• $summary_{type}$: 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 = $\frac{I_C}{I_E}$ (0.95-0.99), beta = $\frac{I_C}{I_B}$ (20-200), relation: beta = $\frac{alpha}{1-alpha}$. In common-emitter mode: small $I_B$ change produces large $I_C$ change (amplification). Voltage gain = beta*$\frac{R_C}{r_in}$. 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.