Discover why LEDs emit different colours, how bandgap energy determines wavelength, and how forward current controls brightness — with real-time I-V curve plotting, emission spectrum, and glowing LED visualisation.
LEDs emit light through electroluminescence — when a forward voltage is applied, electrons from the n-region recombine with holes from the p-region at the junction. Each recombination event releases energy equal to the bandgap energy (Eg) as a photon of light. The frequency of the photon is given by: f = Eg/h, where h is Planck's constant.
The colour (wavelength) of emitted light depends entirely on the bandgap energy (Eg) of the semiconductor material. λ = hc/Eg. A larger bandgap → shorter wavelength (blue, UV). A smaller bandgap → longer wavelength (red, IR). Different materials: GaAs (IR), GaAsP (Red/Orange), GaP (Yellow/Green), InGaN (Blue/Green/White), AlGaN (UV).
The threshold voltage (also called cut-in or knee voltage) is the forward voltage at which significant current and light emission begin. It is approximately equal to Eg/e (where e is electron charge). Red LEDs: ~1.8–2.0V. Green: ~2.0–3.5V. Blue/White: ~3.0–3.6V. UV: ~3.5–4.5V. This is higher than ordinary silicon diodes (0.7V) because LEDs use wider-bandgap materials.
Above the threshold, luminous intensity is proportional to forward current. Doubling the current roughly doubles the brightness. However, at very high currents, efficiency drops due to heating and non-radiative recombination — this is called efficiency droop. LEDs are always operated with a series resistor to limit current safely.
As temperature increases, the forward voltage decreases (negative temperature coefficient, like all p-n junctions). The threshold voltage shifts lower by approximately 2mV/°C. At higher temperatures, non-radiative recombination increases, reducing efficiency and brightness for the same current. This is why LEDs require thermal management in high-power applications.
A series resistor Rs is always connected with an LED to limit forward current. Without it, a small increase in voltage causes a huge current surge (exponential I-V). Rs = (Vs − Vf) / If. For example, driving a red LED (Vf = 2V) from 5V with If = 20mA: Rs = (5−2)/0.02 = 150Ω. This is fundamental to all LED circuit design.
A Light Emitting Diode (LED) is a forward-biased p-n junction semiconductor device that converts electrical energy directly into light through the process of electroluminescence. Unlike conventional diodes that dissipate energy as heat, LEDs are designed using direct-bandgap semiconductor materials where electron-hole recombination produces photons of light instead of heat.
The I-V characteristics of an LED are similar to a conventional p-n junction diode but with a higher threshold (turn-on) voltage determined by the bandgap of the semiconductor material. In the reverse bias region, virtually no current flows and no light is emitted. In the forward bias region below the threshold voltage, only a tiny leakage current flows and the LED remains off. Above the threshold voltage (Vth), current increases exponentially and the LED emits light with intensity proportional to forward current.
The colour of light emitted by an LED is determined by the bandgap energy (Eg) of its semiconductor material, through the relation: λ = hc/Eg, where h is Planck's constant and c is the speed of light. Semiconductors with a larger bandgap emit photons at shorter wavelengths (higher energy — blue, violet, UV), while materials with smaller bandgaps emit longer wavelengths (lower energy — red, infrared). Key materials: GaAs for infrared, GaAsP for red and orange, GaP for yellow and green, InGaN for blue, green, and white, AlGaN for ultraviolet LEDs.
The LED forward voltage (Vf) is approximately equal to the bandgap energy divided by the electron charge: Vf ≈ Eg/e. This is why blue LEDs (larger bandgap ≈ 2.65 eV) require a higher forward voltage (~3.0–3.6V) than red LEDs (smaller bandgap ≈ 1.9 eV, ~1.8–2.2V). The threshold voltage marks the knee of the I-V curve — the point at which current and light emission begin to rise sharply. This is a key parameter in LED circuit design and the LED experiment in FYUGP Physics and Electronics.
The LED I-V characteristics experiment is an important practical in Kerala University FYUGP Electronics and Physics labs. Students plot the I-V curve by varying forward voltage and recording current, identify the threshold voltage for different LEDs, observe how luminous intensity varies with current, and compare the forward voltages of different coloured LEDs to verify the relationship λ = hc/Eg. This simulation provides a complete interactive environment to perform and understand this experiment before entering the physical lab.