V-I Characteristics of LED in Forward Bias

Aim

To study the V-I characteristics of a Light Emitting Diode (LED) in forward bias, plot the forward characteristic curve, and determine its turn-on (threshold) voltage.

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.

LED Component

A standard red LED from the simulator's component library, plus a current-limiting resistor (~330 Ω) to keep current within the LED's rated limit.

Theory & Principle

A Light Emitting Diode (LED) is a PN junction diode made from a direct-bandgap compound semiconductor (such as gallium arsenide phosphide for red LEDs) rather than plain silicon. When forward biased beyond its turn-on voltage, electrons and holes recombine at the junction and release energy directly as photons of visible (or infrared/ultraviolet) light.

Key Insight: Because LEDs use a wider bandgap material than silicon, their forward turn-on voltage is significantly higher than an ordinary silicon diode's ~0.7 V. The turn-on voltage depends on the emitted colour (bandgap energy): approximately 1.8–2.2 V for red, 2.0–2.5 V for yellow/green, and 2.8–3.5 V for blue/white LEDs.

The general shape of the V-I curve follows the same exponential diode equation as a regular PN junction diode, just shifted to a higher voltage.

Circuit / Setup Diagram

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

Procedure

Open the simulator and connect a battery, a current-limiting resistor (330 Ω), and a red LED in series.
Connect the LED in forward bias: anode (longer lead) to the positive terminal, cathode (shorter lead / flat edge) to the negative terminal through the resistor.
Insert a milliammeter in series with the LED and a voltmeter across it.
Increase the source voltage gradually from 0 V, noting when the LED first becomes visibly lit, and record V and I at each step, taking finer steps near the turn-on region (1.6–2.0 V).
Continue until the current reaches about 25 mA — the typical rated forward current for a standard 5 mm LED.
Plot the graph of forward current against forward voltage and mark the point at which the LED visibly begins to glow.

Tip: Note the voltage at which light first becomes visible — it typically appears just after the current starts rising steeply, well before the current reaches its rated maximum.

Observation Table

S. No.	Forward Voltage, V₀ (V)	Forward Current, I₀ (mA)	Visible Light Output
1	0.0	0.00	None
2	1.0	0.00	None
3	1.5	0.03	None
4	1.7	0.60	None
5	1.75	1.50	Very faint
6	1.80	4.00	Faint glow
7	1.90	13.00	Clearly visible
8	2.00	22.00	Bright
9	2.05	25.00	Bright (rated I_F)

V-I Characteristic Graph

Figure 2: Forward V-I characteristic of a red LED. The turn-on knee occurs near 1.8-1.9 V, much higher than a silicon rectifier diode.

Calculations

Turn-on Voltage from Graph

The knee of the curve occurs at approximately V ≈ 1.8–1.9 V for the red LED, compared to ~0.7 V for a silicon rectifier diode.

Series Resistor Check

For a 5 V supply with LED drop ≈ 2.0 V and desired current 20 mA: R = (5 − 2.0) / 0.020 = 150 Ω (minimum), so a 330 Ω resistor used here safely limits current below the rated maximum.

Result

Result: The forward V-I characteristic of the red LED was plotted. The turn-on voltage was found to be approximately 1.8–1.9 V, considerably higher than a silicon rectifier diode's cut-in voltage of ~0.7 V, due to the LED's larger semiconductor bandgap.

Viva Voce Questions

Q1: Why does an LED have a higher forward voltage than a silicon diode?

An LED is made from a direct-bandgap compound semiconductor with a larger energy gap than silicon. Since the forward voltage required to make carriers cross the junction and recombine is related to the bandgap energy (approximately E_g/e), a larger bandgap material requires a higher forward voltage before significant current — and light emission — begins.

Q2: Why do LEDs of different colours have different turn-on voltages?

The colour of light emitted depends on the photon energy released during electron-hole recombination, which equals the semiconductor's bandgap energy (E = hc/λ). Since bandgap energy increases as wavelength decreases, red LEDs (longer wavelength, lower energy) have a lower turn-on voltage than blue or white LEDs (shorter wavelength, higher energy).

Q3: Why must an LED always be used with a current-limiting resistor?

Like any diode, an LED has a very low dynamic resistance once forward biased beyond its turn-on voltage. Without a series resistor, even a small increase in supply voltage could cause a very large increase in current, permanently damaging the LED through excessive heating.

Q4: What happens if an LED is connected in reverse bias?

An LED conducts negligible current in reverse bias, similar to an ordinary diode, and does not emit light. However, LEDs typically have a much lower reverse breakdown voltage (often only a few volts) than rectifier diodes, so applying more than a few volts in reverse can permanently damage the LED.

Q5: How is the light output of an LED related to its forward current?

Within the rated operating range, the light intensity (luminous output) of an LED increases roughly in proportion to the forward current passing through it, since more current means a higher rate of electron-hole recombination and hence more photons emitted per second. Exceeding the rated current, however, causes excessive heating that can reduce light output over time or damage the LED.

Related Experiments

Continue exploring semiconductor devices with the following experiment:

← V-I Characteristics of Photodiode in Reverse Bias Mode

V-I Characteristics of LED in Forward Bias Mode

Aim