Field Effect Transistor

What is a Field Effect Transistor?
Field Effect Transistor abbreviation (FET) is referred to as field effect tube. There are two main types: junction FET (junction FET—JFET) and metal-oxide semiconductor FET (metal-oxide semiconductor FET, MOS-FET for short). Participated in conduction by majority carriers, also known as unipolar transistors. It is a voltage-controlled semiconductor device. It has the advantages of high input resistance (107～1015Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, and wide safe working area. Now a strong contender for bipolar transistors and power transistors.
Field Effect Transistor (FET) is a semiconductor device that controls the output loop current by controlling the electric field effect of the input loop, and is named after it.
Because it conducts electricity only by the majority carriers in the semiconductor, it is also called a unipolar transistor.

Features of FETs

Compared with bipolar transistors, field effect transistors have the following characteristics.
(1) The field effect transistor is a voltage control device, which controls ID (drain current) through VGS (gate-source voltage);
(2) The control input current of the FET is extremely small, so its input resistance (107～1012Ω) is very large.
(3) It uses majority carriers to conduct electricity, so its temperature stability is better;
(4) The voltage amplification factor of the amplifier circuit composed of it is smaller than the voltage amplification factor of the amplifier circuit composed of the triode;
(5) The field effect tube has strong radiation resistance;
(6) Since there is no shot noise caused by the diffusion of electrons in chaotic motion, the noise is low.

Classification of FETs

Field effect transistors are divided into two categories: junction field effect transistors (JFETs) and insulated gate field effect transistors (MOS transistors).
According to the channel material type and the insulating gate type, there are two types: N-channel and P-channel; According to the conduction mode: depletion type and enhancement type, junction field effect transistors are all depletion type, and insulated gate field effect transistors have both depletion type and enhancement type.
Field effect transistors can be divided into junction field effect transistors and MOS field effect transistors, and MOS field effect transistors are divided into N-channel depletion type and enhancement type; There are four types of P-channel depletion and enhancement.

Junction Field Effect Transistor (JFET)
1. Classification of junction field effect transistors: junction field effect transistors have two structural forms, which are N-channel junction field effect transistors and P-channel junction field effect transistors.
The junction field effect transistor also has three electrodes, which are: gate; drain; source.
The arrow direction of the gate in the circuit symbol can be understood as the forward conduction direction of the two PN junctions.

2. The working principle of the junction field effect transistor (taking N-channel junction field effect transistor as an example): The structure and symbol of the N-channel structure field effect transistor, because the carriers in the PN junction have been depleted, the PN is basically non-conductive, forming a so-called depletion region. When the drain power supply voltage ED is constant, if the gate voltage is more negative, the depletion region formed at the PN junction interface will be thicker, and the conductive channel between the drain and source will be narrower, and the drain current ID will be smaller ; Conversely, if the gate voltage is not so negative, the channel becomes wider and ID becomes larger, so the change of drain current ID can be controlled by gate voltage EG. That is to say, the FET is a voltage control element.

Insulated Gate Field Effect Transistor

1. Classification of insulated gate field effect transistors (MOS transistors): Insulated gate field effect transistors also have two structural forms, which are N-channel type and P-channel type. No matter what the channel is, they are divided into enhancement type and depletion type.
2. It is composed of metal, oxide and semiconductor, so it is also called metal-oxide-semiconductor field effect transistor, or MOS field effect transistor for short.
3. The working principle of the insulated gate field effect transistor (taking N-channel enhancement MOS field effect transistor as an example): It uses UGS to control the amount of "induced charges" to change the condition of the conductive channel formed by these "induced charges", and then achieve the purpose of controlling the drain current. When the tube is manufactured, a large number of positive ions appear in the insulating layer through the process, so more negative charges can be induced on the other side of the interface, and these negative charges connect the N region of the high-osmosis impurity to form a conductive channel. Road, even when VGS=0, there is a large drain current ID. When the gate voltage changes, the amount of charge induced in the channel also changes, and the width of the conductive channel also changes accordingly, so the drain current ID changes with the change of the gate voltage.
There are two working modes of field effect transistors: when the gate voltage is zero, there is a large drain current called depletion mode; When the gate voltage is zero, the drain current is also zero, and the drain current must be added after a certain gate voltage is called enhanced.

