Light-Emitting Diodes (LEDs) are efficient semiconductors that generate light through a process known as electroluminescence. They are smaller, longer lasting, and more reliable than incandescent or fluorescent lamps. With applications in lighting, displays, and specialized fields, LEDs offer high performance and energy savings. This article provides information on how LEDs work, their characteristics, lifetimes, and advanced types.
LED Overview
A Light-Emitting Diode (LED) is a semiconductor device that generates light when current flows through it in the forward direction. Unlike incandescent bulbs, which glow by heating a filament, or fluorescent lamps that rely on gas excitation, LEDs function through electroluminescence, the direct emission of photons as electrons recombine with holes inside the semiconductor. This process makes them far more efficient and reliable than older technologies. LEDs stand out due to their compact design, long service life, durability against shock and vibration, and minimal power consumption.
Light Emission in Semiconductors
This image explains the process of light emission in semiconductors, which is the working principle behind LEDs. When a semiconductor is excited either by electrical current or optical injection, electrons move from the valence band to the conduction band, creating a separation between electrons and holes. This energy difference is called the band gap (Eg).
Once excited, the electron in the conduction band eventually recombines with a hole in the valence band. During this recombination process, the lost energy is released in the form of a photon. The energy of the emitted photon corresponds exactly to the band gap of the material, meaning the wavelength (or color) of the light depends on the semiconductor’s band gap.
LED Electrical Characteristics
| LED Color | Forward Voltage (Vf) | Forward Current (mA) | Notes |
|---|---|---|---|
| Red | 1.6 – 2.0 V | 5 – 20 mA | Lowest Vf, highly efficient |
| Green | 2.0 – 2.4 V | 5 – 20 mA | Slightly higher Vf |
| Blue | 2.8 – 3.3 V | 5 – 20 mA | Requires more voltage |
| White | 2.8 – 3.5 V | 10 – 30 mA | Made with blue LED + phosphor coating |
LED Luminous Output and Efficacy
| Light Source | Efficacy (Lumens per Watt) | Notes |
|---|---|---|
| Incandescent bulb | \~10–15 lm/W | Most energy is lost as heat |
| Halogen lamp | \~15–25 lm/W | Slightly better than incandescent |
| Fluorescent tube | \~50–100 lm/W | Requires ballast, contains mercury |
| Compact Fluorescent (CFL) | \~60–90 lm/W | Small form factor, being phased out |
| Modern LED | 120–200 lm/W | Available in consumer lighting |
| High-end LED prototypes | 250–300+ lm/W | Lab-tested, showing future potential |
LED Color and Rendering Quality
Correlated Color Temperature (CCT)
• Warm White (2700K–3500K): Produces a yellowish glow, best for living rooms, restaurants, and cozy indoor settings.
• Neutral White (4000K–4500K): Balanced and comfortable, often used in offices, classrooms, and retail spaces.
• Cool White (5000K–6500K): Crisp, bluish daylight-like light, excellent for outdoor lighting, workshops, and task-heavy environments.
Color Rendering Index (CRI)
• CRI ≥ 80: Suitable for household and commercial lighting.
• CRI ≥ 90: Required in areas demanding precise color judgment, such as art studios, medical facilities, and high-end retail.
LED Lifetime and Lumen Maintenance
The L70 Standard
LED lifetime is measured by the L70 standard. This value represents the number of operating hours until the LED’s light output drops to 70% of its original brightness. At this point, the LED is still functional but no longer provides its intended illumination quality. L70 ensures a consistent way to compare LED performance across manufacturers.
LED Lifetimes
• Consumer LEDs: 25,000 – 50,000 hours of use.
• Industrial LEDs: 50,000 – 100,000+ hours, designed for harsher conditions and higher duty cycles.
LED Thermal Management
Junction Temperature (Tj)
The junction temperature is the internal temperature at the point where light is generated inside the LED chip. Manufacturers specify a safe operating range below 125 °C. If this value is exceeded, the LED’s brightness, efficiency, and lifetime are reduced. Keeping Tj low ensures the LED can meet its rated performance.
Junction-to-Ambient Thermal Path
Heat produced inside the LED must travel from the junction to the surrounding air. This pathway is called the junction-to-ambient path. Designers measure its effectiveness using thermal resistance (RθJA), expressed in °C/W. A lower thermal resistance means heat is transferred more efficiently, keeping the LED cooler and more stable.
Cooling Methods
• Heat Sinks - Aluminum fins absorb and spread heat away from the LED.
