where - thermal current of emitter p-n - transition, measured at the of base and a collector closed ; - thermal current of collector p-n - transition, measured at nakorotko conclusions of a and the emitter.
In the real transistor except thermal currents through transitions generation currents — recombinations, currents and currents of leak proceed. Therefore, are, as a rule, unknown. On transistors usually give values of the return currents of p in specifications - n - transitions. defined as current of the corresponding transition at not connected conclusion of other transition.
Emitter and collector p-n - transitions of the transistor are similar to p-n - transition of the diode. At separate connection of tension to each transition their volt-ampere is defined as well as in case of the diode. However if - transitions to put tension to one of p-n, and conclusions of other p-n - transition to close among themselves , the current proceeding through p-n - transition to which tension is attached, the of nonbasic carriers of a charge in base will increase because of change. Then:
which shows, in how many time should be changed for receiving the same change of current what gives change of tension. The minus sign means that for providing = const of an increment of tension have to have opposite polarity. The coefficient is rather small () therefore at practical calculations on emitter often neglect influence of collector tension.
From (11 it is visible that in the mode of a deep cut-off current of a collector has the minimum value equal to current of p - n - the transition displaced in the opposite direction. Current of the emitter has an opposite sign and much less current of a collector, as. Therefore in many cases it is considered equal :.
If p - n - transition is displaced in the opposite direction, instead of thermal current it is possible to substitute value of the return current, i.e. to consider, as. As a first approximation it can be done and at direct shift of p - n - transition. Thus for silicon transistors instead of it is necessary to substitute where the coefficient of m considers influence of currents of real transition (m = 2-. Taking into account this equation (1, (1 often write down in other look which is more convenient for calculation of chains with real transistors: