How to check a mosfet: examining a transistor with a multimeter

In modern electronics, MOSFETs are among the most widely used radioelements. Despite their reliability, they often fail, which is associated with violations of the regime in their work. At the same time, the search for a faulty element in connection with the specifics of the field-effect transistor device causes certain difficulties. But knowing the principle of operation of the radio component, it is not so difficult to check the mosfet with a multimeter.

Features of the MOSFET

The difference between a field-effect transistor and a classic bipolar transistor is that its operation depends on the applied voltage, and not the current. In the literature, such a radio element is often called a MOS transistor (metal oxide semiconductor) or MOS transistor (metal dielectric semiconductor). In the English version, its name sounds like a mosfet, formed from a MOSFET (Metal-Oxide-Semiconductor-Field-Effect-Transistor).

Field-effect transistors are active elements, that is, their operation is impossible without applying voltage to the terminals. For the first time, the idea of ​​creating a device, the flow of charge carriers in which is controlled by the magnitude of the applied voltage, was proposed by the Austro-Hungarian scientist Julius Lilienfeld. However, the lack of technology for creating such a device made it possible to release a prototype only in 1960. Since 1977, mosfets began to be used in the production of electronic computers, thereby increasing the productivity of the latter.

Various scientists of the world are constantly conducting research to improve the operation of an electronic device, therefore, to date, several types of field-effect transistors have been invented and introduced into production. Each of them has its own advantages and disadvantages, but the general principle of their work is the same.

Types and design

Mosfets are divided into two groups. Depending on the type of the control electrode, they can be: with a p-n junction and an insulated gate. Recently, the first type of elements are starting to be used less and less. Transistors with a control p-n junction are structurally a semiconductor base, the main charge carriers of which can be both holes (p-type) and electrons (n-type).

At the ends of the base, conclusions are made, called drain and source. The controlled part of the circuit is connected to these contacts. The device is controlled through the third terminal of the transistor (gate), formed by connecting it to the base of the reverse conductor. Thus, The p-n transistor has three pins:

1. The source is the entrance through which the main carriers of energy enter.
2. A drain is a device outlet through which the main energy carriers leave.
3. The shutter is an output that controls the passage of charges through the device.

Depending on the type of conductivity of the gate electrode, such mosfets are divided into n and p types.

A radio element with an insulated gate has a different structure. Its gate is separated from the base by a dielectric layer. In the manufacture of the device, a semiconductor with a high resistivity is used. It is called a backing or shutter. Two zones with a reverse type of conductivity are created on it - drain and source. Thus, there are three areas. The distance between the controlled electrodes is very small, and the shutter separated from them is covered with a dielectric layer of the order of 0.1 micrometer. Typically, a SiO2 compound is used as a dielectric.

Depending on the manufacturing method, devices with an insulated contact are divided into two types: depleted and enriched. The former are only available in n-type and can have two gates, while the latter are available in both n and p-type.

Enriched type devices are called induced channel transistors. In them, the controlled contacts are not connected by a conductive layer. Therefore, the drain current appears only when a certain potential difference is applied to the gate relative to the source. Depleted transistors in their design contain a built-in channel, which is why the transistor reacts to voltage of both positive and negative polarity.

Radioelement characteristics

In the diagrams and in the literature, it is customary to designate the mosfet by the Latin letters VT, followed by its serial number in the diagram. Graphically, a field element is depicted as a circle, in the middle of which straight lines are drawn, indicating the path of current flow. The type of conductivity is indicated on the gate terminal in the form of an arrow. The gate, drain and source are signed, respectively, with the letters of the Latin alphabet - S, D, G.

Field devices are characterized by many parameters. But among the main ones, the following characteristics are distinguished:

1. Voltage between controlled electrodes. Shows the amount of voltage that the transistor can withstand without degrading its parameters. That is, in practice, this is the maximum voltage of the power source, for which the transistor is designed to work.
2. Drain current strength. Usually, the maximum value is indicated for a certain amount of DC voltage applied to the gate - source.
3. On-state drain-source channel impedance. The higher this value is, the worse the transistor works, since energy losses occur on the resistance, and the heating of the mosfet increases.
4. Dissipation power. Depends on the ambient temperature. This parameter is depicted as a characteristic showing the dependence of power on temperature.
5. Saturation level of the source-gate channel. Indicates the boundary value of the potential difference, upon overcoming which the current does not pass through the channel.
6. Switch-on threshold. This is the minimum voltage that must be applied to the transistor to open its conductive channel.
7. Shutter capacity. A significant drawback of field-effect transistors is associated with this parameter. So, because of the parasitic capacitance, the use of devices in high-frequency circuits is limited, reducing the speed of switching operating modes.

