Field effect transistors and the principle of their operation

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

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 type 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 p-n-junction.

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

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

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 the electronic circuit it can not be verified. And then, following certain instructions, start checking.

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

The key operating mode of the transistor is this,at which the transistor is in two states - in fully open or in fully 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 the cut-off mode, that is, when the transistorlocked, its resistance between "drain / source" leads to infinity. The loss of power in the closed state is the product of the voltage value by the value of the current 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,of course, some resistance "drain / source" 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 a dynamic, in the case of a transistorcloses or opens, its linear region boosts the operating point where the current passing through the transistor conditionally makes up half 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, a long-termtransistor in a dynamic mode is much less than the length of stay in the static mode. As a result, the efficiency of the transistor cascade, which operates in a key mode, is very high and can be from ninety-three to ninety eight percent.

Field effect transistors that work inabove-mentioned mode, have a fairly wide application in power converter systems, in sources of pulsed power supply, in output stages of certain transmitters, etc.