Distinguish between avalanche and zener breakdown.

II. Short Answer Questions · Q8

Q. Distinguish between avalanche and zener breakdown.

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Step 1. Doping level and depletion-layer width are the key distinguishing factors: Zener breakdown needs a HEAVILY doped junction with a very narrow depletion layer ( $<10^{-6}$ m), while avalanche breakdown needs a LIGHTLY doped junction with a wide depletion layer.

Step 2. In Zener breakdown, the narrow layer concentrates the reverse voltage into an extremely strong field (order $3\times10^7\ \text{V m}^{-1}$ ) that directly ruptures covalent bonds in the lattice, generating electron-hole pairs without any carrier collisions being needed.

Step 3. In avalanche breakdown, the field alone is too weak to rupture bonds directly; instead, existing thermally generated minority carriers are accelerated across the WIDE depletion region, gain enough kinetic energy to collide with lattice atoms and break bonds on impact, and the newly freed carriers are themselves accelerated to cause further collisions -- a self-multiplying (avalanche) chain reaction. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2023Set ANNUAL1 mark
$Q. Why is a semiconductor damaged by a strong current?$
›Reveal solutionSolution

Excess current -> excess heat -> the tiny junction overheats and its structure is destroyed (thermal runaway).

A semiconductor device (such as a diode or transistor) has a very small junction area and mass compared to typical circuit components, so it has a limited capacity to dissipate heat. When a strong current flows through it, the power dissipated as heat ( $P = I^2R$ ) can raise the junction temperature rapidly. In a semiconductor, higher temperature also generates more thermally-excited charge carriers, which further increases the current — a positive-feedback effect called thermal runaway — until the temperature rise permanently damages or destroys the crystal structure/junctio …

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CBSE 2022Set ANNUAL1 mark
$Q. Why does zener diode work in reverse bias?$
›Reveal solutionSolution

A zener diode's useful, voltage-regulating breakdown only happens under reverse bias.

A Zener diode is a heavily-doped p-n junction diode designed so that, under reverse bias, once the reverse voltage reaches a certain sharply-defined value (the Zener voltage $V_Z$ ), the reverse current increases very sharply while the voltage across the diode stays essentially constant at $V_Z$ . This constant-voltage behaviour under reverse breakdown is exactly what makes it useful as a voltage regulator: any device connected across it sees a stable voltage regardless of variations in the supply or load current. In forward bias, the diode behaves just like an ordinary p-n junction diode (conducting once the barrier potential is exceeded) and shows n …

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CBSE 2020Set 55/3/11 markMCQ
$Q. A zener diode has (A) heavily doped p-side and lightly doped n-side. (B) heavily doped n-side and lightly doped p-side. (C) heavily doped n-side as well as p-side. (D) lightly doped n-side as well as p-side.$
›Reveal solutionSolution
A Zener diode is designed to operate in reverse breakdown at a precise, low voltage. This requires both sides to be heavily doped so the depletion region is very thin, enabling a sharp, controlled breakdown. The correct option is (C).

The key to this question lies in understanding why a Zener diode behaves differently from an ordinary diode. An ordinary diode, when reverse-biased beyond a certain voltage, undergoes avalanche breakdown — a high-energy process that can destroy the diode. A Zener diode, however, is engineered to break down at a much lower, precisely controlled voltage and to survive it repeatedly.

That controlled breakdown is achieved by making the depletion region extremely narrow. When the depletion region is thin, even a modest reverse voltage creates a very strong electric field across it — strong enough to pull electrons directly out of their covalent bonds (a process called Zener breakdown, or tunnelling). This happens at a sharp, repeatable voltage.

How do you make the depletion region thin? The depletion region width depends on the doping concentration. Heavier doping means more charge carriers are available, so the space-charge region (depletion layer) becomes narrower. For a Zener diode, both the p-side and the n-side are heavily doped. This gives the thinnest possible depletion region, ensuring breakdown occurs cleanly at a low voltage.

Let’s walk through the options:

Option (A): heavily doped p-side and lightly doped n-side — This would make the depletion region extend mostly into the lightly doped n-side, making it wider overall. Not suitable for a sharp, low-voltage breakdown.

Option (B): heavily doped n-side and lightly doped p-side — Symmetric problem; the depletion region would be wide on the p-side. Again, not ideal.

Option (C): heavily doped n-side as well as p-side — Correct. Both sides heavily doped → very thin depletion region → strong electric field at low reverse voltage → controlled Zener breakdown. …
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CBSE 2019Set ANNUAL1 markMCQ
$Q. Avalanche breakdown is primarily dependent on the phenomenon of :$
(a) doping
(b) recombination
(c) collision
(d) ionisation
›Reveal solutionSolution

Avalanche breakdown is a chain-reaction process triggered by high-energy carrier collisions with lattice atoms, distinguishing it from Zener breakdown's direct field-ionisation mechanism.

In a p-n junction diode under large reverse bias, avalanche breakdown occurs in lightly-doped junctions with a relatively wide depletion region. The small reverse saturation current present in the junction consists of minority charge carriers (thermally generated electron-hole pairs). Under the strong reverse electric field, these carriers are accelerated to high velocities and hence high kinetic energies as they cross the depletion region.

When these energetic carriers collide with the semiconductor's lattice atoms, they transfer enough energy to break covalent bonds and knock valence electrons free, creating new electron-hole pairs (impact ionisation). These newly-freed carriers are, in turn, also accelerated by the field and cause further collisions, releasing still more carriers -- a multiplying chain reaction known as avalanche multiplication. The result is a sudden, very large increase in reverse current: avalanche breakdown.

…

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Q.Distinguish between avalanche and zener breakdown.

Concept understanding — Breakdown Mechanisms: Zener and Avalanche

Q.Distinguish between avalanche and zener breakdown.

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