Classification & structure of FET

Field Effect Transistor (FET)

A field effect transistor is abbreviated as a FET. It is a semiconductor device in which the current is controlled by an electric field. A FET is an example of a unipolar device it works only depending on the majority carriers. FETs are voltage controlled devices, in contrast to bipolar transistor which are current controlled.

Classification of FET

FETs are basically classified in to two categories.

1.       Junction field effect transistors ( JFET simply called as FET )

2.       Metal Oxide semiconductor Field Effect Transistor ( MOSFET )

The MOSFETs are also called as insulated gate field effect transistor (IG GET). MOSFETs are again two types.

(a)      Enhancement mode MOSFET

(b)     Depletion mode MOSFET

Structure of FET

FET can be of n channel or p channel. The structure of n-channel and p-channel FETs are shown in fig. let us consider-channel FET. It consists of an n-type semiconductor bar sandwiched between two heavily doped p-type regions. Ohmic contacts are made at the two ends of n-channel bar. This n-type bar termed as channel, will have flow of current from one end the other end on application of voltage between two ends. If p-type bar is sandwiched between heavily doped n-region it will be called p-channel FET. The current is carried by majority carriers in any FET. Hence in p-channel FET the current by holes and in N channel FET by electrons. One end of the channel is called source and other end is called the drain. The source and drain can be interchanged.

Source: it is the terminal from which the majority carriers enter the bar.

Drain: drain is the terminal from which the majority carriers leave the bar.

Gate: on the both sides of the n-type bar heavily doped p-region have been formed. These p-region form the control electrode and termed the gate. The gate and channel form a p-n junction.

Structure of FET