FET action

1. Field effect tubes can be used for amplification. Since the input impedance of the FET amplifier is very high, the coupling capacitor can have a small capacity, and it is not necessary to use an electrolytic capacitor.
   2. The high input impedance of the FET is very suitable for impedance transformation. It is often used for impedance transformation in the input stage of multi-stage amplifiers.
   3. Field effect transistors can be used as variable resistors.
   4. The FET can be conveniently used as a constant current source.
   5. Field effect transistors can be used as electronic switches.

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Advantages of using field effect tubes

Field-effect transistors are voltage-controlled elements, while transistors are current-controlled elements. In the case where only less current is allowed to be taken from the signal source, a field effect tube should be selected;
However, under the condition that the signal voltage is low and more current is allowed to be taken from the signal source, a transistor should be selected. Field effect transistors use majority carriers to conduct electricity, so they are called unipolar devices, while transistors use both majority carriers and minority carriers to conduct electricity, so they are called bipolar devices.
The source and drain of some field effect transistors can be used interchangeably, and the gate voltage can also be positive or negative, which is more flexible than a triode.
Field effect transistors can work under conditions of very small current and very low voltage, and its manufacturing process can easily integrate many field effect transistors on a silicon chip. Therefore, field effect transistors have been widely used in large-scale integrated circuits.

Comparison of field effect tubes and triodes (respective application characteristics)

1. The source s, gate g, and drain d of the field effect transistor correspond to the emitter e, base b, and collector c of the triode respectively, and their functions are similar.
   2. The FET is a voltage-controlled current device, and iD is controlled by vGS, and its amplification factor gm is generally small, so the FET’s amplification capability is poor; the triode is a current-controlled current device, and iC is controlled by iB (or iE).
   3. The gate of the field effect transistor hardly draws current (ig» 0); while the base of the triode always draws a certain current when it is working. Therefore, the gate input resistance of the FET is higher than that of the triode.
   4. The field effect tube is conducted by multiple sub-conductors; The triode has two types of carriers, multi-carrier and few-carrier, involved in conduction. The carrier concentration is greatly affected by factors such as temperature and radiation, so field effect transistors have better temperature stability and stronger radiation resistance than transistors. Field effect transistors should be selected when environmental conditions (temperature, etc.) vary greatly.
   5. When the source metal and the substrate of the field effect transistor are connected together, the source and drain can be used interchangeably, and the characteristics do not change much; However, when the collector and emitter of the triode are used interchangeably, their characteristics are very different, and the β value will be greatly reduced.
   6. The noise figure of the field effect tube is very small, and the field effect tube should be selected in the input stage of the low noise amplifier circuit and the circuit requiring a high signal-to-noise ratio.
   7. Field effect transistors and triodes can be used to form various amplification circuits and switching circuits. However, the former is widely used in large-scale and ultra-large-scale integrated circuits due to its simple manufacturing process, low power consumption, good thermal stability, and wide operating power supply voltage range.
   8. The on-resistance of the triode is large, and the on-resistance of the field effect tube is small, only a few hundred milliohms. In current electrical devices, field effect transistors are generally used as switches, and their efficiency is relatively high.

   
   

   

Comparison of Field Effect Transistors and Bipolar Transistors

The field effect transistor is a voltage control device, and the gate basically does not take current, while the transistor is a current control device, and the base must take a certain current. Therefore, when the rated current of the signal source is extremely small, a field effect tube should be selected.
The field effect transistor is multi-carrier conduction, and the two types of carriers of the transistor participate in conduction. Since the concentration of minority carriers is very sensitive to external conditions such as temperature and radiation, it is more appropriate to use field effect transistors for occasions where the environment changes greatly.
In addition to being used as an amplifier device and a controllable switch like a transistor, a field effect tube can also be used as a voltage-controlled variable linear resistor.
The source and drain of the field effect transistor are symmetrical in structure and can be used interchangeably. The gate-source voltage of the depletion MOS transistor can be positive or negative. Therefore, the use of FETs is more flexible than transistors.

