Question

Consider a pn junction. Why does the depletion layer become bigger when you reverse bias the junction?

Answer 1

Because in reverse bias the carriers move away from the junction following the electric field to balance it. And this electric field goes from N region to P region in reverse bias. That means that in the anode (P) the holes will move in the same direction of the electric field (away from junction) while in the katode (N) the electrons will move against the electric field (away from junction)

Answer 2

Ok, thanks: two more questions.
1) Is the widening of the depletion layer the cause of the increase in the drift current?
2) When the junction isn't biased, the depletion layer coincides with a region in which there are positive charges because of the positive ions formed in the n region and negative charges because of the ions formed in the p region when electrons and holes recombine. Does the region formed by these fixed charges widens too when we reverse bias the junction? (Reading your previous answer, I would say no, because there is no diffusion when the junction is in reverse bias...)