Semiconductors: Si (1.1 eV, 0.7 V) and Ge (0.67 eV, 0.3 V) are the key materials; n-type (pentavalent dopant, $e^{-}$ majority) and p-type (trivalent dopant, hole majority) are both electrically neutral; mass action law: n_e × n_h = n_$i^{2}$ always applies.

p-n Junction: Depletion region + barrier potential formed at junction; forward bias → narrows + current; reverse bias → widens + tiny current until breakdown at high voltage.

Special Diodes (bias/purpose): Zener = reverse (voltage regulator); Photodiode = reverse (light detector); LED = forward (light emitter, λ = 1.24/E_g μm); Solar cell = none (generates EMF).

Rectifiers: Half-wave → f_out = f_in; Full-wave → f_out = 2 × f_in (most tested NEET number from rectifiers).

Logic Gates: NAND (Y=0 only when all=1) and NOR (Y=1 only when all=0) are BOTH universal gates; De Morgan: break the bar, flip the operator — (A+B)'=A'·B' and (A·B)'=A'+B'.