (a); its output characteristics with induced (b) and built-in (c) channels: I steep region; II - flat area, or saturation area; III - breakdown area; 1 - lunch layer

Like field-effect transistors with a control junction, MIS transistors at low voltages (in the region of Fig. 2.43, b, c) behave like a linearized controlled resistance. As the voltage increases, the channel width decreases due to a voltage drop across it and a change in the resulting electric field. This is especially pronounced in that part of the channel, which is located near the drain (Fig. 2.43, a). The voltage drops created by the current lead to an uneven distribution of the electric field strength along the channel, and it increases as it approaches the drain. Under voltage, the channel near the drain becomes so narrow that a dynamic equilibrium occurs, when an increase in voltage causes a decrease in the width of the channel and an increase in its resistance. As a result, the current changes little with a further increase in voltage. These processes of changing the channel width depending on the voltage are the same as in field-effect transistors with a control p-n junction.

The output characteristics of MIS transistors are similar to those of field-effect transistors with a control (Fig. 2.43, b, c). They can be divided into steep and flat regions, as well as the breakdown region. In a steep region, the MIS transistor can act as an electrically controlled resistance. The flat region II is usually used in the construction of amplifying cascades. Analytical Approximations volt-ampere characteristics MIS transistors are not very convenient and are rarely used in engineering practice. For approximate estimates of the drain current in the saturation region, you can use the equation

For transistors with a built-in channel, equations (2.79) can be used if we replace and take into account the signs of voltages and .. They characterize the parameters of a field-effect transistor, which, for a given measurement mode, is represented by an equivalent circuit in Fig. 2.44, e. It reflects the features of the transistor worse, but its parameters are known or can be easily measured (input capacitance, through capacitance, output capacitance).

The operator equation for the slope of the characteristics of MOS transistors has the same form as for field-effect transistors with a control signal. In this case, the time constant is . In a typical case, with a channel length of 5 μm, the limiting frequency at which the slope of the characteristic decreases by a factor of 0.7 lies within a few hundred megahertz.

The temperature dependence of the threshold voltage and the cutoff voltage is due to a change in the position of the Fermi level, a change in the space charge in the depletion region, and the effect of temperature on the value of the charge in the dielectric. MOS transistors also have a thermally stable operating point where the drain current is little affected by temperature. At different transistors the value of the drain current at the thermostable point is within . An important advantage of MIS transistors over bipolar ones is the low voltage drop across them when switching small signals. So, if in bipolar transistors in saturation mode the voltage

When decreasing, it can be reduced to a value tending to zero. Since MIS transistors with a silicon dioxide dielectric are widely used, we will further call them MOS transistors.

Currently, the industry also produces MOSFETs with two insulated gates (tetrode), for example. The presence of a second gate allows you to simultaneously control the current of the transistor using two control voltages, which facilitates the construction of various amplifying and multiplying devices. Their characteristics are similar to the characteristics of single-gate field-effect transistors, only their number is greater, since they are built for the voltage of each gate with a constant voltage at the other gate. Accordingly, the slope of the characteristic for the first and second gates, the cutoff voltage of the first and second gates, etc. are distinguished. Applying voltage to the gates is no different from applying voltage to the gate of a single-gate MOSFET.

Must exceed the threshold. Otherwise, the channel will not appear and the transistor will be locked.

Field effect transistors are semiconductor devices. Their feature is that the output current is controlled by an electric field and a voltage of one polarity. The control signal is applied to the gate and regulates the conductivity of the transistor junction. In this they differ from bipolar transistors, in which the signal is possible with different polarity. Another distinctive property of a field-effect transistor is the formation of an electric current by the main carriers of the same polarity.

Varieties

There are many different types of field effect transistors, operating with their own characteristics.

• conduction type. The polarity of the control voltage depends on it.
• Structure: diffusion, alloy, MIS, Schottky barrier.
• Number of electrodes: there are transistors with 3 or 4 electrodes. In the version with 4 electrodes, the substrate is a separate part, which makes it possible to control the passage of current through the junction.
• Production material: devices based on germanium, silicon became the most popular. In the marking of the transistor, the letter means the material of the semiconductor. In transistors produced for military equipment, the material is marked with numbers.
• Type of application: indicated in reference books, not indicated on the label. In practice, five groups of field workers are known: in low and high frequency amplifiers, as electronic keys, modulators, DC amplifiers.
• Operating Parameter Range: A set of data within which field workers can operate.
• Features of the device: unitrons, gridistors, alkatrons. All devices have their own distinctive data.
• Number of structural elements: complementary, twin, etc.

