Okay, so my basic understanding of an NP junction (diode) is as follows. When no external voltage is applied, some of the electrons in the N type material jump over to the "holes" in the P type material. The holes then end up moving back into the P type material further, and you end up with negatively charged ions in the P type and positively charged ions in the N type. Since there aren't many free electrons around in the depletion zone, it contains a rather high resistance. When you apply a forward biased voltage to the NP junction, the electrons get pushed into the positively charged ions and cancel with them, and get pulled out of the negatively charged ions, therefore shrinking the depletion zone and lowering the resistance so current flows. When you apply a negative biased voltage to the NP junction, the electrons fill even more of the holes in the P type material and get pulled out of the N type material, so the depletion zone or charged ions grows, as does the resistance, allowing a negligible amount of current to flow. Now I'm confused over NPN bipolar junction transistors. In particular, if a forward biased voltage is applied between the base and emitter, and a larger reverse biased voltage is applied between the base and the collector. I understand that the emitter-base depletion zone breaks down and current flows, but I don't understand why the base-collector depletion zone doesn't grow as a reverse biased diode would. Could someone try and explain this?