Semiconductor Electronics: Materials, Devices and Simple Circuits

The chapter that ends Class 12 physics by building the modern world — concept-heavy, graph-rich and reliably worth 4-6 Board marks plus 1-2 questions in JEE/NEET. It opens with why solid-state devices killed the vacuum tube, then classifies solids by resistivity AND by energy bands: metals (overlapping/partially filled bands), insulators ($E_g$ > 3 eV) and semiconductors ($E_g$ < 3 eV — C: 5.4 eV, Si: 1.1 eV, Ge: 0.7 eV). Intrinsic Si/Ge conduct by thermally generated electron-hole pairs ($n_e = n_h = n_i$; I = $I_e + I_h$); doping transforms them — pentavalent donors (As, Sb, P) make n-type ($n_e \gg n_h$, donor level just below $E_C$), trivalent acceptors (In, B, Al) make p-type ($n_h \gg n_e$), with $n_en_h = n_i^2$ ruling both. Joining p to n creates the chapter's key structure: diffusion vs drift building a ~0.1 micrometre depletion region and barrier potential. Forward bias (barrier → $V_0 - V$) floods the junction with mA of minority-carrier injection past the cut-in voltage (0.2 V Ge, 0.7 V Si); reverse bias (barrier → $V_0 + V$) leaves microamps of saturation current until breakdown. The one-way V-I characteristic makes the diode a rectifier — half-wave (output 50 Hz on 50 Hz input) and centre-tap full-wave (100 Hz) with capacitor filters smoothing to dc. Zener regulation, LEDs, photodiodes, solar cells and the five logic gates — trimmed from the rationalised Board text but alive in JEE/NEET — are covered as tagged extras. Every band diagram, bias rule, waveform and exam trap is covered.