scope:

This publication is intended for power GaAs FET applications requiring high reliability. An accurate measurement of thermal resistance is extremely important to provide the user with knowledge of the FETs operating temperature so that more accurate life estimates can be made. FET failure mechanisms and failure rates have, in general, an exponential dependence on temperature (which is why temperature-accelerated testing is successful). Because of the exponential relationship of failure rate with temperature, the thermal resistance should be referenced to the hottest part of the FET.