Electronic Devices
What this domain tests
Semiconductor physics: energy bands, doping, the p–n junction and diode characteristics, rectification, zener regulation, photodiodes, LEDs, solar cells, transistors and logic gates.
Core rules
- Intrinsic semiconductors have equal electron and hole densities. Pentavalent doping gives n-type (electrons majority), trivalent gives p-type (holes majority); both remain electrically neutral.
- Depletion layer: barrier potential about 0.7 V for silicon, 0.3 V for germanium. Forward bias narrows it; reverse bias widens it and leaves only a small reverse saturation current.
- Half-wave rectifier conducts on one half-cycle, full-wave on both; a capacitor across the load smooths the ripple.
- A zener diode in reverse breakdown acts as a voltage regulator.
- Transistor: I_E = I_B + I_C; β = I_C/I_B; α = I_C/I_E = β/(1+β). The common-emitter configuration gives voltage and current gain, hence power gain.
- Gates: AND (Y = A·B), OR (Y = A+B), NOT, NAND, NOR; NAND and NOR are universal.
Worked example
A transistor has β = 100 and base current 20 μA. Find the collector current, emitter current and α. I_C = βI_B = 100 × 20 μA = 2.0 mA. I_E = 2.0 mA + 0.02 mA = 2.02 mA. α = I_C/I_E = 2.0/2.02 ≈ 0.99. Convert the base current to mA before adding so the units match.
Traps that cost marks
- Treating a transistor as two back-to-back diodes - it is not.
- Saying n-type material is negatively charged; it is neutral.
- Reporting α greater than 1, or forgetting α = β/(1+β).
- Assuming an LED works in reverse bias; it is forward biased and its colour is set by the band gap.