It is also important to know that mosfets are sensitive to static electricity, especially with insulated gate devices. Therefore, when checking the field-effect transistor with a multimeter, you should wear antistatic bracelets on both hands, and you should also not wear woolen clothes.

Principle of operation

The essence of the operation of a radioelement with an insulated gate is to control the amount of current passing through it by changing the potential difference. When a voltage is applied to the source and gate, an electric field transverse to the applied one is generated in the device. This field increases the number of free charge carriers in the surface layer.

Because of this, a significant amount of charge carriers begins to accumulate near the dielectric, as a result of which a conduction band is formed. A current begins to flow through this area, that is, between the controlled outputs. When the voltage is removed from the open gate, conductivity will disappear, and the flow of current will stop.

Slightly different processes occur in the operation of a field-effect transistor with a pn junction. If a voltage opposite to the main charge carriers is applied to this junction, its region begins to expand. An increase in the transition leads to a narrowing of the thickness of the conducting channel, which means an increase in resistance. As a result, the current passing between the drain and the source decreases. Thus, by changing the voltage level, the current passing through the transistor also changes.

Measurement methods

For measuring the parameters of field-effect transistors specialized devices are used. Their work is based on the use of a microcontroller and a built-in generator. A signal of a certain type is applied to the contacts of the transistor, as a result of which it changes. Using a built-in analyzer, the device evaluates these changes and converts the data into easy-to-read information. The whole point of using such a meter comes down to installing a mosfet in special contact pads and pressing the start button.

In everyday life, home-made devices are often used by radio amateurs. So, the simplest type of device of several elements allows you to measure the resistance of the channels. To do this, use: a voltmeter, a car light, a voltage source and a resistor of about 100 ohms. Having assembled such a circuit, you can easily measure the Rds of the radioelement, thereby checking the mosfet for operability.

But the easiest and fastest way to diagnose a radio element is to use a multimeter. With its help, it is easy to check the mosfet for the ability to work in a key mode. And if, according to the results of the check, it opens and closes normally, then the probability of its serviceability is very high.

Gate transistor

For a better understanding of the mosfet verification process, it can be represented in the form of an equivalent circuit like a triangle. Two sides of such a triangle are two diodes, and the third is a resistor. In this case, the connection point of the diodes is considered the gate, and their connection to the resistor is the drain and source.

Having presented the equivalent diagram, you can begin to validate the element. For example it is convenient to consider one of the types of conductivity, for example, n-type:

1. Measurement of channel resistance. To do this, using the measurement selection switch, the multimeter is set to resistance test mode. The measurement limit is chosen to be about two megohms. The probes of the device touch the drain and source of the transistor. As a result, a number equal to the junction resistance will appear on the multimeter screen. Then the polarity of the probes is changed, and the resistance is measured again. With a working mosfet, these values ​​should be approximately the same. Such a connection in the equivalent circuit corresponds to the position when the resistance value of the resistor would be measured.
2. Checking the gate-source transition. For this, the multimeter switches to diode continuity mode. The measuring wire, connected to the positive of the tester, touches the gate, and the negative - to the source. The result of this action will be the measurement of the voltage drop across the open junction with a multimeter. Its value should be approximately 600-700 millivolts. The next step is to reverse the polarity of the attached wires. If the mosfet is working properly, the tester will show infinity. This will indicate that the transition is closed.
3. Study of the drain-gate transition. The multimeter is left in diode continuity mode. But the positive probe touches the shutter, and the negative probe touches the drain. In this case, the tester should show a voltage drop across the junction of the order of 600-700 millivolts. When the polarity is reversed, if the transistor is operational, the tester will show infinity.

If all three points are completed correctly, the mosfet is considered to be operational. Checking a radio element of another type is carried out in the same way, only the polarity of the connection of the probes changes.

Insulated gate mosfet

This type of transistor has a built-in diode in its case, located between the source and drain, therefore, it is he who is initially checked for serviceability. To check it, the multimeter switches to the diode test mode, and its probes are connected to the drain and source. In the forward direction, the device should show a voltage drop, and in the case of a polarity change - infinity.

The main test of the transistor is to simulate its operation in key mode. In the case of an n-type radio element its diagnosis is carried out as follows:

1. The multimeter switches to a diode test.
2. A probe connected to the minus is touched to the source, and to the plus - to the shutter.
3. The positive wire is transferred to the drain. If the mosfet is working, then the resistance of the transition will be very low, that is, the channel will become open.
4. Next, the positive probe is connected to the source, and the negative probe is connected to the gate. After these steps, the transistor will close.

Based on the measurement results, a conclusion is made about the performance of the element. Thus, observing the sequence of the above steps, you can test any type of mosfet for operability using a multimeter.