BJT meaning and npn active mode

Why "bipolar"

A transistor is called a Bipolar Junction Transistor because its current is carried by both types of charge carriers, electrons and holes, and it has two p-n junctions (emitter-base and collector-base). In a field-effect transistor, only one carrier type conducts (unipolar).

npn transistor in active mode

In the active region, the emitter-base junction is forward biased and the collector-base junction is reverse biased.

     E (n)        B (p)        C (n)
   +--------+  +--------+  +----------+
   | e- ---->  thin  ---->  collected |
   +--------+  +--------+  +----------+
     VEE (+ forward)     VCC (reverse)
         IE = IB + IC

Operation:

1. The heavily doped emitter injects a large number of electrons into the base, because the forward bias lowers the barrier at the emitter-base junction.
2. The base is very thin and lightly doped, so only about 1-5% of the electrons recombine with holes there. This forms the small base current $I_B$.
3. The rest (95-99%) diffuse across the base and reach the collector-base junction. The reverse bias there creates a strong field that sweeps them into the collector, giving collector current $I_C$.
4. Currents obey

$$I_E=I_B+I_C, \qquad I_C=\alpha I_E+I_{CBO}, \qquad I_C=\beta I_B$$

with $\alpha=0.95$-$0.99$ and $\beta=\alpha/(1-\alpha)$ (typically 50-300).

A small change in base current causes a large change in collector current, so the transistor acts as a current amplifier.