Electronic Devices
Electronic devices tests semiconductors, p-n junctions, diodes, rectifiers, Zener regulators, transistors, and logic gates. The exam expects band-picture reasoning, bias conditions, and simple current-gain calculations.
Core rules and concepts
- Intrinsic semiconductors have equal electron and hole concentrations. n-type doping adds donors; p-type adds acceptors. Doping leaves the crystal electrically neutral overall.
- In a p-n junction, diffusion creates a depletion region and barrier potential. Forward bias lowers the barrier and allows current; reverse bias widens it and permits only small leakage.
- A diode conducts mainly in forward bias. A half-wave rectifier uses one diode; a full-wave rectifier uses two or four depending on transformer configuration. A Zener diode operates in reverse breakdown for voltage regulation.
- A transistor has emitter, base, and collector. Current gain beta = I_C/I_B and alpha = I_C/I_E, with I_E = I_B + I_C and beta = alpha/(1 - alpha).
- Common-emitter configuration gives voltage and current gain and phase inversion. Logic gates: AND, OR, NOT, NAND, NOR; NAND and NOR are universal.
Worked example
A transistor has beta = 100 and base current 20 microA. Then I_C = beta I_B = 100 x 20 microA = 2.0 mA. Emitter current is I_E = 2.0 mA + 0.020 mA = 2.02 mA. Alpha is I_C/I_E = 2.0/2.02 = 0.990.
Traps that cost marks
- Saying doped semiconductors are charged.
- Reversing forward and reverse bias characteristics.
- Forgetting that a Zener is used in reverse breakdown.
- Mixing beta and alpha definitions.
- Writing a truth table for NAND as if it were AND.