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The transient temperature response and thermal profiles near the junction of a transistor or other semiconductor device are nearly impossible to measure experimentally. These response characteristics can be accurately predicted, however, through the use of computeraided simulation. In this paper results from the simulation of two microwave power transistors are presented and discussed. In particular, the first device contains a single large junction area, and the second contains multiple junction areas. Thermal characteristics within the chip itself, that is, from the transistor junction to the chip .carrier, are emphasized. This region usually contains, by far, the greatest fraction of the total T between the junction and the final heat sink, especially under pulsed power operation. Transient temperature response curves, internal temperature distributions and the effects of temperature dependent material properties are described. The effects of varying the power dissipation rate, pulse length and duty cycle are shown, and final1y, methods of improving heat transfer, such as the use of a diamond heat-spreader, are discussed.