Semiconductor Electronics: Materials, Devices and Simple Circuits
What the exam tests
This domain tests energy bands, intrinsic and extrinsic semiconductors, p-n junction, diode characteristics, rectifiers, Zener diode, transistors, biasing and logic gates. Numerical questions often use transistor current relations.
Core rules
- Conductors have overlapping bands, insulators a large band gap, semiconductors a small gap of about 1 eV.
- Intrinsic semiconductors have n_e = n_h. Doping with pentavalent donors gives n-type; trivalent acceptors give p-type. The crystal remains electrically neutral overall.
- In a p-n junction, diffusion creates a depletion region and barrier potential. Forward bias lowers the barrier and width; reverse bias raises them.
- A diode conducts in forward bias above the cut-in voltage. A half-wave rectifier uses one diode; a full-wave rectifier uses two or four.
- Zener diode works in reverse breakdown to provide voltage regulation.
- Transistor active bias: emitter-base forward, collector-base reverse. α = Ic/Ie, β = Ic/Ib, and β = α/(1 − α).
Worked example
A transistor has β = 100 and base current 20 μA. Collector current is Ic = βIb = 100×20 μA = 2 mA. Emitter current is Ie = Ic + Ib = 2.02 mA. Then α = Ic/Ie = 2/2.02 = 0.990. This confirms α is slightly less than 1 while β can be much larger.
Traps
Doping does not give the semiconductor a net charge; it changes carrier concentration. Forward bias decreases the depletion width, not increases it. Zener breakdown is a reverse-bias effect. Do not confuse α and β. In a transistor, the base is thin and lightly doped, while the emitter is heavily doped.