Transistor Action in the Common Base Mode

In the forward active mode, the emitter-base junction is forward biased by $V_{EB}$ (shrinking that junction's depletion region) while the collector-base junction is simultaneously reverse biased by $V_{CB}$ (widening that junction's depletion region) -- essentially, a BJT can be thought of as two p-n junction diodes connected back-to-back, one forward biased and one reverse biased at the same time. Taking an NPN transistor: because the emitter is heavily doped, forward bias drives a large flow of electrons from the emitter into the base, constituting the emitter current $I_E$. The base region is deliberately very thin and lightly doped, so only a small fraction of these injected electrons find a hole to recombine with there (this recombination current is exactly what constitutes the base current $I_B$, which is why $I_B$ is so much smaller -- microamps versus milliamps -- than $I_E$ or $I_C$); the overwhelming majority of the electrons instead survive across the thin base and reach the collector-base junction, where the reverse bias's field sweeps them into the collector, constituting the collector current $I_C$. Applying Kirchhoff's current law to the transistor as a whole gives $I_E=I_B+I_C$, and since $I_B$ is so small, $I_E\approx I_C$ to a good approximation. (There is also a small additional contribution to $I_C$ from thermally generated minority carriers, the reverse saturation current $I_{CO}$, which is temperature-sensitive and must be kept in mind for stability at high operating temperatures.) The ratio of collector current to emitter current is the transistor's DC forward current gain in this configuration, $\alpha_{dc}=I_C/I_E$; $\alpha$ is always less than unity, typically between 0.95 and 0.99, meaning the collector current is 95-99% of the emitter current -- the higher the $\alpha$, the better the transistor, since it means fewer carriers were lost to recombination in the base. A PNP transistor works the same way with electrons and holes swapped: …