Hole Injection

Author: Leonard Krugman

To understand hole and electron flow in the point-contact transistor, observe that in Fig. 2-3 the surface-bound electrons near the emitter contact are immediately removed by the positive emitter electrode. This is due to the intense emitter field which breaks down covalent bonds of atoms in the vicinity of the emitter electrode. The liberated electrons are immediately attracted to and enter the emitter terminal. These electrons are the emitter current carriers. For every electron which leaves the pellet, a hole is left behind. This creation of holes is called hole injection, since the effect is the same as if holes were injected into the transistor through the emitter. The holes immediately diffuse toward the collector because of the negative potential at that terminal.

The need for the extremely close spacing between the emitter and collector is now apparent. Many of the holes may meet with and be cancelled by the free electrons in the N-type material. Therefore, the flow path between the emitter and collector must be small to keep the hole and electron recombinations to a minimum.

Fig. 2-3 (top). Magnified view of hole and electron flow into point-contact transistor.

At the collector electrode, the potential hill produced by the surface-bound electrons limits the current flow. However, holes that reach the collector area combine with the surface-bound electrons and reduce the potential hill. This permits the collector to inject more electrons into the germanium, thus increasing the collector current.

Holes travel through the transistor from emitter to collector in many indirect paths. The holes set up a net positive space charge in the areas of their flow paths, due to the combined effects of their positive charges. The resultant positive space charge attracts electrons from the more remote areas of the N-type transistor into the hole flow path between the collector and base, thus effectively increasing the electron flow. While some of the electrons emitted by the collector neutralize holes, the majority flow toward and enter the base terminal. The electrons which flow between the collector and the base are the collector current carriers.