Semiconductor Physics and Diode Applications
What this domain tests
This unit covers band theory, intrinsic and extrinsic conduction, the p-n junction, diode V-I characteristics, and the circuits diodes make possible: rectifiers, filters, clippers, clampers and Zener regulators. KSEAB papers usually ask for one numerical on rectifier output, ripple factor, efficiency or PIV, plus a short theory question on barrier potential or Zener regulation.
Core rules
- Barrier potential: about 0.7 V for silicon, 0.3 V for germanium; the depletion layer widens on reverse bias and narrows on forward bias.
- Mass action law: n0·p0 = ni². Doping raises one carrier and suppresses the other.
- Dynamic resistance rd = 26 mV / ID at room temperature, so the diode behaves as a small resistor once conducting.
- Half-wave rectifier: Vdc = Vm/π, ripple factor 1.21, efficiency 40.6 %, PIV = Vm, ripple frequency = supply frequency.
- Full-wave (centre-tap or bridge): Vdc = 2Vm/π, ripple factor 0.48, efficiency 81.2 %, ripple frequency = 2f. PIV is 2Vm for centre-tap and Vm for bridge.
- Zener regulation: the Zener is reverse biased in breakdown; series resistance Rs = (Vin − Vz) / (Iz + IL).
Worked example
A bridge rectifier is fed from a 12 V rms secondary at 50 Hz into a 1 kΩ load. Vm = 12 × √2 = 16.97 V. Ignoring drops, Vdc = 2Vm/π = 10.8 V; subtracting two diode drops (2 × 0.7 V) gives about 9.4 V. Then Idc = 9.4 V / 1 kΩ = 9.4 mA, ripple frequency = 100 Hz, and each diode must withstand PIV = Vm = 16.97 V.
Traps that cost marks
- Substituting rms voltage where the formula needs peak Vm.
- Forgetting the two diode drops in a bridge but remembering one in a centre-tap circuit.
- Swapping PIV values between bridge and centre-tap topologies.
- Claiming the ripple frequency stays at 50 Hz in a full-wave circuit.