Unit
- Semiconductor basis: LEDs are built from compound semiconductors such as gallium nitride (GaN) for blue/green or aluminum gallium indium phosphide (AlGaInP) for red/yellow. The bandgap energy determines the emitted wavelength and thus the color.
- P-N junction: A forward-biased p-n junction injects electrons and holes into the active region. Recombination releases energy as photons through electroluminescence. The efficiency of this process defines the LED's internal quantum efficiency.
- Wavelength control: By adjusting the composition of the semiconductor alloy, manufacturers tune the bandgap to produce specific colors. Indium concentration in InGaN shifts emission from near-UV to green.
- Luminous efficacy: This measures how well an LED converts electrical power to visible light, expressed in lumens per watt. Modern white LEDs achieve over 200 lm/W, far surpassing incandescent bulbs.
- Forward voltage: LEDs require a minimum voltage to turn on, typically 1.8V to 3.3V depending on color. Exceeding the maximum forward voltage damages the chip.
- Thermal effects: Junction temperature rises with current and ambient conditions. Higher temperatures reduce light output and shift wavelength, shortening lifespan. Efficient heat sinking is critical.
- Color binning: Due to process variations, LEDs from the same wafer vary in color and brightness. Manufacturers sort them into bins using chromaticity coordinates and flux ranges to ensure consistency in final products.
- Bin codes: Each bin is labeled with a code specifying dominant wavelength, color temperature, and luminous flux. Designers specify bin codes to match LEDs in a single luminaire.
Design Principle
Principle: Bandgap Engineering
Definition: Deliberately adjusting the semiconductor alloy composition to achieve a target emission wavelength. The bandgap energy (Eg) in electron volts is inversely proportional to wavelength (λ = 1240/Eg nm). Small composition changes produce significant color shifts.
Application: For a warm white LED, manufacturers combine a blue InGaN chip with a yellow phosphor. To achieve specific color temperatures like 3000K vs 5000K, they modify phosphor thickness and composition while ensuring the chip's wavelength remains within a tight tolerance of ±5 nm.
Quick Revision
- LEDs use compound semiconductors with bandgaps tuned to specific wavelengths.
- Internal quantum efficiency drives electroluminescent performance.
- Thermal management is essential to maintain output and longevity.
- Color binning ensures consistency across production batches.