V-I Characteristics of Zener Diode in Forward Bias

Aim

To study the V-I characteristics of a Zener diode in forward bias, plot its forward characteristic curve, and verify that it behaves similar to a standard PN junction diode when forward biased.

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 of any rated Zener voltage (e.g. 6V2, part number BZX55C6V2) from the simulator's component library, plus a current-limiting resistor (~1 kΩ).

Theory & Principle

A Zener diode is a heavily doped PN junction diode specially designed to operate safely in reverse breakdown. However, in forward bias it conducts exactly like an ordinary rectifier diode: negligible current below the cut-in voltage, followed by a sharp exponential rise once the cut-in voltage (typically ~0.6–0.7 V for silicon) is exceeded.

I = I₀ (e^V/ηV_T − 1)

Same Shockley diode equation applies to the Zener diode in forward bias

Key Insight: The Zener effect (controlled breakdown at a precise voltage) is a reverse-bias phenomenon only. The Zener rating (e.g. "6V2") refers to its reverse breakdown voltage — it has no special significance in forward bias.

Circuit / Setup Diagram

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

Procedure

Open the simulator and place a battery, a current-limiting resistor (1 kΩ), and a Zener diode in series.
Connect the Zener diode in forward bias: anode to the positive terminal, cathode (banded end) to the negative terminal through the resistor.
Insert a milliammeter in series with the diode and a voltmeter across the diode.
Increase the source voltage gradually, taking closely spaced readings near the knee (0.6–0.75 V), and record V and I at each step.
Continue until the current reaches about 30 mA to stay within safe limits.
Plot the graph of forward current against forward voltage and compare its shape with that of an ordinary PN junction diode.

Tip: Compare this graph with the PN junction diode forward-bias experiment — you should find the curves are essentially identical, confirming that the Zener effect only applies in reverse bias.

Observation Table

S. No.	Forward Voltage, V₀ (V)	Forward Current, I₀ (mA)
1	0.0	0.00
2	0.3	0.00
3	0.5	0.10
4	0.6	0.80
5	0.65	2.10
6	0.70	5.80
7	0.74	14.20
8	0.78	25.10
9	0.80	30.00

V-I Characteristic Graph

Figure 2: Forward V-I characteristic of a Zener diode — identical in shape to an ordinary silicon diode.

Calculations

Cut-in Voltage from Graph

The knee of the curve occurs at approximately V ≈ 0.7 V, the same cut-in voltage as an ordinary silicon PN junction diode.

Result

Result: The forward V-I characteristic of the Zener diode was plotted and found to be essentially identical to that of an ordinary PN junction diode, with a cut-in voltage of approximately 0.7 V. This confirms that the special Zener (controlled breakdown) behaviour applies only in reverse bias.

Viva Voce Questions

Q1: Does the Zener voltage rating (e.g. 6V2) have any meaning in forward bias?

No. The Zener voltage rating refers specifically to the reverse breakdown voltage at which the diode is designed to conduct in a controlled manner. In forward bias, the diode behaves like any ordinary silicon diode with a cut-in voltage of about 0.7 V, regardless of its Zener rating.

Q2: How is a Zener diode different from an ordinary diode in construction?

A Zener diode is more heavily doped than an ordinary rectifier diode. Heavy doping makes the depletion region very thin, so a relatively low reverse voltage creates a very high electric field, enabling a sharp, controlled, and reversible breakdown (Zener or avalanche effect) at a precisely defined voltage.

Q3: Why is the forward characteristic of a Zener diode rarely used in circuits?

Zener diodes are almost always used in reverse bias, exploiting their sharp, stable breakdown voltage for voltage regulation and reference applications. Their forward characteristic offers no advantage over an ordinary diode and is rarely used deliberately in circuit design.

Q4: What is the ideality factor in the diode equation?

The ideality factor (η) accounts for deviations from ideal diode behaviour due to recombination within the depletion region. For an ideal diode η = 1; for practical silicon diodes it typically ranges from 1 to 2, depending on the dominant current mechanism at a given voltage.

Q5: Can a Zener diode be damaged if used incorrectly in forward bias?

Yes — just like an ordinary diode, if used in forward bias without a current-limiting resistor, excessive current can flow once the cut-in voltage is exceeded, causing overheating and permanent damage to the junction.

Related Experiments

Continue exploring semiconductor devices with the following experiment:

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

V-I Characteristics of Zener Diode in Forward Bias Mode

Aim