Field-effect transistors are semiconductor devices whose principle of operation is based on modulation of the resistance by the transverse electric field of the semiconductor material.
A distinctive feature of devices of this type is that field-effect transistors have a high voltage gain and high input resistance.
For these devices, only carriers of the same type of charge (electrons) participate in the creation of an electric current .
There are two types of field effect transistors:
- having a TIR structure, i.e. metal, followed by a dielectric, then a semiconductor (MIS);
- having a control pn junction.
The simplest field-effect transistor includes a wafer made of a semiconductor material having only a pn junction in the center and non-rectifying contacts along the edges.
The electrode of such a device through which charge carriers pass to the conductive channel is called the source, and the electrode through which the electrodes exit the channel is called the drain.
Sometimes it happens that such powerful key devices fail. Therefore, during the repair of any electronic equipment, a field transistor check is often necessary.
For this, it is necessary to unsolder the device, as on the electronic circuit it cannot be verified. And then, following certain instructions, start checking.
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 - fully open or completely closed. But at the same time, 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 terminals "drain" and "source" tends to zero.
The power loss when open is represented by the product of the voltage (equal to zero) by the amount of current. Therefore, the dissipation power is zero.
In the cutoff mode, that is, when the transistor is locked, its resistance between the drain / source terminals tends to infinity. Lost power when closed is the product of the voltage value by the current value equal to zero. Accordingly, the power loss = 0.
It turns out that in the key mode the power loss of the transistors is zero.
In practice, with the transistor open, of course, some drain / source resistance will be present. When the transistor is closed, a current of small magnitude still flows through these conclusions. Therefore, in static mode, the power loss in the transistor is minimal.
And in the dynamic, if the transistor closes or opens, its linear region is forced by the operating point, where the current passing through the transistor conditionally is half the drain current. But the voltage "drain / source" most often reaches half the maximum value. Consequently, the dynamic mode of the transistor provides the release of a huge power loss, nullifying the remarkable properties of the key mode.
But, in turn, the long-term stay of the transistor in dynamic mode is much less than the duration of stay in static mode. As a result, the efficiency of the transistor cascade, which operates in the key mode, is very high and can turn out from ninety-three to ninety-eight percent.
Field-effect transistors that operate in the above mode are widely used in power converters, in pulsed power sources, in the output stages of certain transmitters, etc.