Semiconductor Devices and Diodes
What this domain tests
This domain links doping and the p-n junction to the four diode circuits the syllabus returns to: rectifiers, filters, Zener regulators and optoelectronic devices. Expect barrier-potential values, rectifier waveforms, PIV and ripple reasoning.
Core rules
- Intrinsic silicon conducts poorly. Pentavalent doping (P, As) gives n-type with electrons as majority carriers; trivalent doping (B, Al) gives p-type with holes.
- The depletion region has a barrier potential of about 0.7 V for silicon and 0.3 V for germanium. Forward bias narrows it; reverse bias widens it.
- A diode conducts above the knee voltage in forward bias and blocks in reverse until breakdown. Reverse saturation current is tiny and roughly doubles every 10 °C.
- Half-wave rectifier: one diode, ripple at 50 Hz on 50 Hz mains, PIV = Vm. Full-wave centre tap: two diodes, 100 Hz ripple, PIV = 2Vm. Bridge: four diodes, 100 Hz ripple, PIV = Vm, but two diode drops in the conducting path.
- A Zener regulates only in reverse breakdown, with a series resistor to drop the surplus voltage.
- LEDs drop 1.8–3.3 V depending on colour and always need a series resistor.
Worked example
A 12 V rms transformer feeds a bridge rectifier. Peak secondary voltage = 12 × 1.414 = 17.0 V. Two diodes conduct at any instant, so the no-load DC output is about 17.0 − 1.4 = 15.6 V, and the ripple frequency is 2 × 50 = 100 Hz. Adding a capacitor input filter pulls the average towards the peak and reduces ripple as C and load resistance increase.
Traps that cost marks
Forgetting the diode drop; using rms instead of peak when computing PIV; expecting a Zener to regulate in forward bias; connecting an LED without a resistor; mixing the centre-tap PIV (2Vm) with the bridge PIV (Vm).