Surface-Bound Electrons

Author: Leonard Krugman

The fundamental concepts of current flow in the point-contact transistor are illustrated in Fig. 2-2. Physicists have found that those electrons which diffuse to the surface of the germanium pellet not only lose their ability to return to the interior of the germanium but also form a skin-like covering over the surface. Because of this phenomenon, they are called surface-bound electrons. For the N-type transistor illustrated, the surface-bound electrons combine with the layer of donor atoms just below to form a potential hill.

Fig. 2-2. Basic point-contact transistor operation.

The proper battery connections for a transistor can be determined as follows: The emitter is always biased in the forward or low resistance direction. Since this is accomplished by reducing the potential hill, the positive battery terminal is connected to the emitter. Conversely, the collector is always biased in the reverse, or high-resistance, direction. Therefore, the negative battery terminal is connected to the collector in order to increase the potential hill.