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Commonly used FET and MOS FET detection methods


There are two types of FETs: junction type and insulated gate type. The junction field effect transistor (JFET) gets its name because it has two PN junctions, and the insulated gate field effect transistor (JGFET) gets its name because the gate is completely insulated from other electrodes. At present, among the insulated gate type field effect transistors, the most widely used is MOS field effect transistor, referred to as MOS transistor (ie metal-oxide-semiconductor field effect transistor MOSFET); in addition, there are PMOS, NMOS and VMOS power field effect transistors, etc.


According to the different channel semiconductor materials, the junction type and the insulated gate type are divided into two types: channel and P-channel. If divided according to the conduction mode, the FET can be divided into depletion type and enhancement type. The junction field effect transistors are all depletion type, and the insulated gate field effect transistor has both depletion type and enhancement type.


Field effect transistors can be divided into junction field effect transistors and MOS field effect transistors. The MOS field effect transistors are divided into four categories: N-channel depletion type and enhancement type; P-channel depletion type and enhancement type.


What are the commonly used field effect tubes, as follows.


1. MOS field effect transistor


That is, the metal-oxide-semiconductor type field effect transistor belongs to the 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 high input resistance. It is also divided into N-channel tube and P-channel tube, usually the substrate (substrate) and the source S are connected together. According to the different conduction modes, MOSFETs are divided into enhancement mode and depletion mode.


The so-called enhancement type means: when VGS=0, the tube is in an off state. After adding the correct VGS, the majority of carriers are attracted to the gate, thereby "enhancing" the carriers in this area and forming a conductive channel. . The depletion type means that when VGS=0, the channel is formed, and when the correct VGS is added, the majority of carriers can flow out of the channel, thus "depleting" the carriers and turning the tube off.


Taking the N channel as an example, it is formed on a P-type silicon substrate with two highly doped source diffusion regions N+ and drain diffusion regions N+, and then the source S and the drain D are drawn out respectively. The source electrode and the substrate are connected internally, and the two always maintain an equal potential. When the drain is connected to the positive pole of the power supply and the source is connected to the negative pole of the power supply and VGS=0, the channel current (ie the drain current) ID=0. As VGS gradually increases, attracted by the positive gate voltage, negatively charged minority carriers are induced between the two diffusion regions, forming an N-type channel from drain to source. When VGS is greater than the tube When the turn-on voltage VTN (generally about +2V), the N-channel tube starts to conduct, forming the drain current ID.


Because the input resistance of the MOS field effect transistor is very high, and the capacitance between the gate and the source is very small, it is very easy to be charged by the induction of external electromagnetic fields or static electricity, and a small amount of charge can form a relatively high voltage on the capacitance between the electrodes. (U=Q/C), the tube will be damaged.


Therefore, all the pins are twisted together at the factory, or installed in metal foil, so that the G pole and the S pole are equipotential to prevent the accumulation of electrostatic charges. All leads should also be shorted when the tube is not in use. Be extra careful when measuring, and take corresponding anti-static measures.


Detection method of MOS field effect transistor


(1) Preparation work


Before measurement, 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 connect with the earth, so that the human body and the earth maintain an equal potential. Separate the pins again and remove the wires.


(2) Judgment electrode


Set the multimeter to the R×100 gear, and first determine the grid. If the resistance of a pin and other pins are infinite, it proves that this pin is the gate G. The resistance value between S-D should be several hundred ohms to several thousand ohms, and when the resistance value is smaller, the black test lead is connected to the D pole, and the red test lead is connected to the S pole.


(3) Check the amplification capability (transconductance)


Put the G pole in the air, connect the black test lead to the D pole, the red test lead to the S pole, and then touch the G pole with your finger, the needle should have a large deflection. The double gate MOS field effect transistor has two gates G1 and G2. In order to distinguish them, the G1 and G2 poles can be touched respectively by hand, and the G2 pole is the one with the larger deflection of the needle to the left.


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