Field-effect transistor: type and designation, advantages and disadvantages, principle of operation for dummies

In electronics and radio engineering, semiconductor devices are very often used, which include transistors. Field effect transistors (FETs) consume significantly less electrical energy, due to which they are used in various low-power devices. In addition, there are models that operate at high currents with low consumption of the supply voltage (U).

General information

FET or FET is a semiconductor device that, when the control U changes, regulates I (current). This type of transistor is also called unipolar. It appeared later than the usual transistor (bipolar), but with the growth of technology it became widespread among digital devices due to its low power consumption. The main the difference lies in the control method I. In the bipolar one, the I regulation is carried out with the help of the control I, and in the field - with the U (Figure 1).

Figure 1 - The difference between field and bipolar T.

The PT has no I control, and it has a high input impedance (R), which reaches several hundred GΩ (GigaOhm) or TOM (TerraOhm). In order to find out the scope of the PT, you need to carefully study it. The charge carriers are electrons or holes, while the bipolar charge is electrons and holes.

Classification and device

PTs are of several types, have different characteristics and device. They are divided into 2 types:

1. With control pn - junction (JFET).
2. Insulated gate (MOSFET).

In addition, each type comes with N and P channels. For a PT with an N-channel, the charge carriers are electrons, and for a P-channel one, holes. The principle of operation for P and N is similar, the only difference is in supplying U of a different polarity as a control.

The JFET PT device (Figure 2) is simple. Area N forms a channel between areas P. Electrodes are connected to the ends of channel N, which are conventionally called drain (C) and source (I), since everything depends on the connection scheme. The gate (Z) is a type of electrode that is formed when the semiconductors P are short-circuited. This is due to the electrical connection when exposed to U. Near C and I there is a region of increased concentration or doping of (N +) electrons, which leads to an improvement in the channel conductivity. The presence of the doping zone significantly reduces the formation of parasitic pn - junctions formed upon the addition of aluminum.

Figure 2 - Schematic device of the JFET type PT.

MOFSET is called MOS or MDP, and are also divided into types - with built-in and induced channels. Each of these types has models with P and N channels. The field-effect transistor, the designation of which is shown in Figure 3, sometimes has 4 leads.

Figure 3 - Designation of the MOS transistor.

The device is quite simple and is shown in Figure 4. For a FET with an N-channel, the substrate (coated with SiO2) has a P-type conductivity. Through the dielectric layer, drain and source electrodes from the doped zones are passed, as well as an output that is short-circuited with the source. The gate layer is located above the dielectric.

Figure 4 - Typical PT device with an induced channel.

How JFET works

JFET operates in 2 modes. This feature is due to the fact that the voltage of the positive and negative components is applied to the gate (Fig. 5). When connecting U> 0 to the drain, and the ground to the source, it is necessary to connect the gate to the ground (Uzi = 0). During a gradual increase in U between C and I (Uis), the PT is an ordinary conductor. At low values ​​of Uis, the channel width is maximum.

At high values ​​of Uis, large values ​​of current flow between the source and drain through the channel (Iis). This state is called the ohmic region (OO). In an N-type semiconductor, namely in the p-n-junction zones, a decrease in the concentration of free electrons occurs. The asymmetric growth of the layer of decrease in the concentration of free electrons is called the depletion layer. The overgrowth occurs from the side of the connected power supply. A strong narrowing of the channel occurs with an increase in Uis, as a result of which Iis grows insignificantly. The operation of the PT in this mode is called saturation.

Figure 5 - Scheme of JFET operation (Uzi = 0).

When a low negative U is supplied at the gate, a strong narrowing of the channel occurs and a decrease in Iis. With a decrease in U, the channel will be closed, and the PT will operate in the cutoff mode, and U, at which the supply of Iis stops, is called the cutoff voltage (Uotc). Figure 6 shows a graphical representation of the operation of the PT at Uzi <0:

Figure 6 - Graphical representation of the principle of operation of a field-effect transistor of the JFET type.

When used in saturation mode, the signal is amplified (Fig. 7), since with insignificant changes in Uis there is a significant change in If:

Figure 7 - Example S JFET.

This parameter is the gain of the JFET and is called the gate slope (S). The unit of measurement is mA / V (milesAmp / Volt).

Features of MOFSET work

When U is connected between electrodes C and And of any polarity to MOFSET with an induced N-channel, the current is not will flow, since between the legitimate layer there is a layer with conductivity P, which does not transmit electrons. The principle of operation with the P-type channel is the same, only it is necessary to supply negative U. If positive Uz is applied to the gate, then an electric field will appear, pushing holes out of the P zone in the direction of the substrate (Fig. 8).

Under the gate, the concentration of free charge carriers will begin to decrease, and their place will be taken by electrons, which are attracted by the positive charge of the gate. When Uzi reaches the threshold value, the electron concentration will be much higher than the hole concentration. As a result of this, a channel with N-type conductivity will be formed between C and I, through which Iis will flow. It can be concluded that the dependence of Ic on Uz is directly proportional: with an increase in U, the channel expands and Ic increases. This process is one of the PT modes - enrichment.

Figure 8 - Illustration of the operation of a PT with an induced channel (type N).

