• Tags: transistor, npn, pnp, gain
• Difficulty: Moderate

A bipolar junction transistor (BJT) has three doped regions: emitter (E, heavily doped), base (B, thin and lightly doped), collector (C, moderately doped, larger area). Two types: npn (more common) and pnp. In common-emitter configuration (npn): E-B junction is forward-biased, C-B junction is reverse-biased. Emitter injects electrons into the thin base; most (~95-99%) pass through to the collector (base is too thin to recombine all). $I_E$ = $I_B$ + $I_C$. Current gain: beta = $\frac{I_C}{I_B}$ (typically 20-200), alpha = $\frac{I_C}{I_E}$ (0.95-0.99). Relation: beta = $\frac{alpha}{1-alpha}$. Small changes in $I_B$ cause large changes in $I_C$ — this is amplification. Three regions of operation: (1) Active: E-B forward, C-B reverse (amplifier). (2) Saturation: both forward (switch ON). (3) Cutoff: both reverse (switch OFF).