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The results obtained from a computer-aided thermal analysis of a microwave power transistor are presented. The transistor is somewhat unique in that it has a junction-to-carrier thermal time constant of only about 50µs. The application for which this transistor was desired required that it handle a pulsed power dissipation wherein each pulse has the shape of a fourth power cosine curve with a 7µs on-time add 130 watts peak power. The half-power pulsewidth is, therefore, only about 2.5µs. Although the pulse duration is quilte short, the junction Undergoes temperature excursions of about 30°C during any single pulse. These temperature excursions cannot possibly be detected by either direct electrical measurements or infrared experimental techniques but, nevertheless, have an important effect on the life and reliability of the transistor. The computer-aided analysis technique and the results are discussed in general throughout the paper. Points of interest include the temperature distribution both on and within the chip, the effects of temperature dependent material properties, the use of a heat spreader under the chip, and, finally, the meaning and significance of the thermal time constant for the junction, the chip, the carrier, and other parts of the electronic package.