npn and pnp transistors

The transistor, which drove the great growth of electronics, is made of two p-n junctions: three semiconductor regions next to each other. There are only two ways to arrange them: pnp and npn.

Figure to be added

Figure 11.20 — the structure of (a) a pnp and (b) an npn transistor, with a lead from each region.

Emitter (E)

The end region that sends out the carriers.

Base (B)

The middle region, which controls the carriers passing from emitter to collector.

Collector (C)

The other end region, which collects them.

Figure to be added

Figure 11.21 — (a) the carriers and current directions in pnp and npn transistors; (b) their symbols, the emitter marked by an arrowhead.

pnp

The carriers are holes (like + charges).

Current flows from emitter to collector: the arrow points in.

npn

The carriers are electrons.

Current flows from collector to emitter: the arrow points out.

Either way the carriers always travel from the emitter to the collector; the arrowhead marks the emitter and shows the direction of the current.