Two questions with BJTs

Thread Starter


Joined Mar 8, 2011
1) The structure of a BJT, why the base-emitter voltage(Vbe) be expected to be about .6-.7V, or the typical built-in voltage of a silicon diode?

2) Why, given base and collector currents that enter the BJT, the emitter current must be exiting the BJT and positive. What happens if this is not the case?


Joined Mar 6, 2009
For a silicon transistor the base emitter junction is effectively equivalent to a silicon diode junction. One needs to forward bias this junction to allow charge flow across the junction.

Since charge cannot be created or destroyed, the nett charge flow into one or more of the three BJT transistor terminals must equal the nett charge flow out of the remaining terminals. Charge balance per unit time equates with current balance.


Joined Dec 26, 2010
In the case of an NPN transistor, conventional current exiting the emitter represents the normal direction of conduction, with the collector-base junction reverse biased and the base-emitter junction forward biased. For PNP transistors, all the current directions are opposite to those for NPN.

Reversal of the normal emitter current flow can occur if the base-emitter junction is reverse biased. As for other diodes, a typically very small leakage current will flow at moderate reverse voltages. Much more current can pass if the base-emitter reverse breakdown voltage is exceeded, and for many devices this voltage is of the order of 6V or less. For some types of transistor, such breakdown can result in permanently degraded performance, even if the current that passes is not sufficient to cause overheating.

If the connections to the collector and emitter are interchanged in an otherwise normal bias arrangement, the current direction in the emitter is reversed as it behaves more like a collector. Subject to not exceeding the breakdown voltage, reverse transistor operation may be possible. Typically the current gain and frequency response are much poorer than in normal operation, but in some circumstances a lower saturation voltage may be obtained. To the best of my knowledge this is an obsolete method, as FET switches now provide better means of achieving this purpose.