The Junction Transistor
R. L. Havill, A. K. Walton · 1985
The basic ingredient of junction transistors is the P—N junction, which has already been considered extensively in chapter 1. A junction transistor consists of two such junctions formed back-to-back in the same single crystal and separated by much less than a minority-carrier diffusion length. The two junctions must be formed within one continuous crystal; two separate junctions with their surfaces stuck together in some way or electrically connected will not suffice! Two distinct types of junction transistor are possible, NPN (see figure 3.1a) or PNP depending on the conductivity type of the central region. Their operation is conceptually identical except for the roles of electrons and holes being interchanged and the polarities of the bias potentials reversed. Thus it will suffice to discuss the NPN transistor. One junction, called the emitter junction, is biased in the forward direction while the other, called the collector junction, is biased in the reverse direction. Pursuing the nomenclature further, the central region between the junctions is called the base , while the outer regions adjacent to the emitter and collector junctions are respectively known as the emitter and collector. Each region is provided with an ohmic contact to permit external connections. In its simplest mode of operation the transistor is connected to a circuit essentially as shown in figure 3.1a. This particular arrangement is referred to as the common-base configuration because the base lead is common to bath the input and output circuits.