Semiconductor Electronics: Materials, Devices and Simple Circuits - Quick Revision
Classification of solids
- By conductivity/resistivity: metals (rho ~ 10^-2 to 10^-8 ohm m), semiconductors (intermediate), insulators (rho ~ 10^11 to 10^19 ohm m).
- By energy bands: valence band (filled), conduction band (above), separated by band gap Eg.
- Metal: Eg approx 0 (bands overlap or conduction band partly filled). Insulator: Eg > 3 eV. Semiconductor: Eg between 0.2 eV and 3 eV (Si 1.1 eV, Ge 0.7 eV, C/diamond 5.4 eV, Sn 0 eV = metal).
Intrinsic semiconductor
- Pure Si or Ge; carriers created in electron-hole pairs by thermal energy.
- ne = nh = ni (intrinsic carrier concentration). Hole = vacancy in a bond, effective +q charge, moves opposite to electrons.
- Total current I = Ie + Ih. Acts like an insulator at T = 0 K.
Extrinsic semiconductor (doping)
- Doping a few ppm of impurity increases conductivity hugely.
- n-type: pentavalent donors (As, Sb, P); electrons are majority, holes minority; ne >> nh.
- p-type: trivalent acceptors (B, Al, In); holes are majority, electrons minority; nh >> ne.
- Mass-action law: ne nh = ni^2 always; crystal stays overall neutral.
p-n junction
- Formed by diffusion (carriers cross due to concentration gradient) and drift (field-driven, opposite direction).
- Depletion region of immobile ion cores creates a barrier potential V0; at equilibrium no net current.
Semiconductor diode
- Forward bias (p to +): barrier (V0 - V) reduced, depletion narrows, large current (mA).
- Reverse bias (n to +): barrier (V0 + V) raised, depletion widens, tiny reverse saturation current (microamp); breaks down at Vbr.
- Cut-in voltage approx 0.2 V (Ge), 0.7 V (Si). Dynamic resistance rd = dV/dI; dc resistance R = V/I (low forward, very high reverse).
Rectifiers and filters
- Half-wave: one diode, conducts one half-cycle, output frequency = input frequency.
- Full-wave: centre-tap transformer + two diodes (or bridge of four), both half-cycles rectified, output (ripple) frequency = 2 x input frequency.
- A capacitor across the load (large RC time constant) filters the ripple, giving a smoother dc near the peak voltage.
Logic gates
- OR: Y = A + B; AND: Y = A.B; NOT: Y = A-bar; NAND and NOR are the universal gates (any logic can be built from them).