Field Effect Transistor Detailed Introduction

Field Effect Transistor (FET) is abbreviated as field effect tube. General transistors are conducted by two polarity carriers, namely majority carriers and minority carriers of opposite polarity, so they are called bipolar transistors, while FETs are conducted by only majority carriers, which is opposite to bipolar transistors and is also called unipolar transistors. It is a voltage-controlled semiconductor device with high input resistance (108~109Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, and wide safe working area. It has become a strong competitor of bipolar transistors and power transistors.

1. Classification of field effect
transistors Field effect transistors are divided into two categories: junction type and insulated gate type. Junction field effect transistors (JFET) are named because they have two PN junctions, and insulated gate field effect transistors (JGFET) are named because the gate is completely insulated from other electrodes. At present, the most widely used insulated gate field effect transistors are MOS field effect transistors, referred to as MOS tubes (i.e. metal-oxide-semiconductor field effect transistors MOSFET); in addition, there are PMOS, NMOS and VMOS power field effect transistors, as well as the recently launched πMOS field effect transistors and VMOS power modules.
According to the different channel semiconductor materials, junction type and insulated gate type are divided into channel and P channel respectively. If divided by the conduction method, field effect transistors can be divided into depletion type and enhancement type. Junction field effect transistors are all depletion type, and insulated gate field effect transistors are 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 four categories: N channel depletion type and enhancement type; P channel depletion type and enhancement type. See the figure below.

2. Field Effect Transistor Model Naming Method
There are currently two naming methods. The first naming method is the same as the bipolar transistor. The third letter J represents the junction field effect transistor, and O represents the 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 N-channel field effect transistor, and 3DO6C is an insulated gate N-channel field effect transistor.
The second naming method is CS××#, CS represents the field effect transistor, ×× represents the model number with numbers, and # represents different specifications in the same model with letters. For example, CS14A, CS45G, etc.

3. Field Effect Transistor Parameters
There are many parameters for field effect transistors, including DC parameters, AC parameters and limit parameters, but generally the following main parameters are concerned when used:
1. I DSS — saturated drain-source current. It refers to the drain-source current when the gate voltage U GS=0 in a junction or depletion-type insulated gate field effect transistor.
2. UP — pinch-off voltage. It 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. It refers to the gate voltage when the drain-source is just turned on in an enhancement-type insulated gate field effect transistor.
4. gM — transconductance. It indicates the control ability of the gate-source voltage U GS — on the drain current ID, that is, the ratio of the change in the drain current ID to the change in the gate-source voltage UGS. gM is an important parameter for measuring the amplification ability of the field effect transistor.
5. BUDS — drain-source breakdown voltage. It refers to the maximum drain-source voltage that the field effect transistor can withstand for normal operation when the gate-source voltage UGS is constant. This is a limit parameter. The working voltage applied to the field effect tube must be less than BUDS.
6. PDSM - Maximum dissipated power. It is also a limit parameter. It 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 field effect tube should be less than PDSM and leave a certain margin.
7. IDSM - Maximum drain-source current. It is a limit parameter. It refers to the maximum current allowed to pass between the drain and the source when the field effect tube is working normally. The working current of the field effect tube should not exceed IDSM

The main parameters of several commonly used field effect transistors

4. The role of field effect tubes
1. Field effect tubes can be used for amplification. Since the input impedance of field effect tube amplifiers is very high, the coupling capacitor can be smaller and there is no need to use electrolytic capacitors.
2. The high input impedance of field effect tubes is very suitable for impedance transformation. It is often used in the input stage of multi-stage amplifiers for impedance transformation.
3. Field effect tubes can be used as variable resistors.
4. Field effect tubes can be conveniently used as constant current sources.
5. Field effect tubes can be used as electronic switches.