The I - V characteristic of a FET with an insulated gate is approximately the same as with a control junction (Fig. 9). The area in which Iis grows in direct proportion to the growth of Uis is the ohmic region (saturation). The area at the maximum channel expansion, in which Ic does not grow, is an active area.

When the threshold value U is exceeded, the pn junction breaks through, and the PT is an ordinary conductor. In this case, the radio component fails.

Figure 9 - I - V characteristic of a PT with an insulated gate.

The difference between FETs with built-in and induced channels is the presence of a conductive channel between C and I. If you connect U of different polarity to a PT with a built-in channel between drain and source and leave the gate is turned on (Uzi = 0), then Iis will flow through the channel (the flow of free charge carriers is electrons). When U <0 is connected to the gate, an electric field appears that pushes electrons towards the substrate. There will be a decrease in the concentration of free charge carriers, and the resistance will increase, therefore, Ic - will decrease. This state is the impoverishment mode.

When U> 0 is connected to the gate, an electromagnetic field arises, which will attract electrons from the drain, source and substrate. As a result of this, the channel will expand and its conductivity will increase, and Iis will increase. The PT will start working in the enrichment mode. The current-voltage characteristic (VAC) is shown in Figure 10.

Figure 10 - I - V characteristic of a PT with a built-in channel.

Despite their versatility, PTs have advantages and disadvantages. These disadvantages follow from the device, the way of execution and the I - V characteristic of the devices.

Advantages and disadvantages

Advantages and disadvantages are conventional concepts taken from the comparison of field-effect and bipolar transistors. One of the properties of the PT is a high resistance Rin. Moreover, for MOFSET, its value is several orders of magnitude higher than that of JFET. FETs practically do not consume current from the signal source that needs to be amplified.

For example, if you take an ordinary circuit that generates a signal based on a microcircuit microcontroller. This circuit controls the operation of the electric motor, but has a low current value, which is not enough for this purpose. In this case, an amplifier is needed that consumes a small amount of I and generates a high current at the output. In an amplifier of this type, a JFET should be used, which has a high Rin. JFET has a low U gain. When building an amplifier based on JFET (1 pc.), The maximum gain will be about 20, when using a bipolar one - several hundred.

High quality amplifiers use both types of transistor. With the help of the FET, amplification occurs in I, and then, with the help of a bipolar, the signal is amplified in U. However, PTs have a number of advantages over bipolar ones. These benefits are as follows:

1. High Rin, due to which there is a minimum consumption of I and U.
2. High gain in I.
3. Operational reliability and noise immunity: in the absence of I flow through the gate, as a result of which the gate control circuit is isolated from the drain and the source.
4. High speed of transition from one state to another, which allows the use of PT at high frequencies.

In addition, despite its widespread use, FETs have several drawbacks that do not allow completely displacing bipolar transistors from the market. The disadvantages include the following:

1. Increased drop in U.
2. Destruction temperature of the device.
3. Consuming more energy at high frequencies.
4. The emergence of a parasitic bipolar transistor (PBT).
5. Sensitive to static electricity.

The increased drop in U occurs due to the high R between drain and source during the open state. The PT is destroyed when the temperature exceeds 150 degrees Celsius, and the bipolar one - 200. The PT has low power consumption only at low frequencies. Above 1.6 GHz, power consumption increases exponentially. Based on this, the frequencies of microprocessors have ceased to grow, and the emphasis is on creating machines with a large number of cores.

When a powerful PT is used, a PBT is formed in its structure, upon opening which the PT fails. To solve this problem, the substrate is short-circuited with I. However, this does not completely solve the problem, since a jump in U can lead to the opening of the PBT and the failure of the PT, as well as the chains of parts that are connected to it.

A significant disadvantage of PTs is their sensitivity to static electricity. This disadvantage comes from the design features of the PT. The dielectric (insulating) layer is thin and very easily destroyed by static electricity, which can reach hundreds or thousands of volts. To prevent failure when exposed to static electricity, the substrate is grounded and short-circuited with the source. In addition, in some types of FETs, there is a diode between the drain and the source. When working with integrated microcircuits on PTs, antistatic measures should be used: special bracelets and transportation in vacuum antistatic packages.

Connection diagrams

The PT is connected in about the same way as an ordinary one, but there are some peculiarities. There are 3 schemes for switching on field-effect transistors: with a common source (OI), drain (OS) and gate (OZ). Most often, a connection scheme with an OI is used (diagram 1). This connection allows for significant power gain. However, the OI connection is used in low frequency amplifiers, and also has a high input capacitive characteristic.

Scheme 1 - Switching on with OI.

When turned on with the OS (scheme 2), a cascade with a follower is obtained, which is called the source. The advantage is the low input capacity. It is used for the manufacture of buffer dividing cascades (for example, a piezoelectric sensor).

Scheme 2 - Connection with OS.

When connected with OZ (Scheme 3), there is no significant current amplification, the power gain is lower than when connected with OI and OS. However, with this type of connection it is possible to completely avoid the Miller effect. This feature allows you to increase the maximum gain frequency (microwave gain).

Scheme 3 - Switching on with OZ.

Thus, PTs are widely used in the field of information technology. However, bipolar transistors could not oust bipolar transistors from the radio components market. This is primarily due to the shortcomings of the PT, which lie in the principle of operation and design features. The main disadvantage is the high sensitivity to static electricity fields.