Reverse Biased PN Junction

REVERSE BIASED PN JUNCTION

We connect the voltage source to a PN junction, such that positive terminal of the voltage source is connected to the N-region and negative to the P-region, the PN junction is said to be reverse biased. Figure 3.25 shows a reverse biased PN junction.

When a PN junction is reverse biased, as shown in Figure 3.25(a) the holes in the P-region are attracted towards the negative terminal of the voltage source. And the electrons in the N-region are attracted to the positive terminal of the voltage source. Thus the majority carriers are drawn away from the junction. This widens the depletion layer and increases the barrier potential as shown in Figure 3.25 (b).

The increased barrier potential makes it very difficult for the majority carriers to diffuse across the junction. Thus there is no current due to majority carriers in a reverse biased PN junction. In other words, the junction offers very high resistance under reverse biased condition. However, the barrier potential helps the minority carriers in crossing the junction. Hence a small amount of current does flow through the reverse biased PN junction.

The amount of this current depends upon the number of minority carriers diffusing across the junction. The current is known as reverse saturation current,