5. Field Effect Transistor Testing
1. Identification of the pins of the junction field effect tube:
The gate of the field effect tube is equivalent to the base of the transistor, and the source and drain correspond to the emitter and collector of the transistor respectively. Set the multimeter to the R×1k position, and use two test pens to measure the forward and reverse resistance between each two pins. When the forward and reverse resistance between two pins are equal, both are several KΩ, then these two pins are the drain D and the source S (interchangeable), and the remaining pin is the gate G. For a junction field effect tube with 4 pins, the other pole is the shielding pole (grounded during use).
2. Determine the gate
Touch one electrode of the tube with the black test pen of the multimeter, and touch the other two electrodes with the red test pen. If the resistance values ​​measured twice are very small, it means that they are both forward resistances. The tube belongs to an N-channel field effect tube, and the black test pen is also connected to the gate.
The manufacturing process determines that the source and drain of the field effect tube are symmetrical and can be used interchangeably, which does not affect the normal operation of the circuit, so there is no need to distinguish them. The resistance between the source and the drain is about several thousand ohms.
Note that this method cannot be used to determine the gate of the insulated gate field effect tube. Because the input resistance of this tube is extremely high, and the inter-electrode capacitance between the gate and the source is very small, as long as there is a small amount of charge during measurement, a very high voltage can be formed on the inter-electrode capacitance, which can easily damage the tube.
3. Estimating the amplification capacity of the field effect tube
Set the multimeter to the R×100 position, connect the red test lead to the source S, and the black test lead to the drain D, which is equivalent to adding a 1.5V power supply voltage to the field effect tube. At this time, the needle indicates the DS inter-electrode resistance value. Then pinch the gate G with your fingers and add the induced voltage of the human body as the input signal to the gate. Due to the amplification effect of the tube, both UDS and ID will change, which is equivalent to the change of the DS inter-electrode resistance. It can be observed that the needle has a large swing. If the needle swings very little when the hand pinches the gate, it means that the amplification capacity of the tube is weak; if the needle does not move, it means that the tube is damaged.
Since the 50Hz AC voltage induced by the human body is relatively high, and different field effect tubes may have different working points when measured with a resistance scale, the needle may swing to the right or to the left when the gate is pinched by hand. The RDS of a few tubes decreases, causing the needle to swing to the right, while the RDS of most tubes increases, causing the needle to swing to the left. Regardless of the direction of the needle's swing, as long as there is an obvious swing, it means that the tube has the ability to amplify.
This method is also applicable to measuring MOS tubes. In order to protect the MOS field effect tube, you must hold the insulating handle of the screwdriver with your hand and touch the gate with a metal rod to prevent the human body induced charge from being directly added to the gate and damaging the tube.
After each measurement of the MOS tube, a small amount of charge will be charged on the GS junction capacitance, establishing the voltage UGS. When the measurement continues, the needle may not move. At this time, short-circuit the GS poles.

The pin order of the commonly used junction field effect transistors and MOS insulated gate field effect transistors is shown in the figure below.

6. Commonly used field effect tubes
1. MOS field effect tubes
Metal-oxide-semiconductor field effect tubes, abbreviated as MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor), are insulated gate type. Its main feature is that there is a silicon dioxide insulating layer between the metal gate and the channel, so it has a very high input resistance (up to 1015Ω). It is also divided into N-channel tubes and P-channel tubes, as shown in Figure 1. Usually the substrate (substrate) is connected to the source S. According to the different conduction methods, MOSFET is divided into enhancement type and depletion type. The so-called enhancement type means that when VGS=0, the tube is in the cut-off state. After adding the correct VGS, most carriers are attracted to the gate, thereby "enhancing" the carriers in this area and forming a conductive channel. Depletion type means that when VGS=0, a channel is formed. When the correct VGS is added, most carriers can flow out of the channel, thereby "depleting" the carriers and turning the tube to cut-off.
Taking the N-channel as an example, it is made of two highly doped source diffusion regions N+ and drain diffusion regions N+ on a P-type silicon substrate, and then the source S and drain D are led out respectively. The source and substrate are connected internally, and the two always maintain the same potential. The front direction in the symbol of Figure 1 (a) is from the outside to the inside, indicating that the N-type channel is pointed from the P-type material (substrate). When the drain is connected to the positive pole of the power supply, the source is connected to the negative pole of the power supply and VGS=0, the channel current (i.e., the drain current) ID=0. As VGS gradually increases, it is attracted by the positive gate voltage, and negatively charged minority carriers are induced between the two diffusion regions, forming an N-type channel from the drain to the source. When VGS is greater than the tube's turn-on voltage VTN (generally about +2V), the N-channel tube begins to conduct, forming a drain current ID.

