Semiconductor Devices
Semiconductor Devices tests energy bands, doping, the p-n junction and its biasing, diode applications such as rectifiers and Zener regulation, photodiodes, LEDs, solar cells and logic gates. Expect a rectifier or Zener numerical and a truth-table question.
Core rules
- Conductors have overlapping bands, insulators a large gap (above about 3 eV), semiconductors a small gap (Si ≈ 1.1 eV, Ge ≈ 0.7 eV).
- Doping: pentavalent donors give n-type (majority electrons), trivalent acceptors give p-type (majority holes); the crystal remains electrically neutral.
- p-n junction: diffusion creates a depletion layer and barrier potential (≈0.7 V for Si, 0.3 V for Ge). Forward bias narrows the layer; reverse bias widens it and allows only a small leakage current.
- Half-wave rectifier output frequency equals the supply frequency; a full-wave rectifier doubles it. Ripple is reduced by a filter capacitor.
- Zener diode works in reverse breakdown, holding a constant voltage; a series resistor drops the excess voltage.
- Photodiode is reverse biased and its current rises with illumination; an LED is forward biased and emits photons on recombination; a solar cell converts light to emf without an external bias.
- Logic gates: OR gives 1 for any 1; AND gives 1 only for both 1; NOT inverts; NAND and NOR are universal gates.
Worked example
A Zener regulator has V(in) = 10 V, V(z) = 6 V, series resistance 200 Ω and load 1 kΩ. Current through the resistor = (10 − 6)/200 = 20 mA; load current = 6/1000 = 6 mA; so the Zener carries 20 − 6 = 14 mA.
Traps
- Assuming the Zener regulates even when the load current exceeds the available series current.
- Reversing the bias conditions for photodiode and LED.
- Thinking a full-wave rectifier keeps the same ripple frequency as the input.