On the temperature response of semiconductor devices

This paper is somewhat tutorial in nature although the results shown here are derived from the thermal analysis of a specific microwave power transistor. Computer-aided thermal simulation techniques are used to derive heat transfer characteristics and to estimate the operating temperature of the device for a time varying sequence of power pulses. The technique and results are discussed to point out important thermal effects that occur and are common to most semiconductor devices. Points of interest include the temperature distribution both on and within the chip, effects of temperature dependent material properties, the effect of adding a heat spreader under the chip, and finally, the meaning and significance of the effective thermal time constant for the junction, the chip, the carrier and other parts of the electronic package. Some effects of unwitting temperature measurement by experimental techniques are also indicated where appropriate. These results form somewhat of a sequel to an earlier paper, which discussed similar results for other semiconductor devices. The main difference of this device compared with those in the earlier paper is that the effective junction time constant is much shorter since it is only on the order of 50 ms. As a result, junction temperature excursions become quite large even for power pulses of only several microseconds duration.