In addition to the main classification of "field workers", there is a special classification that has the principle of operation:

• FETs with p-n transition that manages.
• Field-effect transistors with a Schottky barrier.
• "Field workers" with an insulated shutter, which are divided:
  - with induction transition;
  - with built-in transition.

An auxiliary classification has been proposed in the scientific literature. It says that the semiconductor based on the Schottky barrier must be allocated to a separate class, since this is a separate structure. The same transistor can include an oxide and a dielectric at once, as in the KP 305 transistor. Such methods are used to form new properties of a semiconductor, or to reduce their cost.

In the diagrams, field workers have terminal designations: G - gate, D - drain, S - source. The substrate of a transistor is called a "substrate".

Design features

The control electrode of a field-effect transistor in electronics is called a gate. Its transition is performed from a semiconductor with any type of conductivity. The polarity of the control voltage can be with any sign. An electric field of a certain polarity releases free electrons until the junction runs out of free electrons. This is achieved by applying an electric field to the semiconductor, after which the current value approaches zero. This is the action of the field effect transistor.

Electric current flows from the source to the drain. Let's analyze the differences between these two terminals of the transistor. The direction of the electrons does not matter. Field-effect transistors have the property of reversibility. In radio engineering, field-effect transistors have found their popularity, since they do not form noise due to the unipolarity of charge carriers.

The main feature of field-effect transistors is a significant input resistance. This is especially noticeable in alternating current. This situation is obtained due to the control by the reverse Schottky transition with a certain bias, or by the capacitance of the capacitor near the gate.

The substrate material is an undoped semiconductor. For "field workers" with a Schottky transition, instead of a substrate, gallium arsenide is laid, which in its pure form is a good insulator.

In practice, it is difficult to create a structural layer with a complex composition that meets the necessary conditions. Therefore, an additional requirement is the ability to slowly build up the substrate to the desired size.

Field-effect transistors with p-ntransition

In such a design, the type of gate conduction differs from that of the junction. In practice, various improvements are applied. The shutter can be made from several areas. As a result, the smallest voltage can control the passage of current, which increases the gain.

Used in various schemes reverse view offset transition. The larger the offset, the smaller the width of the transition for the passage of current. At a certain voltage value, the transistor closes. The use of forward bias is not recommended as the high power drive circuit may affect the gate. During an open transition, a significant current, or increased voltage, passes. Normal mode work is created by right choice poles and other properties of the power source, as well as the selection of the point of operation of the transistor.

In many cases direct gate currents are specifically used. This mode can also be used by transistors in which the substrate forms a junction type p-n. The charge from the source is split into a drain and a gate. There is an area with a large current amplification factor. This mode is shutter controlled. However, as the current increases, these parameters drop sharply.

A similar connection is used in the frequency gate detector circuit. It applies channel and gate transition rectification properties. In this case, the forward bias is zero. The transistor is also driven by gate current. A large signal amplification is generated in the drain circuit. The voltage for the gate varies according to the law of the input and is blocking for the gate.

The voltage in the drain circuit has the following elements:

• Constant. Not applicable.
• Carrier signal. Discharged to ground using filters.
• Modulating frequency signal. Is processed to obtain information from it .

As a disadvantage of the shutter detector, it is advisable to single out a significant distortion factor. The results for him are negative for the strong and weak signals. A slightly better result shows a phase detector made on a transistor with two gates. The reference signal is applied to one of the control electrodes, and the information signal, amplified by the field worker, appears on the drain.

Despite significant distortion, this effect has its purpose. In selective amplifiers that pass a certain dose of a certain frequency spectrum. Harmonic vibrations are filtered and do not affect the quality of the scheme.

MeP transistors, which means metal-semiconductor, with a Schottky junction practically do not differ from transistors with a p-n junction. Since the MeN junction has special properties, these transistors can operate at an increased frequency. And also, the MeP structure is easy to manufacture. The frequency characteristics depend on the charge time of the gate element.

MIS transistors

The base of semiconductor elements is constantly expanding. Each new development changes electronic systems. New instruments and devices appear on their basis. The MOS transistor operates by changing the conductivity of the semiconductor layer using an electric field. From this came the name - field.

The designation MIS stands for metal-insulator-semiconductor. This gives a description of the composition of the device. The gate is isolated from the source and drain by a thin dielectric. The MIS transistor of the modern type has a gate size of 0.6 microns, through which only an electromagnetic field can flow. It affects the state of the semiconductor.

