Why doesn't a diode pass current when reverse biased?

Question

If one tacks a P-type semiconductor to an N-type semiconductor, a P-N junction forms in which electrons diffuse from the N-type to the P-type causing a diffusion current. Some time passes, and an electric field in the junction appears causing a drift current. At equilibrium, the diffusion current equals the drift current, hence a resultant of zero current is established.

Since in the forward biased mode, the built in electric field is weakened allowing more more diffusion current, I would expect that in the reverse biased mode more drift current should pass, but this is not the case. Why?

In this discussion, I am assuming no avalanche breakdown has yet occurred.

Another question I would like to ask at this point is that, if in the unbiased P-N junction, the depletion layer is depleted of any mobile charges, where does the drift current come from?

Answer 1

I would expect that in the reverse biased mode more drift current should pass, but this is not the case. Why?

In the reverse biased mode, drift current across the junction would be higher, but it is limited by the rate at which minority carriers travel from the metal electrodes to the PN junction. In a diode there are few of the minority carriers. Hence the drift current is low. (It does exist however). However, in a transistor, many minority carriers are injected into the base from the emitter, and easily cross the base-collector junction whether it is forward or reverse biased.

Another question I would like to ask at this point is that, if in the unbiased P-N junction, the depletion layer is depleted of any mobile charges, where does the drift current come from?

In an unbiased PN junction, there is a greater concentration of holes in P-type quasi-neutral region, and a greater concentration of mobile electrons in the N-type quasi-neutral region. Diffusion causes holes to migrate to the N side of the junction and mobile electrons to migrate to the P side. This sets up an electric field. The electric field causes some of the holes to drift back to the P side and some of the mobile electrons to drift back to the N side.