PH-03 Key Concepts — Must-Know Facts for NEET 2026 — Summary

Band gap classification: Conductor ( $E_g$ = 0), Semiconductor (0.1–3 eV), Insulator (> 3 eV)
Silicon band gap: $E_g$ = 1.1 eV; Germanium: $E_g$ = 0.67 eV; Diamond: $E_g$ = 5.4 eV
Intrinsic semiconductor: Pure Si or Ge; $n_e$ = $n_h$ = $n_i$ ; conductivity increases with temperature (negative temperature coefficient)
n-type: Pentavalent dopant (P, As, Sb — group 15); majority carriers = electrons; donor level just below conduction band
p-type: Trivalent dopant (B, Al, Ga, In — group 13); majority carriers = holes; acceptor level just above valence band
Both n-type and p-type are electrically NEUTRAL (critical NEET trap — most commonly tested neutrality fact)
Mass action law: $n_e$ × $n_h$ = n_ $i^{2}$ — holds for all semiconductor types (intrinsic AND extrinsic)
p-n junction barrier potential: Si ≈ 0.7 V; Ge ≈ 0.3 V
Forward bias: Narrows depletion region; reduces barrier; current flows above knee voltage
Reverse bias: Widens depletion region; increases barrier; tiny reverse saturation current until breakdown
Zener diode: Operates in reverse bias at $V_Z$ ; used as voltage regulator
Photodiode: Operates in reverse bias; photons create e-h pairs; used in optical communication
LED: Operates in forward bias; e-h recombination emits photons; λ = $\frac{hc}{E_g}$
Solar cell: No external bias; photovoltaic effect generates EMF (~0.5–1 V per cell)
Half-wave rectifier: $f_{out}$ = $f_{in}$ ; Full-wave rectifier: $f_{out}$ = 2 × $f_{in}$
OR gate: Y = A+B; AND gate: Y = A·B; NOT: Y = A'; NAND: Y = (A·B)'; NOR: Y = (A+B)'
Universal gates: BOTH NAND and NOR are universal (any Boolean function from either alone)
De Morgan's theorems: (A+B)' = A'·B' and (A·B)' = A'+B'