When the desired potential occurs at the gate, an electromagnetic field arises, which affects the resistance of the drain-source section.

The advantages of this use of the device are:

• Increased input resistance of the device. This property is relevant for use in low current circuits.
• The small capacitance of the drain-source section makes it possible to use the MIS transistor in high-frequency devices. No distortion is observed during signal transmission.
• Advances in new semiconductor manufacturing technologies have led to the development of IGBT transistors, which include positive points bipolar and field devices. Power modules based on them are widely used in soft starters and frequency converters.

When developing such elements, it must be taken into account that MIS transistors are more sensitive to high voltage and static electricity. The transistor can burn out if you touch its control pins. Therefore, when installing them, it is necessary to use special grounding.

These field-effect transistors have many unique properties (for example, control of an electric field), so they are popular in electronic equipment. It should also be noted that transistor manufacturing technology is constantly being updated.

A field-effect transistor is an electrical semiconductor device whose output current is controlled by a field, hence voltage, of the same sign. The shaping signal is fed to the gate, regulates the conductivity of the n or p-type channel. Unlike bipolar transistors, where the signal is of variable polarity. The second sign is the formation of the current exclusively by the main carriers (of the same sign).

Classification of field-effect transistors

Let's start with the classification. There are many types of field effect transistors, each works according to the algorithm:

In addition to the general classification, a specialized one was invented that defines the principles of work. Distinguish:

1. Field-effect transistors with a control p-n-junction.
2. Field-effect transistors with a Schottky barrier.
3. Insulated gate field effect transistors:

• With built-in channel.
• With induced channel.

In the literature, the structures are additionally ordered as follows: it is inappropriate to use the designation MOS, structures based on oxides are considered a special case of MIS (metal, insulator, semiconductor). The Schottky barrier (MeB) should be singled out separately, since it is a different structure. Reminds properties p-n-junction. Let us add that structurally, a dielectric (silicon nitride), an oxide (quadrivalent silicon) can simultaneously enter into the composition of the transistor, as happened with KP305. Such technical solutions used by people seeking methods obtaining unique properties of the product, reducing the cost.

Among foreign abbreviations for field-effect transistors, the combination FET is reserved, sometimes it denotes the type of control - with a p-n junction. In the latter case, along with this, we will meet JFET. Words-synonyms. Abroad, it is customary to separate oxide (MOSFET, MOS, MOST - synonyms), nitride (MNS, MNSFET) field-effect transistors. The presence of a Schottky barrier is labeled SBGT. Apparently, the material meaning, domestic literature, the meaning of the fact is hushed up.

The electrodes of field-effect transistors in the diagrams are indicated: D (drain) - drain, S (source) - source, G (gate) - gate. The substrate is called substrate.

FET device

The control electrode of a field effect transistor is called a gate. The channel is formed by a semiconductor of an arbitrary type of conductivity. Accordingly, the polarity of the control voltage is positive or negative. The field of the corresponding sign displaces free carriers until the isthmus under the gate electrode is completely empty. It is achieved by exposing the field to either a p-n junction or a homogeneous semiconductor. The current becomes zero. This is how a field effect transistor works.

The current flows from the source to the drain; beginners are traditionally tormented by the question of distinguishing between the two indicated electrodes. There is no difference in which direction the charges move. The field effect transistor is reversible. The unipolarity of charge carriers explains the low noise level. Therefore, field-effect transistors occupy a dominant position in technology.

The key feature of the devices is the large input resistance, especially to alternating current. An obvious fact arising from the control of a reverse-biased p-n junction (Schottky junction), or the capacitance of a technological capacitor in the region of an insulated gate.

The substrate is often favored by an undoped semiconductor. For field-effect transistors with a Schottky gate - gallium arsenide. In its pure form, a good insulator, to which the following requirements are imposed as part of the product:

It is difficult to create a significant thickness of the layer that meets the list of conditions. Therefore, a fifth requirement is added, which consists in the possibility of gradually building up the substrate to the required dimensions.

Field-effect transistors with a control p-n-junction and MeP

In this case, the type of conductivity of the gate material differs from that used by the channel. In practice, you will find various improvements. The shutter is made up of five areas recessed in the channel. A lower voltage can control the flow of current. Meaning gain increase.

bipolar transistor

The circuits use the reverse bias of the p-n junction, the stronger, the narrower the channel for the current to flow. At a certain voltage value, the transistor is locked. Forward bias is dangerous to use because a powerful controlled circuit can affect the gate circuit. If the junction is open, a large current will flow, or a high voltage. Normal mode provided the right choice polarity and other characteristics of the power supply, the choice of the operating point of the transistor.

