OK I'm just having a bit of trouble getting my head around this concept. I'm doing a small talk on it as a mini-assignment for solid state, and so I really need to understand this. A semiconductor in thermal equilibrium Now I know that the recombination and generation rates are equal in this case. So this means that the net charge carrier density remains constant. MY QUESTION But what is the significance of that? Are there some cases where you'd want it to be constant and then for other applications you'd want excess charge carriers? I mean I assume that if you were wanting a semiconductor to act more like a metal conductor then you'd want to increase the level of excess charge carriers? Excess charge carrier lifetimes The recombination lifetime measures the period of recovery of carriers from the perturbed state (conduction band) to the equilibrium state (valence band). The generation lifetime refers to the time associated with the generation of charge carriers. MY QUESTION So essentially, if you wanted to have a good conducting semiconductor would you aim to have a long recombination lifetime and short generation lifetime??? I know these are stupid questions, but I'd appreciate any help.