Electronics: MDCAT Notes and MCQs

Semiconductors

• Intrinsic Si/Ge are tetravalent. n-type: doped with pentavalent impurity (P, As, Sb — donors) → majority carriers electrons. p-type: doped with trivalent impurity (B, Al, Ga, In — acceptors) → majority carriers holes. Both remain electrically neutral.

PN junction

• When p and n regions join, electrons and holes diffuse across and recombine, leaving immobile ions → depletion region (no free carriers) with a potential barrier: ≈ 0.7 V for Si, ≈ 0.3 V for Ge.
• Forward bias: p to positive, n to negative terminal → barrier and depletion layer decrease → large current flows (low resistance) once V exceeds the barrier.
• Reverse bias: p to negative, n to positive → depletion layer widens → only tiny leakage current (minority carriers), high resistance. Excess reverse voltage → breakdown.
• Thus a diode conducts in one direction only.

Rectification

Conversion of AC into DC.

• Half-wave rectifier: one diode; conducts only during one half-cycle → output is pulsating DC with gaps; low efficiency.
• Full-wave rectifier: uses both half-cycles — centre-tapped transformer with 2 diodes, or bridge rectifier with 4 diodes (in a bridge, 2 diodes conduct in each half-cycle). Output frequency is double the input (100 Hz for 50 Hz mains).
• A capacitor filter smooths the pulsating DC.