However, in some cases direct gate currents are intentionally used. It is noteworthy that this mode can be used by those MIS transistors, where the substrate forms a p-n junction with the channel. The moving charge of the source is shared between the gate and the drain. You can find a region where a significant current gain is obtained. The shutter mode is controlled. With an increase in current iz (up to 100 μA), the circuit parameters deteriorate sharply.

A similar inclusion is used by the circuit of the so-called gate frequency detector. The design exploits the rectifier properties of the p-n junction between the gate and the channel. The forward bias is small or completely zero. The device is still controlled by the gate current. In the drain circuit, a significant signal amplification is obtained. The rectified voltage for the gate is blocking, it changes according to the input law. Signal amplification is achieved simultaneously with detection. The drain circuit voltage contains the following components:

• constant component. Not used at all.
• Carrier frequency signal. It is brought to the ground by using filter tanks.
• Signal with the frequency of the modulating signal. Processed to extract embedded information.

The disadvantage of the gate frequency detector is considered to be a large coefficient of non-linear distortion. Moreover, the results are equally bad for weak (quadratic dependence of the operating characteristic) and strong (exit to the cutoff mode) signals. Somewhat better is demonstrated by the phase detector on a double-gate transistor. A reference signal is applied to one control electrode, and an information component is formed on the drain, amplified by a field-effect transistor.

Despite the large linear distortions, the effect finds application. For example, in selective power amplifiers, dosed passing a narrow frequency spectrum. Harmonics are filtered out and do not have much effect on the overall performance of the circuit.

Metal-semiconductor (MeS) transistors with a Schottky barrier almost do not differ from those with a p-n junction. At least when it comes to operating principles. But due to the special qualities of the metal-semiconductor transition, the products are able to operate at an increased frequency (tens of GHz, cutoff frequencies in the region of 100 GHz). At the same time, the MeP structure is easier to implement when it comes to production and technological processes. Frequency characteristics are determined by the gate charge time and carrier mobility (for GaAs over 10,000 sq. cm/V s).

MIS transistors

In MIS structures, the gate is reliably isolated from the channel, and control occurs entirely due to the action of the field. Insulation is carried out by silicon oxide or nitride. It is these coatings that are easier to apply on the crystal surface. It is noteworthy that in this case there are also metal-semiconductor junctions in the source and drain regions, as in any polar transistor. Many authors forget about this fact, or mention it in passing by using the mysterious phrase ohmic contacts.

In the topic about the Schottky diode, this question was raised. A barrier does not always appear at the junction of a metal and a semiconductor. In some cases, the contact is ohmic. It depends for the most part on the features of technological processing and geometric dimensions. Specifications real devices strongly depend on various defects of the oxide (nitride) layer. Here are some:

1. The imperfection of the crystal lattice in the surface region is due to broken bonds at the boundary of the change of materials. The influence is exerted both by free atoms of the semiconductor, and there are impurities like oxygen, which is present in any case. For example, when using epitaxy methods. As a result, energy levels appear that lie deep in the bandgap.
2. At the interface between the oxide and the semiconductor (3 nm thick), an excess charge is formed, the nature of which has not yet been explained. Presumably, positive vacancies(holes) of defective atoms of the semiconductor itself and oxygen.
3. The drift of ionized atoms of sodium, potassium and other alkali metals occurs at low voltages on the electrode. This increases the charge accumulated at the layer boundary. To block this effect in silicon oxide, phosphorus oxide (anhydride) is used.

FET A semiconductor amplifying device is called, the resistance of which can change under the influence of an electric field. The change in resistance is achieved by changing the electrical resistivity of the semiconductor layer or by changing the volume of the semiconductor through which the electric current passes.

Field-effect transistors use various effects, such as changing the volume R-P- transition when the blocking voltage acting on it changes; effects of depletion, enrichment with charge carriers, or inversion of the type of conduction in the near-surface layer of a semiconductor. FETs are sometimes referred to as unipolar, because the current flowing through them is due to carriers of only one sign. Field effect transistors are also called canal transistors, since the electric field that controls the operation of the transistor penetrates the semiconductor relatively shallowly, and all processes take place in a thin layer called channel.

The control circuit of the field-effect transistor practically does not consume current and power. This makes it possible to amplify signals from sources with very high internal resistance and low power. In addition, this makes it possible to place hundreds of thousands of transistors on a single microcircuit chip.

