FET is a ______.

Question

Question

This question was previously asked in

ALP CBT 2 Electronic Mechanic Previous Paper: Held on 21 Jan 2019 Shift 2

Answer 1

FET is a voltage-driven/controlled device, i.e. the output current is controlled by the electric field applied.
The current through the two terminals is controlled by a voltage at the third terminal (gate).
It is a unipolar device (current conduction is only due to one type of majority carrier either electron or hole)
It has a high input impedance.
For a FET, either

\({I_D} = {I_{{D_{SS}}}}{\left( {1 - \frac{{{V_{GS}}}}{{{V_P}}}} \right)^2}\)in case of JFET

or

\({I_D} = K{\left( {{V_{GS}} - {V_T}} \right)^2}\)in case of MOSFET

So, a FET is a voltage-controlled current source.

26 June 1

The difference between FET and BJT is explained in the following table:

FET	BJT
Unipolar device: Uses only one type of charge carrier	Bipolar device: Uses both electron and hole
Voltage-controlled device: voltage between gate and source control the current through the device.	Current-controlled device: Base current control the amount of collector current
High input resistance	Low input impedance
Faster in switching	Slower in switching