The Field Effect Transistor, or FET is another three terminal semiconductor device used for switch and amplifier application as it has very similar characteristics to those of the previous bipolar junction transistor. The field effect transistor is a unipolar device since the flow of current through it is carried out by the majority carriers only. However, unlike the previous bipolar transistor which is a current operated device, the field effect transistor is a “voltage” operated device.
We saw previously that there are two basic types of Bipolar Transistor construction, NPN and PNP, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made.
This is also true of field effect transistors as there are also two basic classifications of FET, called the N-channel FET and the P-channel FET.
The Field Effect Transistors basic construction consists of a semiconductor “channel” which corresponds in function to the Collector and the Emitter respectively of the bipolar transistor. This resistive channel is the main current carrying path through the FET and may be made of either a P-type or an N-type semiconductor material.
Like the bipolar transistor, the FET is also a three terminal device with each terminal being given a name to identify it from the other two. These three terminals are known as the Drain ( D ), the Gate ( G ) and the Source ( S ).
Field Effect Transistor Configurations
Comparison of terminals between the Field Effect Transistor and the Bipolar Junction Transistor.
|Bipolar Transistor||Field Effect Transistor|
|Emitter – (E) >> Source – (S)|
|Base – (B) >> Gate – (G)|
|Collector – (C) >> Drain – (D)|
Field Effect Transistor Amplifier
The control of the Drain current by a negative Gate potential makes the Junction Field Effect Transistor useful as a switch and it is essential that the Gate voltage is never positive for an N-channel JFET as the channel current will flow to the Gate and not the Drain resulting in damage to the JFET. The principals of operation for a P-channel JFET are the same as for the N-channel JFET, except that the polarity of the voltages need to be reversed.