• 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 = I_C/I_B (typically 20-200), alpha = I_C/I_E (0.95-0.99). Relation: beta = 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).