Semiconductor Electronics: Materials, Devices and Simple Circuits
What is examined
Semiconductor Electronics tests energy bands, intrinsic and extrinsic conduction, the p–n junction, diode characteristics, rectifiers, Zener regulation, the transistor as an amplifier or switch, current gains and logic gates.
Core rules
- Bands: valence band, conduction band, forbidden gap E_g. Conductors have overlapping bands, semiconductors E_g ≈ 1 eV, insulators E_g > 3 eV.
- Intrinsic: n_e = n_h = n_i. Doping with a pentavalent donor gives n-type (majority electrons); a trivalent acceptor gives p-type (majority holes). 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 lowers the barrier; reverse bias raises it.
- Diode I–V: negligible current below the knee in forward bias; large current in reverse only at breakdown. A half-wave rectifier gives output at the input frequency, a full-wave rectifier at twice that frequency.
- Zener diode works in reverse breakdown; a series resistor drops the excess voltage and the Zener holds V_out constant.
- Transistor: emitter–base forward, collector–base reverse (active region). α = I_C/I_E, β = I_C/I_B, β = α/(1 − α), I_E = I_B + I_C. CE has current and voltage gain and phase reversal.
- Logic gates: NOT, AND, OR; NAND and NOR are universal.
Worked example
A transistor has α = 0.98 and I_E = 10 mA. Find I_C and I_B. I_C = αI_E = 0.98 × 10 = 9.8 mA; I_B = 10 − 9.8 = 0.2 mA. β = α/(1 − α) = 0.98/0.02 = 49, and indeed I_C/I_B = 9.8/0.2 = 49.
Traps
Saying holes are majority carriers in n-type; reversing forward and reverse bias; using β = α/(1 + α); and forgetting that a CE amplifier inverts the output.