A bipolar junction transistor has three separately doped regions and two pn-junctions. A pnp transistor is formed by starting with a p-type substrate. An zz-type region is grown by thermally diffusing do-nor impurities into this substrate. A very heavily doped p+ region is then diffused into the ntype region. The heavily doped p + -region is called the emitter, symbol E in below figure. The narrow central n-region, with lightly doped concentration, is called the base (symbol B). The width of the base is small compared with the minority carrier diffusion length. The moderately doped p-region is called the collector (symbol C). The doping concentration in each region is assumed to be uniform.
The npn transistor is the complementary structure to the pnp transistor : A narrow p region grown into an n type substrate, by thermally diffusing acceptor impurities, forms the base. The heavily doped n + region diffused into the base forms the emitter.
Schematic BJT architectures (a) npn (b) pnp
