Download pnp transistor in reverse active mode

pnp transistor in reverse active mode
(a) A pnp bipolar transistor is biased in the reverse active mode. The emitter is more heavily doped than the base or collector. Draw the minority charge distribution. (b) What limits the speed of a bipolar transistor? (c) What is punch through? a) Minority charge distribution For a bipolar transistor we typically have a short diode condition(concentration is linear). The emitter-­‐base is reverse biased and the base-­‐collector is forward biased. This is not very efficient because of the doping. Emitter-­‐base is reverse biased=> minority carrier concentration decays from the emitter to the base. Base-­‐collector is forward biased=> minority carrier concentration grows from the collector to the base. b)What limits the speed of a bipolar transistor? There are different types of transistor models. The first is the simplest model. A voltage drops through a resistance. Whatever voltage drops through the resistor appers at the output as a current. If you add the parasitic capacities you have a more complex model. When you increase the frequency you see that some of the voltage flows over the capacities and some over the resistor. The voltage through the capacitor gets lost and you loose gain because of this. You need a larger input voltage to get the same output current. .Some of the charge charges the capacity some gets injected into the base. Only the charge injected in the base contributes to the transistor action. c) What is punch through? As you put a larger and larger reverse bias on the base-­‐collector junction the depletion widths increase and the whole base gets depleted. At this point all transistor action is lost, electrons just go in, see the electric field and go through it but they don`t recombine anymore. Hence the controll by the base is lost. A possibility to avoid punch-­‐through is to lightly dope the collector, so that most of the depletion width is in the collector and less is in the base.