CBSE Class 12 Physics: Semiconductors — Diodes & Transistors 2026

Energy Bands in Solids

• Conductor: overlapping valence and conduction bands; free electrons always available
• Semiconductor: small energy gap (~1 eV); Si (1.1 eV), Ge (0.7 eV); thermal excitation creates carriers
• Insulator: large band gap (>3 eV); very few free electrons at room temperature

Intrinsic and Extrinsic Semiconductors

• Intrinsic: pure Si/Ge; equal electrons and holes; n = p = nᵢ
• n-type: doped with pentavalent (P, As, Sb); extra electrons; minority carriers are holes
• p-type: doped with trivalent (Al, In, B); extra holes; minority carriers are electrons

p-n Junction Diode

• Depletion region formed at junction; barrier potential ~0.6 V (Si), 0.3 V (Ge)
• Forward bias: reduces barrier; current flows above threshold
• Reverse bias: increases barrier; only leakage current until breakdown

Rectification

• Half-wave rectifier: one diode; only one half of AC used; efficiency 40.6%
• Full-wave rectifier: 4 diodes (bridge) or 2 diodes + centre tap; both halves used; efficiency 81.2%
• Filter capacitor smooths pulsating DC; output closer to steady DC

Special Purpose Diodes

• Zener diode: reverse breakdown at specific voltage; used as voltage regulator
• LED: forward-biased junction emits light; semiconductor has direct band gap
• Photodiode: reverse-biased; photocurrent proportional to light intensity; used in sensors

Transistor

• NPN and PNP types; three terminals: emitter (heavy doping), base (thin, lightly doped), collector
• CE configuration: most common; β = I_C/I_B; current amplification
• Transistor as switch: saturation (on) and cutoff (off) states; used in digital circuits

Logic Gates

• AND, OR, NOT, NAND, NOR gates: truth tables and Boolean expressions
• NAND and NOR are universal gates: can construct all other gates
• CBSE: convert combinations of logic gates; implement given Boolean expression