Hi to you all, I'm reading about semiconductor devices on Muller-Kamins' "Device Electronics for Integrated Circuits" and I have some problem understanding the base transit time of a bipolar transistor. I read that when the npn bjt is forward biased the emitter injects electron in the base. In the case of high injection, the majority population of the base (holes) increase at the emitter-base junction "because of charge neutrality", Muller says. -So, this is my first question: why it is necessary the hypotesis of charge neutrality? It's maybe for th hypotesis of completely depletion of the space charge region, so that the voltage is 'seen' only in the depleted region(s, if we count the base-collector) and in the other parts of the system the voltage (and so the field) is veeeery small? -Second question: from where this addictional charge comes? From the base lead? -Third question: why they arrive?? That's because the injected electrons create a field that attracts the holes so that they're arranged in that triangular shape? In this case the total charge is 0, but, Muller says, the electrons see the opposite field caused by holes that helps them and so they can move faster than only with the diffusion-method. That's it? Otherwise, can anyone explain me what happened to the fields and to the carrier concentration during the minority injection? Thank you very much [and sorry for my english].