Semiconductor Devices and the PN Junction
This domain tests how doped silicon behaves, how a PN junction forms, and how diodes are used in simple circuits. Expect questions on doping, barrier potential, V–I characteristics, and diode approximations.
Core concepts
- Intrinsic silicon has equal electron and hole concentrations. Pentavalent doping (P, As) gives n-type with electrons as majority carriers; trivalent doping (B, Al) gives p-type with holes as majority carriers.
- Diffusion across the junction leaves a depletion region of immobile ions and a barrier potential: about 0.7 V for silicon, 0.3 V for germanium.
- Forward bias narrows the depletion layer and current rises sharply past the knee voltage. Reverse bias widens it, so only small leakage (minority-carrier) current flows until breakdown.
- Zener breakdown is a heavily doped, sharp effect below about 6 V; avalanche breakdown comes from impact ionisation at higher voltages. A Zener diode works in reverse bias as a shunt regulator.
- Dynamic resistance r_d = ΔV_f / ΔI_f. The ideal model ignores the drop; the second approximation keeps 0.7 V in series.
Worked example
A silicon diode in series with 100 Ω across 5 V. Using the 0.7 V drop, I = (5 − 0.7)/100 = 43 mA. The ideal model would give 50 mA, an error of about 14%, which shows why the second approximation is used when the supply is only a few volts.
Traps
Reversing majority carriers (electrons, not holes, in n-type); forgetting that reverse current is carried by minority carriers; assuming every diode drops 0.7 V regardless of material; treating a diode as an ohmic resistor.