Semiconductor Devices: Diodes and Rectifiers
3. Semiconductor Devices: Diodes and Rectifiers
This domain tests PN junction behaviour, diode characteristics and the conversion of AC to DC - the most heavily numerical part of the syllabus.
Junction and diode
- A PN junction forms a depletion region with a barrier potential of about 0.7 V for silicon and 0.3 V for germanium.
- Forward bias narrows the depletion layer and current rises sharply after the knee; reverse bias widens it and only leakage flows.
- Reverse breakdown is normal for a Zener diode, which is used in reverse bias as a voltage regulator.
- An LED emits light in forward bias; a photodiode works in reverse bias.
Rectifiers
- Half-wave: one diode, output frequency = supply frequency, V_dc = V_m/π ≈ 0.318V_m, ripple factor 1.21, PIV = V_m.
- Full-wave centre-tap: two diodes, ripple frequency 2f, V_dc = 2V_m/π ≈ 0.636V_m, ripple factor 0.48, PIV = 2V_m.
- Bridge: four diodes, same V_dc and ripple factor as centre-tap, but PIV = V_m.
- A shunt capacitor filter raises V_dc towards V_m and reduces ripple.
Worked example
A transformer with a 12 V rms secondary feeds a bridge rectifier. V_m = 12 × √2 = 16.97 V. V_dc = 2 × 16.97/π = 10.8 V. Ripple frequency = 2 × 50 = 100 Hz. PIV across each diode = 16.97 V.
Traps
- Using rms where peak is required, or forgetting the 0.7 V drop in small-signal work.
- Quoting PIV = 2V_m for a bridge rectifier.
- Thinking a lower ripple factor means worse filtering (it means better).
- Connecting a Zener in forward bias and expecting regulation.