• Thermal Vias - Small plated holes in the PCB conduct heat from the LED pad to the copper layers.
• Metal-Core PCBs (MCPCBs) - Used in high-power LEDs, these boards have a metal base that transfers heat efficiently.
• Active Cooling - Fans or liquid cooling systems are used in demanding environments such as projectors, stadium lighting, or industrial fixtures.
LED Driving Methods
Constant Current Drivers
A constant current driver keeps the LED current stable even when the supply voltage fluctuates. This is the most reliable way to power LEDs, as it prevents thermal runaway and maintains consistent light output. High-quality drivers often include protections against short circuits, surges, and overtemperature conditions.
PWM Dimming
Pulse Width Modulation (PWM) controls brightness by turning the LED on and off at very high speeds. By adjusting the duty cycle (the ratio of on-time to off-time), the perceived brightness changes smoothly. Because the switching frequency is above the human eye’s detection range, the light appears steady. Poorly designed systems with low-frequency PWM can cause visible flicker, leading to eye strain or camera artifacts.
Analog Dimming
In analog dimming, brightness is adjusted by changing the amplitude of the current flowing through the LED. This method avoids flicker issues but can shift the LED’s color slightly, especially at very low brightness levels. Analog dimming is often combined with PWM in advanced systems to achieve both smooth color control and precise brightness regulation.
LED Packaging and Optics
Surface-Mount Device (SMD) LEDs
SMD LEDs are the most used type in modern lighting. They are mounted directly on the PCB and come in standard sizes such as 2835 and 5050. SMD LEDs provide good efficiency and flexibility, making them best for LED strips, household bulbs, and panel lights. Their compact size allows easy integration into thin and lightweight fixtures.
Chip-on-Board (COB) LEDs
COB packages mount multiple LED dies directly on a single substrate, creating a dense light source. This design offers higher brightness, smoother light output, and reduced glare compared to individual SMDs. COB LEDs are found in spotlights, downlights, and high-power lamps, where strong directional lighting is required.
Chip-Scale Package (CSP) LEDs
CSP technology eliminates bulky packaging, reducing the LED to nearly the same size as the semiconductor die itself. This allows for smaller, more efficient, and thermally stable designs. CSP LEDs are widely used in automotive headlights, smartphone backlighting, and display panels, where compactness and durability are required.
Optics and Beam Control
The raw light from an LED package is not always suitable for direct use. To shape and direct light, designers use optical elements such as lenses for focusing or spreading light. Reflectors to redirect and control beam angles. Diffusers for soft, uniform illumination.
Specialized LED Types
UV LEDs
Emit ultraviolet light for sterilization, adhesive curing, and counterfeit detection. Safe, compact alternative to mercury UV lamps.
IR LEDs
Produce invisible infrared light for remote controls, night vision, and biometric systems. Efficient and widely used in electronics and security.
OLEDs
Thin, flexible organic LEDs are used in smartphones, TVs, and wearables. Deliver vivid colors and contrast but have shorter lifetimes.
Micro-LEDs
Next-gen displays offering brighter, more efficient, and longer-lasting performance than OLEDs. Best for AR/VR, TVs, and smartwatches.
Laser Diodes
Semiconductor devices that create coherent, high-intensity beams. Used in fiber optics, scanners, medical tools, and laser pointers.
Conclusion
LEDs have developed into versatile components used in lighting, displays, and advanced technologies. Their efficiency, durability, and controllability set them apart from older light sources. Specialized forms such as UV, IR, OLEDs, and micro-LEDs expand their role even further. With continued improvements, LEDs remain central to the future of sustainable and high-performance lighting systems.
Frequently Asked Questions [FAQ]
Q1. What materials are LEDs made of?
LEDs are made from semiconductors like gallium arsenide (GaAs), gallium phosphide (GaP), and gallium nitride (GaN).
Q2. Why do LEDs need resistors?
Resistors limit current flow and protect LEDs from burning out.
Q3. How are white LEDs made?
White LEDs use a blue LED chip with a yellow phosphor coating to create white light.
Q4. Why do LEDs change color over time?
LEDs shift color due to heat and material degradation, as well as phosphor degradation.
Q5. Can LEDs work in extreme environments?
Yes. With proper design, LEDs can run in very cold, hot, humid, or dusty conditions.
Q6. How is LED lifetime tested?
LEDs are tested with thermal, humidity, and electrical stress to estimate lifespan.