Semiconductor Electronics: Materials, Devices and Simple Circuits
This domain covers energy bands, doped semiconductors, p–n junctions, diodes, rectifiers, Zener regulation, transistors and logic gates. The exam favours bias conditions and current relations.
Core rules and concepts
- In solids, allowed energy bands are separated by forbidden gaps. Conductors have overlapping bands; semiconductors have a small gap; insulators have a large gap.
- Intrinsic semiconductors have equal electron and hole concentrations. n-type doping adds pentavalent donors; p-type adds trivalent acceptors. Doping increases conductivity without making the crystal charged.
- A p–n junction has a depletion region and barrier potential. Forward bias reduces the barrier and allows current; reverse bias widens it and permits only small leakage current.
- A diode can rectify AC; a full-wave rectifier uses two diodes per cycle. A Zener diode works in reverse breakdown to regulate voltage with a series resistor.
- Transistor relations: α = IC/IE, β = IC/IB, and β = α/(1 − α). In CE mode, a small base current controls a larger collector current.
- Logic gates: OR, AND, NOT, NAND, NOR; NAND and NOR are universal gates.
Worked example
A transistor has α = 0.95 and base current IB = 20 μA. Find β, IC and IE. β = α/(1 − α) = 0.95/0.05 = 19. IC = βIB = 19 × 20 μA = 380 μA = 0.38 mA. IE = IC + IB = 0.40 mA.
Traps
- Doping does not give the semiconductor a net charge; it changes carrier type and concentration.
- Forward bias lowers the barrier; it does not remove the depletion region completely.
- Zener breakdown is a controlled reverse-breakdown effect and needs a current-limiting resistor.
- α is always less than 1; β can be much larger.
- Do not confuse the truth tables of NAND and NOR.