A brief history of LED

More than half a century ago it was already known that semiconductor materials can emit light. In 1962 Nick Holonyak Jr. of General Electric developed the first practical visible light emitting diode. An LED is a semiconductor device that converts electrical energy into light energy. Its basic construction is a piece of electroluminescent semiconductor material mounted on a lead frame and encapsulated in epoxy resin, forming a solid state package with good resistance to shock and vibration.

LEDs were initially used mainly as indicator lamps in instruments. As technology advanced they were adopted widely in traffic signals and large area displays, bringing significant economic and social benefits. A 12 inch red traffic signal, for example, traditionally used a 140 W incandescent lamp, losing 90 percent of its light output through a red filter. A new design using 18 red LEDs as the light source consumes only 14 W in total while achieving the same light output. Automotive signal lamps are another important LED application.

How an LED chip works

An LED is a solid state semiconductor device that converts electrical energy directly into light. At its core is a semiconductor die connected to a negative terminal at one end and a positive terminal at the other, with the whole die encapsulated in epoxy resin. The die consists of P-type and N-type semiconductor material, in which holes and electrons dominate respectively. When current flows, electrons are pushed into the P region, recombine with holes and emit photons, producing light. The colour of the light depends on the wavelength of the P-N junction material.

1. Light emission principle

2. Internal chip structure

Chip classifications

1. MB chip. Definition: metal bonded chip, a UEC patented product. Characteristics: uses silicon with a high heat dissipation coefficient as substrate for effective thermal management. A metal layer bonds the epitaxial layer and the substrate, reflecting photons and reducing absorption. The conductive silicon substrate replaces the GaAs substrate and offers stronger heat conduction. A metal reflector at the bottom improves brightness and heat dissipation. Larger sizes are possible, making it suitable for high power applications.

2. GB chip. Definition: glue bonded chip, a UEC patented product. Characteristics: transparent sapphire substrate with high light output power. The chip emits light from all four sides, giving excellent pattern effects. Brightness is high but tolerance of high current is slightly poorer.

3. TS chip. Definition: transparent structure chip, an HP patented product. Characteristics: complex process with high reliability. Transparent GaP substrate and high brightness.

4. AS chip. Definition: absorbing structure chip, developed to maturity by the LED optoelectronics industry in Taiwan, China. Characteristics: quaternary chip with high brightness and excellent reliability, widely used.

Epitaxial material types

1. LPE: liquid phase epitaxy, used for GaP/GaP.
2. VPE: vapour phase epitaxy, used for GaAsP/GaAs.
3. MOVPE: metal organic vapour phase epitaxy, used for AlGaInP and GaN.
4. SH: single heterostructure, GaAlAs/GaAs.
5. DH: double heterostructure, GaAlAs/GaAs.
6. DDH: double heterostructure, GaAlAs/GaAlAs.

Composition and light emission

1. LED wafers are composed mainly of arsenic, aluminium, gallium, indium, phosphorus, nitrogen and silicon.

2. Classification of LED wafers.

By brightness: standard brightness (R, H, G, Y, E and similar), high brightness (VG, VY, SR and similar), ultra high brightness (UG, UY, UR, UYS, URF, UE and similar), invisible light (infrared R, SIR, VIR, HIR), infrared receivers (PT), phototransistors (PD).

By composition: binary wafers such as phosphorus and gallium (H, G and similar), ternary wafers such as phosphorus, gallium and arsenic (SR, HR, UR and similar), quaternary wafers such as phosphorus, aluminium, gallium and indium (SRF, HRF, URF, VY, HY, UY, UYS, UE, HE, UG).

AEC-Q102 testing required for automotive LEDs

The Automotive Electronics Council, formed by Ford, Chrysler and General Motors, issued AEC-Q102, the reliability standard for optoelectronic devices. It covers discrete devices including LEDs, laser components, photodiodes and phototransistors in 28 major categories, imposing strict reliability test requirements that give vehicle makers optoelectronic devices with high reliability and long service life. LEDs used in automotive lighting must be automotive grade, with high luminous efficacy, good temperature withstand and extremely high requirements for chip stability and consistency.

Passing AEC-Q102 certification is a long process for a company. A product must be designed against the standard from the outset and tested repeatedly so that defects are found and corrected in time, allowing mass produced units to pass AEC-Q102 certification smoothly.

Products covered by AEC-Q102

LEDs, laser components, laser elements, photodiodes, phototransistors, light emitting diodes, light pipes, photocells, phototriodes, thermistors, thermoelectric generators, thermoelectric coolers, photoresistors, infrared light sources, optocouplers, light emitting numeric displays, and multichip modules that use photoelectric functions together with other components, such as LEDs with integrated circuits, laser components with photodiodes and optocouplers.

AEC-Q102 test items for LEDs

1. Electrostatic discharge (ESD). LED components have lower ESD robustness than ordinary ICs. Unless a Zener diode is added, most component makers cannot guarantee an ESD class, and HBM above 2 kV and MM above 1 kV are commonly used as challenge targets.

2. Moisture sensitivity level (MSL). Automotive LED specifications already require this test. For surface mount devices it helps establish the product moisture class and supports production line management.

3. Thermal resistance and junction temperature estimation. LEDs are highly sensitive to heat, and degradation accelerates as ambient conditions worsen, so effective heat dissipation is essential. Thermal resistance measurement helps customers find suitable cooling methods or materials and extend service life.

4. Lumen maintenance life testing. Testing runs combinations of current, temperature and humidity, and includes the US ENERGY STAR DoE LM-80 verification service.

5. Life estimation. A typical life test calculates the activation energy of the product to obtain a more accurate life estimate.

6. Pulsed testing. A fast and severe method that most faithfully simulates actual use. It can be combined with high or low temperature storage and with temperature cycling, making it the most severe accelerated simulation.

7. Environmental stress testing. The typical stress tests are temperature cycling, which exploits mismatched thermal expansion of materials to expose structural risks in the assembled component, and temperature humidity cycling, which uses the breathing effect to reveal quality problems.

8. Mechanical stress testing. Push and pull force, vibration and shock on the assembled product should be planned according to the intended use and environmental conditions so as to highlight product defects.

9. Corrosive gas testing. Corrosive gases come from two sources: salt spray and acidic gases. Affected by air pollution, acidic gases combine with moisture in the air to form liquids that are extremely harsh on LED materials. They can sharply reduce internal light reflection and also cause external ageing and colour shift.

10. Water and dust protection testing. End products have very high requirements for water and dust protection, especially for outdoor use, so stricter testing is needed to ensure the structure is stable and reliable.

11. Other tests. Other commonly used tests include early failure screening, solderability, pressure cooker test and heat resistance testing.

12. Optical analysis. Temperature controlled integrating spheres of 50 cm and 100 cm and MCD measurement allow measurement of colour temperature, luminous flux, luminous intensity, colour coordinates, colour rendering index and wavelength. This supports customers in solving measurement needs during testing, provides a basis for judging verification results and improves test efficiency and accuracy.

NTEK testing capability against AEC-Q102 requirements