V-I Characteristics of PN Junction Diode in Forward Bias

Aim

To study the V-I (voltage-current) characteristics of a PN junction diode in forward bias, plot the forward characteristic curve, and determine the cut-in (threshold) voltage of the diode.

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.

Diode Component

Silicon PN junction diode (e.g. 1N4007) available in the simulator's component library, plus a current-limiting resistor (~1 kΩ).

Theory & Principle

A PN junction diode is formed by joining a p-type and an n-type semiconductor. In forward bias, the positive terminal of the source is connected to the p-side and the negative terminal to the n-side. This reduces the width of the depletion region and, once the applied voltage exceeds a certain threshold, majority carriers flow easily across the junction, producing a rapidly rising current.

I = I₀ (e^V/ηV_T − 1)

Shockley diode equation: I₀ = reverse saturation current, V_T = thermal voltage (≈26 mV at room temperature), η = ideality factor

Key Insight: For silicon diodes the cut-in voltage (also called knee or threshold voltage) is approximately 0.6–0.7 V; for germanium diodes it is approximately 0.2–0.3 V. Below this voltage the current is negligible; above it, current increases exponentially with a very small further increase in voltage.

Circuit / Setup Diagram

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

Procedure

Open the simulator and place a battery, a current-limiting resistor (1 kΩ), and a silicon diode in series to form the circuit.
Connect the diode in forward bias: anode (p-side) to the positive terminal, cathode (n-side, marked with a band) to the negative terminal through the resistor.
Insert a milliammeter in series with the diode and a voltmeter in parallel across the diode terminals.
Increase the source voltage in small steps (0.1 V initially, then 0.02–0.05 V near the knee) and note the voltmeter (V) and milliammeter (I) readings at each step.
Continue until the current reaches about 30 mA — beyond this the diode's safe operating current is exceeded.
Plot the graph of forward current (I, y-axis) against forward voltage (V, x-axis) and locate the knee point (cut-in voltage).

Tip: Take closely spaced readings (every 0.02–0.05 V) near the knee region (0.6–0.75 V) since the current changes very rapidly there — this is essential for accurately locating the cut-in voltage.

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 silicon PN junction diode. The knee near 0.7 V marks the cut-in voltage.

Calculations

Determining the Cut-in Voltage

The cut-in voltage is read directly from the graph as the voltage at which the curve sharply bends upward (the "knee").

From the plotted graph, the knee occurs at approximately V ≈ 0.7 V, consistent with the known cut-in voltage for silicon diodes.

Forward (Dynamic) Resistance near Operating Point

r_f = ΔV / ΔI = (0.80 − 0.74) / (30.00 − 14.20) × 10⁻³ = 0.06 / 0.0158 ≈ 3.8 Ω

This small dynamic resistance explains why current rises so sharply for a small increase in voltage beyond the knee.

Result

Result: The V-I characteristic of the PN junction diode in forward bias was plotted. The cut-in (threshold) voltage was found to be approximately 0.7 V, matching the expected value for a silicon diode.

Key observations:

Current remains negligible below the cut-in voltage.
Beyond the cut-in voltage, current increases exponentially with voltage.
The forward (dynamic) resistance of the diode is very small near and above the knee.

Viva Voce Questions

Q1: What is the cut-in (threshold) voltage of a diode?

The cut-in voltage is the forward voltage at which the diode begins to conduct appreciably. Below this voltage the current is negligible (in the microamp range); above it, current rises exponentially. For silicon diodes it is about 0.6–0.7 V, and for germanium diodes about 0.2–0.3 V.

Q2: Why is the forward V-I characteristic exponential rather than linear?

The current through a PN junction follows the Shockley diode equation, I = I₀(e^V/ηV_T − 1), which is an exponential function of voltage. This arises because the number of majority carriers able to cross the depletion region increases exponentially with the applied forward voltage, unlike a resistor where current is linearly proportional to voltage (Ohm's Law).

Q3: Why must a current-limiting resistor be used in this experiment?

Once the cut-in voltage is exceeded, the diode's dynamic resistance becomes very small, so even a tiny increase in voltage would cause a very large increase in current, potentially destroying the diode. A series resistor limits the current to a safe value.

Q4: What is the difference between silicon and germanium diodes in forward bias?

Silicon diodes have a higher cut-in voltage (~0.7 V) but lower reverse saturation current and better temperature stability. Germanium diodes have a lower cut-in voltage (~0.3 V) but higher reverse leakage current and are more temperature-sensitive, so silicon diodes are far more commonly used in modern circuits.

Q5: What happens to the diode if the forward current exceeds its rated value?

Excess current causes excessive heating (I²R losses) at the junction, which can permanently damage the diode by melting the junction or causing thermal runaway. This is why diodes are always used with a current-limiting resistor or are rated with a maximum forward current (I_F(max)) that must not be exceeded.

Related Experiments

Continue exploring semiconductor devices with the following experiment:

← V-I Characteristics of PN Junction Diode in Reverse Bias Mode

V-I Characteristics of PN Junction Diode in Forward Bias Mode

Aim