• summary_type: concept
• word_count: 160

A BJT has three layers: emitter (heavily doped, injects carriers), base (very thin ~1 um, lightly doped), collector (moderately doped, larger area). npn transistor: E-B forward biased, C-B reverse biased. Electrons injected from emitter into base; base is so thin that >95% reach the collector before recombining. I_E = I_B + I_C. Current gain: alpha = I_C/I_E (0.95-0.99), beta = I_C/I_B (20-200), relation: beta = alpha/(1-alpha). In common-emitter mode: small I_B change produces large I_C change (amplification). Voltage gain = beta*(R_C/r_in). Three modes: active (E-B forward, C-B reverse — amplifier), saturation (both forward — switch ON, V_CE ≈ 0.2 V), cutoff (both reverse — switch OFF, I_C ≈ 0). pnp works identically but with holes as primary carriers and reversed bias polarities. JEE focuses on alpha-beta relations and simple circuit calculations.