Q. Why FET is called a voltage-controlled device? Why its input resistance is high?
In the case of a FET the output current ID is a function of the voltage VGS applied to the input circuit. Hence FET is known as a voltage controlled device. The equation that governs the working of a FET is called Shockley's equation given by,
I_{D} = I_{DSS}[1 - (V_{GS}/V_{P}) ^{2}
where, ID is the drain current
I_{DSS }is the maximum saturation current
V_{GS} is the voltage applied between gate and source
V_{P} is the pinch-off voltage
A FET requires virtually no input current and hence IG is always made zero. This gives an extremely high input resistance to FET. The range of its input impedance is at a level of 1 to several 100MWs.