Question

Consider carrier transport in a Zener diode in the breakdown region. Which is the dominant transport mechanism for current flow in this case?

• A. Drift
• B. Diffusion
• C. Tunneling
• D. Ballistic transport

Hint:

A Zener diode is doped heavily on both sides, giving a thin depletion region where carriers can cross the barrier directly under a high field.

Answer 1

The Correct Option is C

Step 1: Recall how a Zener diode is built.
A Zener diode is a p-n junction diode with both the p-side and n-side doped heavily, which makes its depletion region very thin compared to an ordinary diode.

Step 2: Recall what happens under reverse bias near breakdown.
As the reverse voltage increases, the electric field across this thin depletion region becomes extremely high, of the order of \(10^{6}\) V/cm or more.

Step 3: Identify the transport mechanism at this field strength.
At such a high field, the depletion region is thin enough and the field strong enough that valence electrons on the p-side can quantum mechanically tunnel straight across the narrow energy barrier into the conduction band on the n-side, without needing extra thermal energy to climb over the barrier. This direct barrier crossing is called tunneling, and it is the mechanism behind what is named Zener breakdown.

Step 4: Rule out the other options.
Drift is charge motion caused by an electric field acting on free carriers already present in a band; it describes ordinary current flow in the diode, not the barrier-crossing event itself. Diffusion is motion driven by a concentration gradient, and it dominates minority-carrier current in the neutral regions, not the breakdown of the thin junction. Ballistic transport describes carriers moving without scattering over distances shorter than their mean free path, a concept used for very short channel devices, not the physics of Zener breakdown.

Step 5: Final conclusion.
\[ \boxed{\text{Tunneling}} \]