Semiconductor Electronics and Devices
Semiconductor Electronics and Devices tests energy bands, intrinsic and extrinsic semiconductors, p-n junctions, diodes, rectifiers, transistors and logic gates. Questions often combine biasing with current gain.
Core rules
- In an intrinsic semiconductor, n_e = n_h; conductivity rises with temperature. Doping with pentavalent atoms gives n-type (majority electrons), trivalent gives p-type (majority holes).
- A p-n junction has a depletion region and barrier potential. Forward bias reduces the barrier and allows diffusion current; reverse bias widens it and permits only small leakage current.
- A half-wave rectifier uses one diode; a full-wave rectifier uses two diodes with a centre-tapped transformer or four in a bridge. A capacitor filter smooths the output.
- Transistor: I_E = I_B + I_C; current gain β = I_C/I_B, α = I_C/I_E. In common-emitter active mode, the base-emitter junction is forward biased and base-collector reverse biased.
- Logic gates: OR gives 1 if any input is 1; AND gives 1 only if all are 1; NOT inverts. NAND and NOR are universal gates.
Worked example
A transistor has β = 100 and base current 20 μA. Then I_C = βI_B = 100(20 μA) = 2000 μA = 2.0 mA. If the collector supply is 10 V and collector resistor 2.0 kΩ, the resistor drop is 2.0 mA × 2.0 kΩ = 4.0 V, so V_CE = 10 - 4 = 6.0 V.
Traps
Reversing forward and reverse bias; using β and α interchangeably; forgetting the base current is much smaller than collector current; and assuming a diode conducts perfectly in reverse bias.