研究生入学考试chapter.pptx

We will use the p-n-p transistor for most illustration. The main advantage of the p-n-p for discussing transistor action is that hole flow and current are in the same direction. Once these basic ideas are established for the p-n-p device, it is simple to relate them to the more widely used the n-p-n transistor.;7.1 Fundamentals of BJT operation;The reverse current depends on the rate at which minority carriers are generated in the neighborhood of the junction.;How to increase the reverse current? optical excitation with light h?Eg;A convenient hole injection device is a forward-biased p+-n junction. In p+-n junction, the current is due primarily to holes injected from the p+ into the n region. If we make use of the same n side for both forward and reverse-biased junction, we can obtain the p+-n-p structure. In this structure, injection of holes from p+-n junction into the n region supplies the minority carrier holes to participate in the reverse current through the n-p junction. To enable the p+-n-p structure to normally operate, it is important that the injected holes do not recombine in the n region before they can diffuse to the depletion layer of the reverse biased junction. So, we must make the n region narrower than the hole diffusion length, WbLp.;Injected holes lost to recombination in the base Holes reaching the reverse-biased collector junction Thermally generated electrons and holes making up the reverse saturation current of the collector junction Electrons supplied by the base contact for recombination with holes Electrons injected across the forward-biased emitter junction;The base current originates from the following three mechanisms: There must be some recombination of injected holes with electrons in the base, even with WbLp. The electrons lost to recombination must be resupplied through the base contact. Some electrons will be injected from n to p in the forward biased emitter junction, even if the emitter is heavily doped compared to the base. Th