The model name of the field effect tube

There are two naming methods.
The first naming method is the same as that of bipolar transistors. The third letter J stands for junction field effect transistor; O stands for insulated gate field effect transistor. The second letter represents the material, D is P-type silicon, and the inversion layer is N-channel; C is N-type silicon P-channel. For example, 3DJ6D is a junction type P-channel field effect transistor, and 3DO6C is an insulated gate type N-channel field effect transistor.
The second naming method is CS××#: CS stands for field effect tube, ×× represents the serial number of the model with numbers, and # represents different specifications in the same model with letters. Such as CS14A, CS45G, etc.

The main parameters of the FET

DC parameters
The saturated drain current IDSS can be defined as: when the voltage between the gate and the source is equal to zero, and the voltage between the drain and the source is greater than the pinch-off voltage, the corresponding drain current.
The pinch-off voltage UP can be defined as: UGS required to reduce ID to a tiny current when UDS is constant.
Turn-on voltage UT can be defined as: UGS required to make ID reach a certain value when UDS is constant.

AC parameter
AC parameters can be divided into two parameters: output resistance and low-frequency mutual conductance. The output resistance is generally between tens of kiloohms and hundreds of kiloohms. The low-frequency mutual conductance is generally in the range of a few tenths to several milliohms, and the special one can reach 100mS, or even higher.
The low-frequency transconductance gm describes the control effect of the gate and source voltages on the drain current.
The capacitance between the three electrodes of the interelectrode capacitance field effect tube, the smaller the value, the better the performance of the tube.
Limit parameter
① The maximum drain current refers to the allowable upper limit of the drain current when the tube is working normally.
② The maximum dissipated power refers to the power in the tube, which is limited by the maximum working temperature of the tube.
③ The maximum drain-source voltage refers to the voltage when the avalanche breakdown occurs and the drain current begins to rise sharply.
④ The maximum gate-source voltage refers to the voltage value when the reverse current between the gate and source begins to increase sharply.
In addition to the above parameters, there are other parameters such as inter-electrode capacitance and high frequency parameters.
Drain and source breakdown voltage When the drain current rises sharply, UDS during avalanche breakdown occurs.
Gate breakdown voltage When the junction field effect transistor is working normally, the PN junction between the gate and the source is in a reverse bias state. If the current is too high, breakdown will occur.

The main parameters of concern when using are:
1. IDSS—saturated drain-source current: refers to the drain-source current when the gate voltage UGS=0 in a junction or depletion insulated gate field effect transistor.
2. UP—Pinch-off voltage: refers to the gate voltage when the drain-source is just cut off in a junction or depletion-type insulated gate field effect transistor.
3. UT—turn-on voltage: refers to the gate voltage when the drain-source is just turned on in the enhanced insulated gate field effect tube.
4. gM—transconductance: It indicates the control ability of the gate-source voltage UGS-to the drain current ID, that is, the ratio of the variation of the drain current ID to the variation of the gate-source voltage UGS. gM is an important parameter to measure the amplification ability of the field effect tube.
5. BUDS—drain-source breakdown voltage: refers to the maximum drain-source voltage that the FET can withstand when the gate-source voltage UGS is constant. This is a limit parameter, and the working voltage applied to the FET must be less than BUDS.
6. PDSM—Maximum Dissipated Power: It is also a limit parameter, which refers to the maximum drain-source dissipated power allowed when the performance of the field effect tube does not deteriorate. When in use, the actual power consumption of the FET should be less than that of the PDSM with a certain margin.
7. IDSM—Maximum Drain-Source Current: It is a limit parameter, which refers to the maximum current allowed to pass between the drain and source when the FET is working normally. The working current of the FET should not exceed IDSM.