Q.Is FET a unipolar device or a bipolar device?

A field effect transistor (FET) is a three-terminal semiconductor device in which the current flowing through a conducting channel is controlled by the electric field produced by the voltage applied to its gate. This makes it a voltage-controlled device, in contrast to the bipolar junction transistor whose output current is set by an input current.

The FET's defining property is that it is unipolar: conduction depends on only one kind of majority carrier — electrons in an n-channel device or holes in a p-channel device — whereas a bipolar transistor uses both electrons and holes. Because the controlling gate junction is kept reverse biased, it draws almost no current, so the FET offers a very high input resistance (of the order of megaohms). It also switches fast, adds little noise, and is easy to fabricate in integrated circuits. …