V-I Characteristics of Zener Diode in Reverse Bias

Aim

To study the V-I characteristics of a Zener diode in reverse bias, plot the reverse characteristic curve, and determine the Zener breakdown voltage (V_z).

Apparatus / Tools Required

Virtual Circuit Simulator

PhET Semiconductor / Circuit Construction Kit, Tinkercad Circuits, or Falstad Circuit Simulator — free and browser-based.

Web Browser

A modern browser such as Chrome, Firefox, or Edge with JavaScript enabled.

Spreadsheet Software

Excel, Google Sheets, or LibreOffice Calc for recording readings and plotting the V-I graph.

Zener Diode Component

A Zener diode rated at a known breakdown voltage (e.g. 8V2, part number BZX55C8V2) and a series protective resistor (~470 Ω) to limit current after breakdown.

Theory & Principle

When a Zener diode is reverse biased, current remains extremely small (just the ordinary reverse saturation current) until the applied reverse voltage reaches the diode's rated Zener breakdown voltage (V_z). At this point, the heavily doped junction undergoes a sharp, controlled breakdown — via the Zener effect (quantum tunnelling, dominant below ~5 V) or the avalanche effect (carrier multiplication, dominant above ~5–6 V) — and current rises almost vertically while the voltage across the diode stays essentially constant at V_z.

Key Insight: This near-constant voltage during breakdown is exactly what makes the Zener diode useful as a voltage regulator or voltage reference: as long as a series resistor limits the current to a safe value, the voltage across the Zener diode remains fixed at V_z over a wide range of currents.

Circuit / Setup Diagram

Figure 1: Zener diode reverse-biased through a protective series resistor, with a milliammeter in series and a voltmeter across the diode.

Procedure

Open the simulator and connect a battery, a series protective resistor (470 Ω), and a Zener diode of known rating (e.g. 8V2).
Connect the Zener diode in reverse bias: cathode to the positive terminal, anode to the negative terminal.
Insert a milliammeter in series and a voltmeter across the diode.
Increase the reverse voltage gradually from 0 V, taking readings every 1–2 V initially, then every 0.05–0.1 V as the current begins to rise sharply.
Continue until the current reaches about 30 mA, ensuring the series resistor keeps the diode within its safe power rating.
Plot the graph of Zener current (I_z) against reverse voltage (V_r) and identify the breakdown knee.

Tip: The breakdown region is very sharp — take extra-fine voltage steps (0.02–0.05 V) right around the expected Zener voltage so the knee of the curve is captured precisely.

Observation Table

S. No.	Reverse Voltage, V_r (V)	Zener Current, I_z (mA)
1	0	0.00
2	2	0.01
3	4	0.02
4	6	0.05
5	7.5	0.15
6	7.8	0.50
7	7.9	2.00
8	8.0	8.00
9	8.1	22.00
10	8.2	30.00

V-I Characteristic Graph

_r (volts) → 0612182430 Zener Current, I_z (mA) → LegendZener Diode 8V2 Current stays negligible until the Zener breakdown voltage (V_z ≈ 8.0 V), then rises almost vertically.

Figure 2: Reverse V-I characteristic of the Zener diode. The near-vertical knee at V_z ≈ 8.0 V is the breakdown point.

Calculations

Zener Breakdown Voltage from Graph

The knee of the curve (where current begins rising almost vertically) is read from the graph at V_z ≈ 8.0 V, matching the diode's rated value.

Zener (Dynamic) Resistance in Breakdown

r_z = ΔV / ΔI = (8.2 − 8.0) / (30 − 8) × 10⁻³ = 0.2 / 0.022 ≈ 9.1 Ω

A low dynamic resistance in breakdown means the Zener voltage stays nearly constant even as current through it varies — the property exploited in voltage regulator circuits.

Result

Result: The reverse V-I characteristic of the Zener diode was plotted. The Zener breakdown voltage was found to be approximately 8.0 V, matching the diode's rated value, with a low dynamic resistance in the breakdown region confirming its suitability for voltage regulation.

Viva Voce Questions

Q1: What is the difference between the Zener effect and the avalanche effect?

Both cause reverse breakdown, but by different mechanisms. The Zener effect dominates in heavily doped diodes with thin depletion regions and breakdown voltages below about 5 V; it occurs when the strong electric field directly pulls valence electrons across the junction (quantum tunnelling). The avalanche effect dominates at higher breakdown voltages (above ~6 V); it occurs when accelerated carriers collide with atoms and knock loose additional electron-hole pairs, which are further accelerated, creating a multiplying 'avalanche' of carriers.

Q2: Why is a series resistor essential when operating a Zener diode in breakdown?

Once breakdown begins, current can rise extremely fast for a very small increase in voltage. Without a current-limiting series resistor, the current would be limited only by the source and circuit resistance, likely exceeding the diode's maximum power rating and destroying it. The resistor sets a safe operating current.

Q3: How does a Zener diode act as a voltage regulator?

When reverse biased with a series resistor from a supply voltage greater than V_z, the Zener diode maintains an almost constant voltage (V_z) across itself even if the supply voltage or load current varies, because its dynamic resistance in breakdown is very low. Any excess voltage is dropped across the series resistor, and any excess current is diverted through the Zener diode.

Q4: What is the temperature coefficient of a Zener diode?

Zener diodes with V_z below about 5 V (dominated by the Zener effect) have a negative temperature coefficient (breakdown voltage decreases as temperature rises). Those above about 6 V (dominated by the avalanche effect) have a positive temperature coefficient. Diodes rated near 5–6 V often have a near-zero temperature coefficient, making them useful as stable voltage references.

Q5: What happens if the Zener diode's maximum power rating is exceeded?

Exceeding the maximum power rating (P = V_z × I_z) causes excessive heating at the junction, which can permanently damage the diode by destroying its crystal structure or melting the bond wires — resulting in an open circuit or a short circuit failure.

Related Experiments

Continue exploring semiconductor devices with the following experiment:

← V-I Characteristics of Zener Diode in Forward Bias Mode

V-I Characteristics of Zener Diode in Reverse Bias Mode

Aim