What is a Zener diode? Draw its V-I characteristic and explain why it is used as a voltage regulator.

1. 1 · Define it. A Zener diode is a heavily doped p-n junction diode made to work in reverse breakdown.
2. 2 · Explain its construction. Heavy doping makes the depletion layer very thin, so a few volts give a very strong field and Zener breakdown at a set voltage Vz.
3. 3 · Describe the characteristic. Forward: like an ordinary diode. Reverse: tiny current until Vz, then a large rise in current at almost constant voltage.
4. 4 · Draw the characteristic. [Graph: forward curve in the first quadrant; reverse current near zero, then a sharp, almost vertical drop at Vz in the third quadrant; Vz labelled]
5. 5 · Real life. Mobile chargers and power supplies use Zener diodes to keep a voltage steady.
6. 6 · Close in one line. So, since its voltage stays at Vz over a wide range of current, a Zener diode is a constant-voltage device.

Scoring tip: Draw the drop at Vz as almost vertical. A sloping line suggests the voltage changes, which loses the point.