How to judge the quality of field effect transistors and test them, engineers must know

1 Qualitative judgment of the quality of MOS field effect tubes

First use the multimeter R×10kΩ gear (built-in 9V or 15V battery), connect the negative probe (black) to the gate (G), and the positive probe (red) to the source (S). Charge between the gate and the source, and the multimeter pointer will deflect slightly. Then use the multimeter R×1Ω gear, connect the negative probe to the drain (D), and the positive probe to the source (S). If the multimeter indicates a few ohms, it means that the field effect tube is good.

2 Qualitative judgment of the electrodes of the junction field effect tube

Set the multimeter to R×100 gear, connect the red probe to any one leg, and the black probe to another leg, leaving the third leg hanging. If the needle is found to swing slightly, it proves that the third leg is the gate. To obtain a more obvious observation effect, you can also use the human body to approach or touch the hanging leg with your fingers. As long as you see the needle deflect greatly, it means that the hanging leg is the gate, and the other two legs are the source and drain.

Judgment reason: The input resistance of JFET is greater than 100MΩ, and the transconductance is very high. When the gate is open, the electromagnetic field in space can easily induce a voltage signal on the gate, causing the tube to tend to be cut off or turned on. If the human body induced voltage is directly added to the gate, the above phenomenon will be more obvious due to the strong input interference signal. If the needle deflects significantly to the left, it means that the tube tends to be cut off, the drain-source resistance RDS increases, and the drain-source current decreases IDS. On the contrary, if the needle deflects significantly to the right, it means that the tube tends to be turned on, RDS↓, IDS↑. However, the direction in which the needle deflects depends on the polarity of the induced voltage (forward voltage or reverse voltage) and the working point of the tube.

Note:

(1) The experiment shows that when both hands are insulated from the D and S poles and only touch the gate, the needle generally deflects to the left. However, if both hands touch the D and S poles respectively and touch the gate with fingers, it is possible to observe that the needle deflects to the right. The reason is that several parts of the human body and resistors bias the field effect tube, causing it to enter the saturation zone.

(2) You can also lick the gate with the tip of your tongue, and the phenomenon is the same as above. 

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3 Identification of transistor pins

The transistor is composed of a tube core (two PN junctions), three electrodes and a tube shell. The three electrodes are called collector c, emitter e and base b. The most common transistor is a silicon planar tube, which is divided into PNP and NPN types. Germanium alloy tubes are now rare.

Here we introduce a simple method of how to use a multimeter to measure the three pins of a transistor. 

(1) Find the base and determine the tube type (NPN or PNP).

For a PNP transistor, the C and E poles are the positive poles of the two PN junctions inside, and the B pole is their common negative pole. For an NPN transistor, it is just the opposite: the C and E poles are the negative poles of the two PN junctions, and the B pole is their common positive pole. Based on the characteristics of the PN junction with small forward resistance and large reverse resistance, it is very convenient to determine the base and tube type. The specific method is as follows: 
Set the multimeter to R×100 or R×1K. Use a red pen to touch a certain pin, and use a black pen to touch the other two pins respectively. In this way, three groups (two times each) of readings can be obtained. When one of the two measurements is a low resistance of several hundred ohms, if the common pin is a red pen, it is touching the base, and the tube type of the transistor is PNP; if the common pin is a black pen, it is also touching the base, and the tube type of the transistor is NPN.

(2) Distinguishing the emitter and collector

Since the doping concentrations of the two P regions or two N regions are different when the triode is manufactured, if the emitter and collector are used correctly, the triode has a strong amplification ability. On the contrary, if the emitter and collector are used interchangeably, the amplification ability is very weak. In this way, the emitter and collector of the tube can be distinguished. After distinguishing the tube type and the base b, the following method can be used to distinguish the collector and emitter. Set the multimeter to the R×1K position. Pinch the base and the other pin together with your hand (be careful not to let the electrodes touch each other directly). To make the measurement phenomenon obvious, you can wet your fingers, connect the red test lead to the pin pinched together with the base, and the black test lead to the other pin. Pay attention to the amplitude of the multimeter pointer swinging to the right. Then swap the two pins and repeat the above measurement steps. Compare the amplitude of the needle swinging to the right in the two measurements and find the one with the larger swing amplitude. For PNP transistors, connect the black test lead to the pin pinched together with the base, repeat the above experiment, and find the time when the needle swings the most. For NPN, the black test lead is connected to the collector, and the red test lead is connected to the emitter. For PNP, the red test lead is connected to the collector, and the black test lead is connected to the emitter.

The principle of this electrode identification method is: use the battery inside the multimeter to add voltage to the collector and emitter of the transistor to give it amplification ability. When the base and collector are pinched by hand, it is equivalent to adding a forward bias current to the transistor through the resistance of the hand, making it conductive. At this time, the swing amplitude of the needle to the right reflects the size of its amplification ability, so the emitter and collector can be correctly identified.