Question

If the doping concentration in a Si-Zener diode is increased, the Zener breakdown voltage:

• A. Decreases
• B. Increases
• C. Remains unchanged
• D. Becomes broader

Hint:

More doping means a thinner depletion layer and a stronger field, so breakdown is reached at a lower voltage.

Answer 1

The Correct Option is A

Step 1: In a Zener diode the reverse breakdown is dominated by the Zener effect (quantum tunnelling) when the diode is heavily doped, and by avalanche when it is lightly doped. The question concerns how the breakdown voltage responds to a change in doping.

Step 2: The width of the depletion region depends on doping. For a junction the depletion width scales roughly as \[W \propto \frac{1}{\sqrt{N}},\] where \(N\) is the doping concentration. So increasing \(N\) makes the depletion layer thinner.

Step 3: A thinner depletion layer means the same reverse voltage produces a much larger electric field across the junction, since \[E \sim \frac{V}{W}.\] A strong field triggers Zener tunnelling at a smaller applied reverse voltage.

Step 4: Therefore heavier doping lowers the reverse voltage needed to reach breakdown. The Zener breakdown voltage decreases as doping is increased. This is exactly why low-voltage Zener diodes (a few volts) are heavily doped, while higher-voltage ones are lightly doped.

Step 5: The correct option is (A), Decreases.

\[\boxed{\text{Breakdown voltage decreases}}\]