A junction transistor consists of two back-toback pn-junctions forming a sandwich structure in which a thin layer of n-type or ptype semiconductor is sandwiched between two layers of opposite type semiconductor.
The three terminals of a transistor connected to its three layers are known as the emitter (E), base (B) and collector (C). One pn-junction is between the emitter and the base while the other pn-junction is between the collector and the base.
The electric current is transported by both type of carriers, electrons and holes; for this reason the device is called a bipolar junction transistor (BJT).
There are two types of junction transistors :
(i) pnp transistor
(ii) npn transistor.
[Note : The point-contact transistor was invented in 1947 by US physicists John Bardeen (1908-91), Walter Brattain (1902-87) and William Shockley (1910-89). A month later Shockley invented the junction transistor.]
