Semiconductor Electronics: Materials, Devices and Simple Circuits
The unit covers energy bands, intrinsic and extrinsic semiconductors, the p–n junction, diode applications (rectifier, Zener regulator, photodiode, LED, solar cell), the transistor as an amplifier and switch, and logic gates with truth tables. Expect a short numerical on current gain, a definition question on the depletion region, and a long answer on the half-wave/full-wave rectifier or on transistor action in CE configuration.
Core rules
- Band gap: conductors overlap, semiconductors ≈1 eV (Si 1.1 eV, Ge 0.7 eV), insulators above about 3 eV.
- Intrinsic semiconductor: n(e) = n(h); conductivity rises with temperature and with doping.
- Pentavalent doping (P, As, Sb) gives n-type with electrons as majority carriers; trivalent (B, Al, In) gives p-type with holes majority.
- p–n junction: diffusion creates a depletion layer and a barrier potential (≈0.7 V for Si, 0.3 V for Ge). Forward bias lowers the barrier and gives a low-resistance path; reverse bias widens it.
- Zener diode works in reverse breakdown as a voltage regulator; a photodiode is reverse biased; an LED is forward biased and emits photons at the junction.
- Transistor gains: α = I(C)/I(E), β = I(C)/I(B), and β = α/(1 − α). CE amplifier output is 180° out of phase with input.
- NAND and NOR are universal gates.
Worked example
A transistor in common-base mode has α = 0.95 with emitter current 10 mA. Find collector and base currents and β. I(C) = αI(E) = 0.95 × 10 = 9.5 mA; I(B) = I(E) − I(C) = 0.5 mA. β = I(C)/I(B) = 9.5/0.5 = 19, which agrees with β = α/(1 − α) = 0.95/0.05 = 19. Note that α is always less than 1 while β is much larger, so the CE configuration gives the greater current gain.
Traps that cost marks
- Confusing majority carriers of n-type (electrons) with the fixed donor ions.
- Using α = β/(1 + β) instead of the correct β = α/(1 − α) relation.
- Forgetting that the Zener diode is used in reverse bias, not forward bias.
- Writing truth tables with the wrong number of rows for a three-input gate (eight, not four).