# Zener Diode Characteristics

> Reverse breakdown at V_Z, the I-V curve and Zener voltage regulator action with NEET Physics numericals.

- Canonical URL: https://prepelephant.com/topics/neet-ug/physics/zener-characteristics-neet
- Exam / course: NEET-UG · Subject: Physics
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Zener Diode Characteristics", PrepElephant, https://prepelephant.com/topics/neet-ug/physics/zener-characteristics-neet

## Direct answer

Reverse-bias a Zener diode and it holds its ground: past a sharp knee, the voltage across it stays pinned at the Zener voltage V_Z while the current through it swings widely — a controlled breakdown that is not destructive because a series resistor limits the current. In breakdown below roughly 5-6 V the mechanism is Zener (field-emission) tunnelling in heavily doped junctions; above that, avalanche multiplication — both reversible if the power rating is respected. Forward-biased, it behaves as an ordinary silicon diode with its 0.7 V drop. The flat reverse characteristic is what makes it a voltage regulator: wired in parallel with the load, it keeps V_Z across the load while the series resistance absorbs supply fluctuations.

## What you must remember

- **I-V shape:** forward — negligible current below about 0.7 V, then a steep rise; reverse — tiny leakage until the knee at V_Z, then near-vertical current rise at almost constant voltage.
- **Two breakdown types:** heavily doped, low-voltage (below about 5-6 V) units break by Zener tunnelling; higher-voltage units by avalanche multiplication — both non-destructive under current limiting.
- **Regulator wiring:** supply V_in > V_Z in series with R_S; Zener in parallel with the load; the load sits at V_Z; R_S carries I_S = I_Z + I_L.
- **Design equation:** R_S = (V_in − V_Z)/(I_L + I_Z) chosen so the Zener always keeps some minimum current in breakdown.
- **Power limit:** the diode survives while V_Z × I_Z stays within its power rating; load removed, the Zener takes the whole series current and must still cope.
- **Numerical anchor:** V_in = 12 V, V_Z = 6 V, R_S = 300 Ω gives I_S = 20 mA; a 600 Ω load draws 10 mA and the Zener passes the other 10 mA.
- **Ideal-versus-real framing:** an ordinary diode is not designed to live in breakdown; the Zener is engineered for a sharp, stable V_Z — that is the entire distinction.

## Regulating a supply, step by step

Build the standard circuit: 12 V supply, 300 Ω series resistor, a 6 V Zener across the load. The series current is fixed at (12 − 6)/300 = 20 mA. Attach a 600 Ω load: it demands 6/600 = 10 mA, and the Zener quietly passes the remaining 10 mA (dissipating 6 × 0.01 = 0.06 W) while holding exactly 6 V across the load. Now stress it. Supply rises to 15 V: series current becomes 30 mA; the load still takes 10 mA, and the Zener absorbs 20 mA — the output stays 6 V, which is the regulation promise kept. Finally overload it: drop the load to 300 Ω, demanding 20 mA — the entire series current; the Zener is starved out of breakdown, and regulation collapses. The Zener regulates only while it carries some current, and that boundary is the number every numerical turns on.

## Where NEET sets the trap

The word "breakdown" baits the misconception that the diode is destroyed — the credited statement is that breakdown is non-destructive provided the current (hence V_Z I_Z) is limited by the series resistor. Circuit-identification items test polarity: in a regulator the Zener is reverse biased, its cathode to the positive side; options show it flipped and expect you to notice. The characteristic-curve question asks which quantity stays constant in the breakdown region — the voltage across the diode, never the current through it. Numerical traps: using V_in instead of (V_in − V_Z) when computing the series current, and forgetting that load and Zener share the series current — the dropout condition (load takes everything) is the concept the exam quietly tests. The 0.7 V forward drop and V_Z are different numbers and never interchangeable in options.

## Frequently asked questions

### What happens when a Zener diode is reverse biased past its knee?

The voltage across it stays essentially constant at V_Z while current varies widely — the flat region that does the regulating.

### Why doesn't breakdown destroy a Zener diode?

The series resistor limits the current, keeping dissipation P = V_Z I_Z within the diode's power rating; only exceeding that rating causes permanent damage.

### How does a Zener hold a 6 V output from a 12 V supply with a 600 Ω load?

With R_S = 300 Ω the series current is (12 − 6)/300 = 20 mA; the load takes 10 mA and the Zener the remaining 10 mA, pinning the output at 6 V.

### What separates Zener breakdown from avalanche breakdown?

Zener (field-emission) tunnelling dominates below roughly 5-6 V in heavily doped junctions; impact-ionisation avalanche dominates higher — both are reversible mechanisms.

### When does a Zener regulator stop regulating?

When the load current grows to equal the series current, the Zener drops out of breakdown — or when the supply falls to V_Z — and the output is no longer held.