Q.Expand the terms FET and MOSFET.

Field effect transistors are classified into two main families according to how the gate is coupled to the channel.

1. Junction FET (JFET) — the gate forms an ordinary reverse-biased pn junction with the channel. It is further divided into the n-channel JFET and the p-channel JFET.
2. Metal-Oxide-Semiconductor FET (MOSFET) — the gate is separated from the channel by a thin insulating oxide layer, so it is also called the Insulated Gate FET (IGFET). The MOSFET itself is subdivided into depletion-type and enhancement-type, each again available as n-channel or p-channel.

Knowing these expansions is a common one-mark exam task: FET = Field Effect Transistor, JFET = Junction Field Effect Transistor, MOSFET = Metal-Oxide-Semiconductor Field Effect Transistor, and IGFET = Insulated Gate Field Effect Transistor.

The key distinction to hold on to is the nature of the gate: in a JFET the gate is a reverse-biased pn junction, while in a MOSFET/IGFET the gate is electrically insulated from the channel by an oxide film. This single difference in gate coupling is what separates the two families and decides how each device is biased and used. This chapter concentrates on the JFET; the MOSFET is developed in a later chapter.