A Beginner's Guide to Radio Amateur: How to Test a Field Effect Transistor

Field-effect transistors are semiconductor devices in which the control of transient processes, as well as the magnitude of the output current, is accomplished by changing the magnitude of the electric field. There are two types of these devices: with an isolated gate (in turn, divided into transistors with a built-in channel and with an induction channel) and with a controlled transition. Field effect transistors, due to their unique characteristics, are widely used in radio electronic equipment: power supplies, television sets, computers, etc.

When repairing such equipment, for sure, every novice amateur radio faced such a question: how to test a field effect transistor? Most often with the verification of such elements can be encountered in the repair of switching power supplies. In this article we will describe in detail how to do it correctly.

How to test a FET with an ohmmeter

First of all, in order to start checking the field-effect transistor, it is necessary to deal with its "pin-hole", that is, with the arrangement of the terminals. To date, there are many different versions of such elements, respectively, the location of the electrodes they have different. Semiconductor transistors with signed contacts are often found. For the marking, the Latin letters G, D, S are used. If there is no signature, then it is necessary to use the reference literature.

So, figuring out the marking of the contacts, let's look at how to test the FET. The next step is to take the necessary security measures, because the field devices are very sensitive to static voltage, and to prevent the failure of such an element, it is necessary to arrange grounding. To relieve the accumulated static charge, usually wear an antistatic grounding bracelet on the wrist.

Do not forget that storing FETs is necessary with closed leads. By removing the static voltage, you can proceed to the verification procedure. This requires a simple ohmmeter. In the case of a good element between all the conclusions, the resistance should tend to infinity, but there are some exceptions. Now we'll look at how to test an n-type field effect transistor.

Apply the positive probe to the gate electrode (G), and the negative probe to the source contact (S). At this moment, the gate capacity starts to charge and the cell opens. When measuring the resistance between the source and drain (D), the ohmmeter will show some resistance value. In different types of transistors this value is different. If the terminals of the transistor are shorted, the resistance between the drain and the source will again tend to infinity. If this does not happen, then the transistor is defective.

If you ask how to test a P-type field-effect transistor, then the answer is simple: repeat the above procedure, only change the polarity. We should also not forget that modern high-power field-effect transistors between the source and drain have a built-in diode, so it only "calls" one way.

FET Multimeter Testing

If there is a "multimeter" device, you can check the field effect transistor. To do this, we place the measuring device in the "continuity" mode of the diodes and introduce the field element into saturation mode. If the transistor is N-type, then the negative probe touches the drain, and the positive one - the shutter. A good transistor is then opened. We transfer the positive probe, without detaching the minus one, to the source, and the multimeter shows some resistance value. After that, we lock the transistor: without detaching the probe from the source, we touch the shutter with the minus one and return it to the drain. The transistor is locked, and resistance tends to infinity.

Many radio amateurs ask: "How to check the FET, not evaporating?" Immediately answer that there is no one hundred percent way. To do this, use a multimeter with a HFE shoe, but this method often fails, and you can waste a lot of time.