Semiconductor Devices and Diodes
What this domain covers
Semiconductor Devices and Diodes examines the physics of the p-n junction and the circuit behaviour of diodes: rectifier diodes, Zener diodes, LEDs and photodiodes. Expect V-I characteristic questions, rectifier comparisons and bias-state reasoning.
Core rules
- A p-n junction has a depletion region with a barrier potential of about 0.7 V for silicon and 0.3 V for germanium.
- Forward bias narrows the depletion region and allows majority-carrier current; reverse bias widens it and only a small leakage current flows.
- A Zener diode is operated in reverse breakdown, where its voltage stays nearly constant, making it a shunt voltage reference.
- Half-wave rectifier: one diode, ripple frequency equal to the supply frequency, ripple factor 1.21. Full-wave centre-tap and bridge: two diodes conduct per cycle, ripple frequency is twice the supply, ripple factor 0.482.
- Bridge rectifier peak inverse voltage per diode is V_m, versus 2V_m for the centre-tap circuit.
Worked example
Design a series resistor for a red LED from a 5 V supply. LED forward voltage 2 V, current 20 mA. Voltage across the resistor = 5 − 2 = 3 V; R = V/I = 3/0.02 = 150 Ω; dissipation = 3 × 0.02 = 60 mW, so a ¼ W resistor is adequate.
Traps that cost marks
Assuming a Zener regulates in forward bias; using 50 Hz as the ripple frequency of a full-wave rectifier; forgetting the two diode drops in a bridge; reversing LED polarity; and confusing PIV with output voltage.