Potential Hills

Author: Leonard Krugman

Fig. 1-4. P-N junction at equilibrium.

Alone, either P- or N-type germanium is capable of bi-directional current flow. This means that reversing the battery will reverse the direction of the current flow, but will not affect the magnitude of the current. When P- and N-type germanium are joined as shown in Fig. 1-4, an effective rectifying device is formed. The junction, designated ab, is called a P-N junction. In the illustration the + and — signs represent holes and electrons, respectively; and the + and — signs with circles around them represent the donor and acceptor atoms, respectively.

It might appear that the holes of the P-region would diffuse into the N-region and the electrons of the N-region would diffuse in the P-region, eventually destroying the P-N junction. Instead, the holes and electrons concentrate away from the junction. This phenomenon is caused by the fixed position of the donor and acceptor atoms in the crystal lattice structure, as compared to the mobility of the electrons and holes. The donor atoms repel the holes to the left in the diagram, while the acceptor atoms repel the electrons to the right. This barrier to the flow of holes and electrons is called a potential hill, and it produces the same effect as a small battery (shown dotted in Fig. 1-4) with its negative terminal connected to the P-region and its positive terminal connected to the N-region. To use the P-N junction as a rectifying device requires connection of an external battery to either aid or oppose the equivalent potential hill battery.