Current of base is useless and even harmful. It is desirable that it was as little as possible. Usually makes emitter current percent, i.e. and, therefore, collector current less current of the emitter is only insignificant. i.e. it is possible to consider. In order that current was as little as possible, base do very thin and reduce in it concentration of impurity which defines concentration of holes. Then the smaller number of will recombine in base with holes.
Considerable current is explained by that some small part of tension is attached to emitter transition as a direct. Thereof emitter current increases, and it in this case also is through current.
At increase in injection of carriers from the emitter in base there is a of nonbasic carriers of a charge in base. i.e. increase in concentration and charge of these carriers. On the contrary, at reduction of injection there is a reduction of concentration and a total charge of nonbasic carriers in it. This process is called a rassasyvaniye of carriers of a charge in base.
The volt-ampere characteristic of emitter transition represents the characteristic of the semiconductor diode at direct current. And the volt-ampere characteristic of collector transition is similar to the characteristic of the diode at the return current.
At increase of tension on collector transition in it is mute there is an avalanche reproduction of carriers of a charge which is mainly shock ionization. This phenomenon and tunnel, effect can cause electric breakdown which at increase of current can pass in thermal transition.
The principle of operation of the transistor is that the direct tension of emitter transition, i.e. a site the base emitter (), significantly influences collector current: the more this tension, the is more currents of the emitter and collector. Thus changes of current of a collector less changes of current of the emitter are only insignificant. Thus, tension, i.e. an entrance, operates collector current. Strengthening of electric fluctuations with the transistor is based on this phenomenon.
If the base had considerable thickness and concentration of holes in it was great, the most part of electrons of emitter current, through base, would recombine with holes and would not reach collector transition. Collector current would almost not increase at the expense of emitter electrons, and only the increase in current of base would be observed.
At substantial increase of tension, current sharply increases and there is an electric breakdown. It should be noted that if, very much, at break of a chain of base in the transistor the fast, avalanche increase in current leading to an overheat and an exit of the transistor out of operation can sometimes be observed (if in a chain of a collector there is no the resistor limiting increase current. In this case there is the following process: the part of tension operating on emitter transition increases current, and current equal to it, on collector transition more carriers, its resistance and tension arrive on is mute decrease and at the expense of it tension on emitter transition that leads to even bigger increase in current, etc. increases. That it did not occur, at operation of transistors it is forbidden to a base chain if food of a chain of a collector is not switched off. It is necessary to include also a food of a chain of base, and then collector chains, but not on the contrary.
It should be noted that the emitter and a collector it is possible to trade places (the so-called inverse mode). But in transistors, as a rule, collector transition becomes with considerably bigger area, than emitter , as the power disseminated in collector transition, much a, than disseminated in the emitter. Therefore if to use the emitter as a collector, the transistor will work, but it can be applied only at much smaller power that is inexpedient. If the areas of transitions are made identical (transistors in this case call ), any of extreme areas can work with identical success as the emitter or a collector.