Reverse and Forward Bias

Author: Leonard Krugman

Fig. 1-5. P-N junction with reverse bias.

The connection of an external battery, illustrated in Fig. 1-5, is an example of reverse bias. The negative terminal attracts holes and concentrates them further to the left, while the positive terminal concentrates the electrons further to the right. There is no flow across the junction, since the effect of this connection is to increase the potential hill barrier.

Fig. 1-6. (A) P-N junction with forward bias.

Consider now the connection illustrated in Fig. 1-6 (A). This is an example of a forward bias connection. The positive terminal pushes the holes towards the N-area, while the negative terminal forces the electrons toward the P-area. In the region around the ab junction, holes and electrons combine. For each combination, a covalent bond near the positive terminal breaks down, and the liberated electron enters the positive terminal. This action creates a new hole which moves toward the N-region. Simultaneously, an electron enters the crystal through the negative battery terminal and moves toward the P-region. The total current (I_o) flowing through the crystal is composed of electron flow (I_N) in the N-area, hole flow (I_P) in the P-area, and a combination of the two (I_N and I_P) in the region near the junction. The forward bias connection, then, reduces the potential hill by a sufficient amount to allow current to flow by a combination of hole and electron carriers, as illustrated in Fig. 1-6 (B).

Fig. 1-6. (B) Carrier conduction in P-N junction.

One may well ask, "How much battery voltage is necessary?" Offhand, since the equivalent battery potential is in the neighborhood of a few tenths of a volt, an external battery of equal value should normally be considered sufficient. Unfortunately, a large part of the battery potential is dropped across the resistance of the P- and N-regions before the potential hill is reached. The voltage drop in these regions is proportional to the current flow through them; as the current increases due to the reduction of the potential hill, the drop across the P- and N-regions also increases, leaving even less of the external voltage available to reduce the junction barrier potential. An external battery of approximately one to two volts is required because of these factors.