Reverse Biased P-N Junction

When battery connections to the semiconductor are made, the junction is said to reverse-biased. In this case, holes are attracted by the negative battery terminal and electrons by the positive terminal so that both holes and electrons move away from the junction and away from each other. Since there is no electron-hole combination, no current flows and the junction offer high resistance.

Another way of looking at the process is that in this case, the applied voltage increases the barrier potential to (V + VB), thereby blocking the flow of majority carriers.

Incidentally, it may be noted that under reverse bias condition, width of depletion layer is increased because of increased barrier potential. Although, in this case, there is practically no current due to majority carriers, yet there is a small amount of current (a few µA only) due to the flow of minority carriers across the junction.

Due to thermal energy, there are always generated some holes in the N-type region and some electrons in the P-type region of the semiconductor. The battery drives these minority carriers across the junction thereby producing a small current called reverse current or reverse saturation current I0 or IS. Since minority carriers are thermally-generated, I0 is extremely temperature dependent.

For the same reason, forward current is also temperature dependent but to a much less degree because minority current forms a very small percentage of the majority current. The name saturation has been used because we cannot get minority current more than what is produced by thermal energy. In other words, IS does not increase with increase in reverse bias.

IS is found to increase approximately 7 percent per 0ºC rise in temperature both for Ge and Si. Since, (1.07)10 = 2, it means that reverse current doubles for every 10ºC rise in temperature. It is worth noting that reverse saturation current is also referred to as leakage current of the P-N junction diode. With reverse bias, energy hill becomes too steep for majority carriers to go up the hill and cross over.

Reverse V/I Characteristic

As said earlier, the reverse saturation current is also referred to as leakage current of the P-N junction. V/I characteristics of a reverse-biased P-N junction, it is seen that as reverse voltage is increased from zero, the reverse current quickly rises to its maximum or saturation value. Keeping temperature constant as the reverse voltage is increased, I0 is found to increase only slightly. This slight increase is due to the impurities on the surface of the semiconductor which behaves as a resistor and hence obeys Ohm’s law. This gives rise to a very small current called surface leakage current. Unlike the main leakage (or saturation) current, this surface leakage current is independent of temperature but depends on the magnitude of the reverse voltage. A reverse-biased junction can be represented by a very large resistance.