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Field Effect Transistor application

Field Effect Transistor is abbreviated as FET. There are two main types: junction FET (JFET) and metal-oxide semiconductor FET (MOS-FET). A majority carrier participates in conduction, also known as a unipolar transistor. It belongs to 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, wide safe operating area, etc. It has become a powerful bipolar transistor and power transistor competitor.

1. How to prevent breakdown of insulated gate field effect transistor

Because the input impedance of the insulated gate field effect transistor is very high, this is its advantage, but it brings new problems in use. Due to the high input impedance, when the charged object is close to the gate, it is difficult to discharge the charge induced in the gate through this resistor. The accumulation of charge causes a rise in voltage, especially when the capacitance between the poles is relatively small under the circumstances, a small amount of charge will generate a higher voltage, so that the transistor has not been used or it has broken down or the index has dropped during welding. Especially for the MOSFET, its insulation layer is very thin and it is easier to break down damage. In order to avoid such an accident, the key is to avoid floating gates, that is, a DC path must be maintained between the gate and source poles. Usually a resistor (less than 100K) is connected between the gate and source poles so that the accumulated charge is not excessive, or a voltage regulator is connected so that the voltage does not exceed a certain value. During storage, the three electrodes should be short-circuited and placed in a shielded metal box. When welding the transistor to the circuit or removing it, the electrodes should also be short-circuited. The soldering iron instrument used in the installation test should have a good grounding.

2. How to judge the electrode of the junction field effect transistor

Put the multimeter in the RX1K block, use the black test pen to contact the assumed gate G pin, and then use the red test pen to contact the other two pins respectively. If the resistance is relatively small (about 5~10 ohms), then exchange the red and black test once again. If the resistance is large (infinite), it means that the resistance is reverse (PN junction is reversed), which belongs to the N-channel transistor, and the black test pen contact with the gate G, and the original assumption is correct. The resistance measured again is very small, indicating that it is a forward resistance, which belongs to the P-channel field effect transistor, and the black test pen is also in contact with the gate G. If the above situation does not occur, you can replace the red and black test pens, and test according to the above method until you judge the gate. Generally, the source and drain of the junction effect transistor are symmetrical during manufacture, so after the gate G is determined, it is not necessary to judge the source S and the drain D, because these two poles can be used interchangeably, so there is no need to judge. The resistance between the source and drain is about several thousand ohms.

3. Estimation of field effect transistor amplification capacity

The RX100 block of a multimeter can be used to estimate the amplification capacity of the field effect transistor. The specific tests are as follows: the red test pen is connected to the source S, and the black test pen is connected to the drain D, which is equivalent to adding a 1.5 V power supply voltage to thefield effect transistor. At this time, the needle indicates the resistance value between the D and S poles. Then hold the insulation handle of the screwdriver with your hand, and touch the gate with a metal rod to prevent the induced charge from being directly added to the gate, causing the gate breakdown of the MOSFET. Due to the amplification effect of the field effect transistor, both Rds and Id will change, which is also equivalent to the change of the resistance between the D-S poles, and it can be observed the needle have large amplitude swings. If needle swings very little, it means that the field effect transistor has weak amplification ability. If the needle does not move, it means that the field effect transistor is damaged.