Field-effect transistors with a control pn junction

The field effect transistor can be made in the form of a semiconductor plate (with P- or R-conductivity), in one of the surfaces of which a layer of metal is fused, called shutter, forming a flat r-p-transition (Fig. 5.1). Leads are attached to the lower and upper ends of the plate, respectively called source and drain. If a blocking voltage is applied to the gate (positive to P-shutter and negative on R-shutter), then depending on its value in the channel ( r-p-transition), a layer depleted in charge carriers appears, which is practically an insulator.

By changing the gate voltage from zero to some sufficiently large voltage, called cutoff voltage (turn-off voltage, or threshold voltage, see fig. 5.6), it is possible to expand the volume of the semiconductor occupied by r-p-transition that it will occupy the entire channel and the movement of charge carriers between the source and the drain will become impossible. The transistor will completely close (Fig. 5.2).

Unlike current-controlled bipolar transistors, FETs are voltage-controlled, and since this voltage is applied to the control r-p- a transition in the reverse (blocking) polarity, then the current in the control circuit practically does not flow (at a voltage of 5 V, the control current does not exceed 10 -10 A).

Insulated gate field effect transistors

field-effect transistors with an induced channel

On fig. 5.3 shows an insulated gate field effect transistor called MIS transistor. This name is due to the design: the gate is made of metal (M) and is separated by a thin layer of dielectric (D) from the semiconductor (P) from which the transistor is made. If the transistor is made of silicon, then a thin film of silicon oxide is used as a dielectric. In this case, the name is changed to MOSFET(metal-oxide-semiconductor).

Shown in fig. 5.3 on the left, the transistor is made on the basis of a plate ( substrates, or grounds) from silicon with R-conductivity. On the surface of the plate, two regions with P-conductivity (source and drain), separated by an area P- channel having a predominant R-conductivity. As a result, when voltage is applied to the transistor, current will not flow between the source and drain, because the drain-base and source-base junctions form two back-to-back r-p- transition, one of which will be closed at any polarity of the applied voltage.

However, if the surface layer R-semiconductor to act with a sufficiently strong electric field by applying a voltage of positive polarity between the gate and the base, then a current will begin to flow between the source and drain. This is explained by the fact that from the surface layer of the semiconductor located under the gate, holes will be pushed aside by the electric field and electrons will collect, forming a channel (with P-conductivity shown in fig. 5.3 with a dotted line), as a result of which r-p- the source-to-channel and channel-to-source junctions will cease to exist. Conductivity P- The channel will be the larger, the greater the voltage applied between the gate and the base.

The transistor of the considered design is called MIS transistor with an induced channel.

The base is usually connected to the source, but sometimes the voltage is applied to it separately, and then the base acts as an additional gate.

If the base is made of P-silicon, the source and drain are formed by heavily doped regions with R- conductivities, and silicon oxide is used as an insulator, it turns out Induced p-channel MOSFET(with conductivity R) (Fig. 5.3 on the right).

field-effect transistors with built-in channel

MOSFETs can be made with an embedded channel. For example, in fig. 5.4 on the left is a diagram of the device of such a transistor with P-channel. The base is made from R-silicon, and the source and drain have P-conductivity and obtained by diffusion method. The source and drain are connected by a relatively thin channel with little R- conductivity.

If the base is made of P-silicon, and source and drain - from R-silicon, then the transistor has a built-in p-channel (Fig. 5.4 on the right) .

work P-channel MOSFET can be explained as follows. If a negative (relative to the base) voltage is applied to the gate, then conduction electrons are displaced from P-channel to the base, and the conductivity of the channel decreases, up to complete depletion and blocking of the channel .

When a positive voltage is applied to the gate P-channel is enriched with electrons, and its conductivity increases (Fig.5.6).

Classification and characteristics of field-effect transistors

Field effect transistors are depleted and enriched type. The former include all transistors with r‑p-transition and P-channel MOSFETs depleted type. Enriched MOSFETs are available as P- channel, and R-channel (Fig. 5.5).

Enriched and depleted type transistors differ only in the value of the so-called threshold voltage, obtained by extrapolation of the rectilinear section of the characteristic (Fig. 5.6.).

output characteristics field-effect transistor are called the dependences of the drain current on the drain-source voltage for various gate-source voltages.

The FET is a very good device in terms of output conductance - at a constant gate-source voltage, the drain current is almost independent of voltage (except in the region of low drain-source voltages). On fig. 5.7 shows typical dependencies i from from u si for a range of values u zi.