Junction temperature is the highest temperature of the actual semiconductor in an electronic device. In operation, it is higher than case temperature and the temperature of the part's exterior. The difference is equal to the amount of heat transferred from the junction to case multiplied by the junction-to-case thermal resistance.
Various physical properties of semiconductor materials are temperature dependent. These include the diffusion rate of dopant elements, carrier mobilities and the thermal production of charge carriers.
At the low end, sensor diode noise can be reduced by cryogenic cooling. On the high end, the resulting increase in local power dissipation can lead to thermal runaway that may cause momentary or permanent device failure.
Maximum junction temperature
Maximum junction temperature (sometimes abbreviated TJMax) is specified in a part's datasheet and is used when calculating the necessary case-to-ambient thermal resistance for a given power dissipation. This in turn is used to select an appropriate heat sink if necessary. Other cooling methods include thermoelectric cooling and Coolants.
In Intel processors, the core temperature is measured by a sensor. If the core reaches its TJMax, this will trigger a protection mechanism to cool the processor. If the temperature rises above the TJMax, the processor will trigger an alarm to warn the computer operator who can then discontinue the process that is causing the overheating or shut down the computer to prevent damage.
An estimation of the chip-junction temperature, TJ, can be obtained from the following equation: TJ = TA + ( R θJA × PD )
where: TA = ambient temperature for the package ( °C )
R θJA = junction to ambient thermal resistance ( °C / W )
PD = power dissipation in package (W)
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