This section explains the distributions for reverse bias in details.
For example, if V = -Vr (p negatively biased with respect to n), we can approximate
- This reverse-bias depletion of minority carriers can be thought of as minority carrier extraction, analogous to the injection of forward bias.
- Physically, extraction occurs because minority carriers at the edges of the depletion region are swept down the barrier at the junction to the other side and are not replaced by an opposing diffusion of carriers.
- For example, when holes at xn0 are swept across the junction to the p side by the ξ field, a gradient in the hole distribution in the n material exists, and holes in the n region diffuse toward the junction.
- The steady state hole distribution in the n region has the inverted exponential shape of Fig. a.
- It is important to remember that although the reverse saturation current occurs at the junction by drift of carriers down the barrier, this current is fed from each side by diffusion toward the junction of minority carriers in the neutral regions.
The rate of carrier drift across the junction (reverse saturation current) depends on the rate at which holes arrive at xn0 (and electrons at xp0) by diffusion
from the neutral material.
- These minority carriers are supplied by thermal generation, and we can show that the expression for the reverse saturation current, represents the rate at which carriers are generated thermally within a diffusion length of each side of the transition region.
- In reverse bias, the quasi-Fermi levels split in the opposite sense than in forward bias (Fig.b).
- Fn moves farther away from Ec (close to Ev) and Fp moves farther away from Ev, reflecting the fact that in reverse bias. we have fewer carriers than in equilibrium, unlike the forward bias case where we have an excess of carriers.
- In reverse bias, in the depletion region, we have
- Hence, the band diagram simply reflects the fact that we have very few holes in this region, even fewer than the already small equilibrium minority carrier hole concentration (Fig. a).
- Similar observations can be made about the electrons.