Typical domestic N-channel MOSFET products include 3DO1, 3DO2, 3DO4 (all of which are single-gate transistors), and 4DO1 (dual-gate transistors). Their pin arrangement (bottom view) is shown in Figure 2.
MOS field effect transistors are relatively "delicate". This is because their input resistance is very high, and the gate-source capacitance is very small. They are easily charged by external electromagnetic fields or static electricity, and a small amount of charge can form a very high voltage (U=Q/C) on the inter-electrode capacitance, which will damage the tube. Therefore, when the factory is in operation, the pins are twisted together or installed in metal foil to make the G pole and the S pole at the same potential to prevent the accumulation of static charge. When the tube is not in use, all leads should also be short-circuited. Be extra careful when measuring and take appropriate anti-static measures.
MOS field effect tube detection method
(1). Preparation
Before measuring, short-circuit the human body to the ground before touching the pins of the MOSFET. It is best to connect a wire on the wrist to the ground to keep the human body and the ground at the same potential. Separate the pins and remove the wires.
(2) Determine the electrode
Set the multimeter to R×100 and first determine the gate. If the resistance of a pin and the other pins is infinite, it proves that this pin is the gate G. Exchange the test leads and measure again. The resistance value between SD should be several hundred ohms to several thousand ohms. The one with the smaller resistance value has the black test lead connected to the D pole and the red test lead connected to the S pole. The 3SK series products produced in Japan have the S pole connected to the tube shell, so it is easy to determine the S pole.
(3) Check the amplification capacity (transconductance)
Leave the G pole suspended, connect the black test lead to the D pole, and the red test lead to the S pole. Then touch the G pole with your finger. The needle should have a large deflection. The dual-gate MOS field effect transistor has two gates G1 and G2. To distinguish them, you can touch the G1 and G2 poles with your hand respectively. The one with the larger deflection of the needle to the left is the G2 pole.
At present, some MOSFET tubes have added a protection diode between the GS poles, so there is no need to short-circuit the pins at ordinary times.

Precautions for using MOS field effect transistors.
MOS field effect transistors should be classified when used and should not be interchanged at will. Due to the high input impedance of MOS field effect transistors (including MOS integrated circuits), they are easily broken down by static electricity. The following rules should be observed when using them:
(1). MOS devices are usually packed in black conductive foam plastic bags when leaving the factory. Do not pack them in any plastic bag at will. You can also use thin copper wire to connect the pins together, or wrap them in tin foil
. (2). The removed MOS devices cannot slide on the plastic board. Metal plates should be used to hold the unused devices.
(3). The soldering iron used for welding must be well grounded.
(4). Before welding, the power line and ground line of the circuit board should be short-circuited, and then separated after the MOS device is welded.
(5). The welding order of the pins of the MOS device is drain, source, and gate. The order is reversed when disassembling the machine.
(6). Before installing the circuit board, use a grounding wire clamp to touch the terminal blocks of the machine, and then connect the circuit board.
(7) If conditions permit, it is best to connect a protection diode to the gate of the MOS field effect transistor. When inspecting the circuit, check whether the original protection diode is damaged.

2. VMOS field effect tube
VMOS field effect tube (VMOSFET) is referred to as VMOS tube or power field effect tube, and its full name is V-groove MOS field effect tube. It is a new high-efficiency, power switching device developed after MOSFET. It not only inherits the high input impedance (≥108W) and small drive current (about 0.1μA) of MOS field effect tube, but also has excellent characteristics such as high withstand voltage (maximum withstand voltage 1200V), large working current (1.5A~100A), high output power (1~250W), good linearity of transconductance, and fast switching speed. It is precisely because it combines the advantages of electron tubes and power transistors that it is widely used in voltage amplifiers (voltage amplification can reach thousands of times), power amplifiers, switching power supplies and inverters.
As we all know, the gate, source and drain of traditional MOS field effect tubes are roughly on the same horizontal plane of the chip, and its working current basically flows in the horizontal direction. VMOS tubes are different. From the lower left figure, we can see its two major structural features: first, the metal gate adopts a V-groove structure; second, it has vertical conductivity. Since the drain is led out from the back of the chip, ID does not flow horizontally along the chip, but starts from the heavily doped N+ region (source S), flows through the P channel into the lightly doped N-drift region, and finally vertically downward to the drain D. The direction of the current is shown by the arrow in the figure. Because the cross-sectional area of ​​the flow is increased, a large current can pass. Since there is a silicon dioxide insulating layer between the gate and the chip, it still belongs to an insulated gate type MOS field effect tube.

