Identification and Judgment of FET Using a Multimeter
Field effect transistor, abbreviated as FET. A typical transistor is composed of two polarities of charge carriers, namely majority charge carriers and minority charge carriers of opposite polarity, participating in conduction, hence it is called a bipolar transistor. In contrast, FET is only composed of majority charge carriers participating in conduction, and it is the opposite of bipolar, also known as a unipolar transistor. It belongs to voltage controlled semiconductor devices and has the advantages of high input resistance (108-109 Ω), 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 to bipolar transistors and power transistors.
Testing of Field Effect Transistors
1. Pin identification of junction field-effect transistor:
The gate of a field-effect transistor is equivalent to the base of the transistor, and the source and drain correspond to the emitter and collector of the transistor, respectively. Place it in the R × 1k range and use two probes to measure the forward and reverse resistance between each two pins. When the forward and reverse resistances between two pins are equal, both in the order of K Ω, these two pins are the drain D and source S (interchangeable), and the remaining pin is the gate G. For a junction field-effect transistor with four pins, the other pole is the shield (grounded during use).
2. Judgment gate
Touch one electrode of the tube with the black probe of the multimeter, and touch the other two electrodes with the red probe. If the resistance values measured twice are both very small, it indicates that they are both forward resistors. This transistor belongs to the N-channel field-effect transistor, and the black probe is also connected to the gate.
The manufacturing process determines that the source and drain of the field-effect transistor are symmetrical, interchangeable, and do not affect the normal operation of the circuit, so there is no need to distinguish them. The resistance between the source and drain is about several thousand ohms.
Note that this method cannot be used to determine the gate of an insulated gate field-effect transistor. Because the input resistance of this type of tube is extremely high and the inter electrode capacitance between the gate and source is very small, a small amount of charge can form a high voltage on the inter electrode capacitance during measurement, which can easily damage the tube.
