Field effect transistors and their operation principle

Field-effect transistors are those semiconductor devices whose operation principle is based on the modulation of resistance by the transverse electric field of a semiconductor material.

A distinctive feature of devices of this type is that the field effect transistors have a high voltage amplification factor and high incoming resistance.

Only carriers of the same charge (electrons) participate in the creation of an electric current in these devices.

There are two types of field-effect transistors:

- having a MIS structure; Metal, followed by a dielectric, then a semiconductor (MDP);

- having control pn junction.

The structure of the simplest field-effect transistor includes a plate made of a semiconductor material with only a pn junction in the center and non-straightening contacts along the edges.

The electrode of such a device, through which charge carriers pass to the conductor, is called the source, and the electrode, through which the electrodes exit the channel, is drained.

Sometimes it happens that such powerful key devices are out of order. Therefore, during the repair of any electronic equipment, it is often necessary to check the field effect transistor.

To do this, it is necessary to evaporate the device, tk. On an electronic circuit it can not be verified. And then, following certain instructions, proceed with the verification.

Field effect transistors have two modes of operation - dynamic and key.

The key mode of operation of the transistor is one in which the transistor is in two states - completely open or completely closed. But the intermediate state, when the component is partially open, is absent.

In the ideal case, when the transistor is "open", i.e. Is in the so-called saturation mode, the resistance between the "drain" and "source" terminals tends to zero.

The loss power in the open state is represented by the product of the voltage (equal to zero) by the current value. Consequently, the power of the scattering is zero.

In cutoff mode, that is, when the transistor is locked, its resistance between the "drain / source" leads to infinity. The loss of power in the closed state is the product of the voltage value by the current value equal to zero. Accordingly, the loss power = 0.

It turns out that in the key mode the power loss of transistors is zero.

In practice, with an open transistor, naturally, some "drain / source" resistance will be present. With a closed transistor, a small current flows through these terminals. Therefore, in the static mode, the power loss in the transistor is minimal.

And in the dynamic, in case the transistor is closed or opened, its linear region forces the operating point where the current passing through the transistor conditionally makes up half of the drain current. But the "drain / source" voltage most often reaches half the maximum value. Consequently, the dynamic mode of the transistor provides the allocation of a huge power loss, which reduces to "no" the remarkable properties of the key mode.

But, in turn, the long-term presence of a transistor in a dynamic mode is much less than the length of time in the static mode. As a result, the efficiency of the transistor cascade, which operates in the key mode, is very high and can be from ninety-three to ninety-eight percent.

Field-effect transistors, which operate in the above mode, are widely used in power conversion systems, in sources of pulsed power supply, in output stages of certain transmitters, etc.