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.
