Transistors

The transistor was invented at Bell Laboratories in 1947. Today there are two main types of transistors. The bipolar junction transistor and the field-effect transistor (FET). The bipolar junction transistor or BJT is used in mainly linear amplifiers or as an electronic switch. The term bipolar means both electrons and holes are used as current carriers.

The BJT is constructed from three doped semiconductors joined together, thus forming two pn junctions. The arrangement of doped semiconductors is either pnp or npn. These three doped semiconductors are called emitter, base and collector as shown in the symbol below for an npn transistor:

Transistor Symbol

 The base region is lightly doped and thin, compared to emitter and collector regions.