The junction temperature refers to the temperature inside the semiconductor element. In the LED, it refers to the temperature of the in-chip light-emitting layer (the position at which the multiple quantum well structure is disposed between the pn junctions). The temperature of the light-emitting layer of the LED chip rises when it is lit. In general, the higher the junction temperature, the lower the luminous efficiency. As the input current increases, the luminous flux increases as the luminous flux increases. As a result, there is a vicious circle in which the temperature (junction temperature) of the light-emitting layer is increased to lower the luminous efficiency, and the power consumption is increased, thereby further increasing the junction temperature. By reducing the thermal resistance of the LED chip package and the package mounting substrate, the heat generated by the chip can be dissipated, and the improvement of the junction temperature can be avoided, and the brightness can be improved.
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The junction temperature is: thermal resistance × input power ambient temperature, so if you increase the maximum rating of the junction temperature, the LED will work normally even if the ambient temperature is very high. For example, in white LEDs, some LED chip varieties have an allowable bonding temperature of up to 185 °C. The bonding temperature can vary greatly depending on how the LEDs are lit. For example, when the LED is driven by a pulse (driving intermittent current drive to the LED, intermittently lighting), the junction temperature is not easily increased, and the LED is continuously driven (steady current driving to the LED input, continuously lighting), and the junction temperature is likely to rise.
When the chip is stored in heat, the light intensity will decrease.
The luminescent layer of the LED chip equipped with the white LED will rise in temperature during lighting. In general, if the temperature of the portion of the light-emitting layer called the junction temperature rises, the luminous efficiency is lowered, even if the input power is not bright. By reducing the thermal resistance of the LED chip package and the package substrate, the heat generated on the chip is dissipated, and the junction temperature is not raised to make the illumination brighter. (The picture is based on the data of the German OSRAM Opto Semiconductors)
If you use an LED chip that has the highest junction temperature rating, you can get a lot of advantages when you install it. For example, the output power can be increased due to the increased input power. It is also possible to reduce the heat sink of the bottom plate and the like.
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