Transistor Action

The biasing arrangement. In its normal, or 'active', mode of operation, a transistor's emitter-base junction is FORWARD biased (so carriers are readily injected from the emitter into the base, exactly as at any forward-biased p-n junction, Section 9.10) while its base-collector junction is simultaneously REVERSE biased (so the same junction that would otherwise block carriers instead actively sweeps them onward, exactly as the reverse-biased junction of Section 9.11 sweeps minority carriers across).

Carrier flow in an n-p-n transistor. Consider an n-p-n transistor under this biasing. The heavily doped n-type emitter injects a LARGE number of free electrons across the forward-biased emitter-base junction into the thin p-type base. Once inside the base, these injected electrons are, for a moment, MINORITY carriers in a p-type region (surrounded mostly by majority holes), and so are subject to recombination with the base's own holes -- but because the base is made deliberately very THIN and only lightly doped (Section 9.18), only a very SMALL fraction of the injected electrons actually recombine there before they reach the far edge of the base. This small recombined fraction supplies the (small) BASE CURRENT $I_B$, flowing out of the base terminal.

Why nearly all carriers reach the collector. The great MAJORITY of the injected electrons -- typically $95$-$99\%$ of them -- survive the brief crossing of the thin base WITHOUT recombining, and arrive at the base-collector junction, where the strong REVERSE-bias field there (which would repel majority carriers of the base, but eagerly sweeps across exactly this kind of minority-type carrier arriving from the base side, precisely as in Section 9.11) sweeps them rapidly across into the collector, where they now become the (large) COLLECTOR CURRENT $I_C$. …