Transistor is invented in the year 1947 by J. Bardeen and W.H. Brattain of Bell Telephone Laboratories, U.*.A. That transistor was a point-contact transistor. The first junction transistor consisting of two back-to-back p-n junctions was invented by William Schockley in 1951. The junction transistor is also called Bipolar Junction Transistor (BJT).
A transistor has three doped regions forming two p-n junctions between them. Obviously, there are two types of transistors, as shown in Fig.
(i) n-p-n transistor : Here two segments of n-type semiconductor (emitter and collector) are separated by a segment of p-type semiconductor (base).
(ii) p-n-p transistor : Here two segments of p-type semiconductor (termed as emitter and collector) are separated by a segment of n-type semiconductor (termed as base). The schematic representations of an n-p-n and a p-n-p configuration are shown in Fig. (a). All the three segments of a transistor have different thickness and their doping levels are also different. In the schematic symbols used for representing p-n-p and n-p-n transistors Fig. (b) the arrowhead shows the direction of conventional current in the transistor. A brief description of the three segments of a transistor is given below:
• Emitter : This is the segment on one side of the transistor shown in Fig. (a). It is of moderate size and heavily doped. It supplies a large number of majority carriers for the current flow through the transistor.
• Base : This is the central segment. It is very thin and lightly doped.
• Collector : This segment collects a major portion of the majority carriers supplied by the emitter. The collector side is moderately doped and larger in size as compared to the emitter.
Depletion regions are formed at the emitter-base junction and the base-collector junction of a transistor.
