Unipolar Transistor

A unipolar transistor is a field effect transistor (FET) that uses only one type of charge for conduction from drain to source i.e. either electrons (n-channel FET) or holes (P-channel FET).

The invention of the bipolar junction transistor in 1948 was the beginning of semiconductor electronics. This device and semiconductor diodes spawned a revolution in electronics. Drastic reductions in size, cost, and power consumption were achieved simultaneously with greatly increased equipment complexity and capability.

At the same time, in 1948, Shockley and Pearson tried fabricating a rudimentary FET using evaporated layers of germanium on dielectric. FETs are also referred to as unipolar transistors.

Unipolar, field effect transistors are, in their conductive tract, made up of a single type of semiconductor, either N-type or P-type. Bipolar transistors have a junction between N-type and P-type semiconductors in their conductive tract, either in a NPN, or PNP configuration. This results in different characteristics and ways of driving for both types of transistors, though they're commonly referred to as FETs and transistors, rather than unipolar and bipolar transistors.

FET or unipolar transistors have three terminals; Drain, source and gate. Current for the critical load flows through the drain and source; this is controlled by a voltage applied to the gate. In depletion-mode, increasing this increases the depletion layer in the FET device, so restricting the flow of drain-source current and therefore conductivity. In enhancement mode, increased gate voltage increases conductivity.

Today, FETs are typically implemented as Metal Oxide Semiconductor FETs (MOSFETs).