GATE 2026 EC Q.14: solution and answer

Official answer: CSource: Official answer key, GATE 2026 EC (IIT Guwahati)

1. Zener diodes are fabricated with heavily doped $p$ and $n$ regions ($N_A$ and $N_D$ both large). The depletion width $W$ scales as $1/\sqrt{N}$ (dominated by the more lightly doped side), so with both sides heavily doped, $W$ is extremely small.

2. Under reverse bias $V_R$, this very thin depletion width produces an extremely high electric field, $E=V_R/W$, across the junction even for a modest reverse voltage.

3. This strong field tilts the energy bands so steeply that the valence band edge on the $p$-side comes into energetic alignment with the conduction band edge on the $n$-side, separated only by a very thin barrier.

4. Electrons filling the valence band on the $p$-side can then pass directly, without gaining kinetic energy, into empty conduction-band states on the $n$-side through this thin barrier. This direct quantum-mechanical tunneling through the barrier is the Zener effect, distinct from carrier acceleration followed by impact ionization (avalanche breakdown).

5. Hence the dominant carrier transport mechanism in the breakdown region of a (heavily doped) Zener diode is tunneling: option (C).