Semiconductor Electronics and Devices
Semiconductor Electronics and Devices covers band theory, diodes, rectifiers, Zener regulation, transistors and logic gates. The paper usually pairs a two-line numerical on transistor gain with a circuit-based question.
Core rules
- Band picture: a full valence band and an empty conduction band separated by a gap. Metals have overlapping bands, semiconductors a small gap (about 1.1 eV for Si), insulators a large gap (above 3 eV).
- Intrinsic material: n_e = n_h = n_i. Pentavalent donors give n-type (majority electrons), trivalent acceptors give p-type; the crystal stays electrically neutral. Mass action: n_e n_h = n_i².
- p-n junction: diffusion builds a depletion layer and barrier potential (about 0.7 V for Si, 0.3 V for Ge). Forward bias narrows the depletion layer, reverse bias widens it.
- Half-wave rectifier ripple frequency is f; full-wave gives 2f. A Zener diode works in reverse breakdown as a voltage regulator.
- Transistor: emitter heavily doped, base thin and lightly doped, collector large. α = I_C/I_E, β = I_C/I_B, and β = α/(1 − α).
- Logic gates: OR, AND, NOT, NAND, NOR; NAND and NOR are universal gates.
Worked example
A transistor has β = 100 and base current 20 μA. Find I_C, I_E and α. I_C = βI_B = 100 × 20×10⁻⁶ = 2.0×10⁻³ A = 2.0 mA. I_E = I_C + I_B = 2.0 + 0.02 = 2.02 mA. α = I_C/I_E = 2.0/2.02 ≈ 0.99, consistent with β/(β + 1).
Traps
Swapping α and β; treating a transistor as two back-to-back diodes; quoting 50 Hz as the ripple frequency of a full-wave rectifier; using a Zener in forward bias for regulation; and believing doping gives the crystal a net charge.