The main domestic manufacturers of VMOS field effect tubes include 877 Factory, Tianjin Semiconductor Device Factory No. 4, Hangzhou Electronic Tube Factory, etc. Typical products include VN401, VN672, VMPT2, etc. Table 1 lists the main parameters of six types of VMOS tubes. Among them, the appearance of IRFPC50 is shown in the upper right figure.

VMOS field effect tube detection method
(1). Determine the gate G.
Set the multimeter to R×1k and measure the resistance between the three pins. If the resistance of a pin and its two pins is infinite, and it is still infinite after exchanging the test leads, it proves that this pin is the G pole, because it is insulated from the other two pins.
(2). Determine the source S and drain D.
As shown in Figure 1, there is a PN junction between the source and the drain. Therefore, according to the difference in the forward and reverse resistance of the PN junction, the S pole and the D pole can be identified. Use the method of exchanging test leads to measure the resistance twice. The one with a lower resistance value (generally several thousand ohms to more than ten thousand ohms) is the forward resistance. At this time, the black test lead is the S pole and the red test lead is connected to the D pole.
(3). Measure the drain-source on-state resistance RDS (on).
Short-circuit the GS pole, select the R×1 position of the multimeter, connect the black test lead to the S pole, and the red test lead to the D pole. The resistance value should be several ohms to more than ten ohms.
Due to different test conditions, the measured RDS (on) value is higher than the typical value given in the manual. For example, using a 500-type multimeter at R×1 to measure an IRFPC50 VMOS tube, RDS (on) = 3.2W, which is greater than 0.58W (typical value).
(4) Check transconductance
Set the multimeter to R×1k (or R×100), connect the red probe to the S pole and the black probe to the D pole, and hold a screwdriver to touch the gate. The needle should have obvious deflection. The greater the deflection, the higher the transconductance of the tube.

Notes:
(1) VMOS tubes are also divided into N-channel tubes and P-channel tubes, but most products belong to N-channel tubes. For P-channel tubes, the position of the test leads should be swapped during measurement.
(2) A few VMOS tubes have protection diodes between GS, and items 1 and 2 in this test method are no longer applicable.
(3) There is also a VMOS tube power module on the market, which is specially used for AC motor speed regulators and inverters. For example, the IRFT001 module produced by IR Corporation of the United States has three N-channel and three P-channel tubes inside, forming a three-phase bridge structure.
(4) The VNF series (N-channel) products currently on the market are ultra-high frequency power field effect tubes produced by Supertex Corporation of the United States. Their maximum operating frequency fp=120MHz, IDSM=1A, PDM=30W, and common source small signal low-frequency transconductance gm=2000μS. It is suitable for high-speed switching circuits and broadcasting and communication equipment.
(5) When using VMOS tubes, they must be equipped with a suitable heat sink. Taking VNF306 as an example, the maximum power can reach 30W after the tube is equipped with a 140×140×4 (mm) heat sink.

VII. Comparison between field effect transistors and transistors
(1) Field effect transistors are voltage-controlled elements, while transistors are current-controlled elements. When only a small amount of current is allowed to be drawn from the signal source, field effect transistors should be used; when the signal voltage is low and more current is allowed to be drawn from the signal source, transistors should be used.
(2) 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. They are called bipolar devices.
(3) The source and drain of some field effect transistors can be used interchangeably, and the gate voltage can be positive or negative, which is more flexible than transistors.
(4) Field effect transistors can work under very small current and very low voltage conditions, and their manufacturing process can easily integrate many field effect transistors on a silicon wafer. Therefore, field effect transistors have been widely used in large-scale integrated circuits.