Knowledge LED Junction Temperature

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Junction temperature (Tj) is the temperature of the active region between the p-n junction of the LED chip. The active region is where charge carriers recombine to generate light. Junction temperature cannot be measured directly but is calculated based on a known case temperature (Tc) and the materials' thermal resistance in accordance with the manufacturer’s instructions. As the highest temperature within the semiconductor chip, junction temperature determines life, efficiency and color point of LEDs.

LEDs are self-heating devices. They do not radiate heat as seen in incandescent sources but dissipate thermal energy from the semiconductor die through conduction. If the thermal energy that is introduced to the junction is not efficiently dissipated away from the LED, junction temperature will rise and accelerate the kinetics of temperature-dependent failure mechanisms. At higher temperatures defects introduced to the epitaxial layers during crystal growth tend to grow larger. Threading dislocations form at the interface of the sapphire substrate and GaN epitaxial layer. Elevated dislocation density contributes to reduced probability of photon generation in the active region, which causes irreversible lumen depreciation (decrease in quantum efficiency). A heated semiconductor die can impose thermal stresses on packaging materials. Package-related failure mechanisms induced by high junction temperatures include carbonization of the encapsulant, encapsulant yellowing, and phosphor thermal degradation. These package-level degradation mechanisms can lead to permanent color shifts and reductions in light extraction and wavelength conversion efficiencies. Continuous operation at elevated temperature will reduce the useful life of an LED by 30% to 50% for every 10 °C increase.

Aside from irreversible failures as a result of operating LEDs beyond the maximum rated junction temperature, there're other LED characteristics related to junction temperature. High junction temperatures may cause diode thermal droop and phosphor thermal quenching, which compounds the loss in light output. The reduction of band gap energy at higher junction temperatures leads to a decrease in forward voltage. The higher the junction temperature the lower the forward voltage. This is because there's a reduction of band gap energy at higher junction temperatures. The negative temperature coefficient of LED forward voltage imposes special considerations in the design of LED power supplies.

Junction temperature also affects the dominant wavelength, or perceived color of light emitted by the LED chip. Junction temperature is inversely proportional to the energy band gap of the semiconductor chip, and the energy band gap is inversely proportional to the wavelength of light emitted from the LED chip. Accordingly, as junction temperature rises, the energy band gap becomes narrower, and thus the dominant wavelength of the emitted light increases. The dominant wavelength will increase one nanometer for every 10°C rise in junction temperature.

LEDs are traditionally tested and binned with the junction temperature set to 25°C, which does not reflect the operating conditions in real world applications. To ensure light output, forward voltage and color point of LEDs are binned within a tight set of bounds, hot-testing has been introduced to improve predictability of the performance of a LED at typical temperature conditions.

The junction temperature of LED is affected by three variables: drive current, thermal path, and ambient temperature.
 
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