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Metamorphic double heterojunction bipolar transistors (MDHBT) using InP or InAlP as metamorphic buffer layers were grown on GaAs substrates. For devices using InP buffer layers on GaAs substrates, measured junction-ambient temperature rise at 7.5 mW power dissipation is comparable to those of devices grown on InP substrates, while much larger temperature rises are observed when InAlP buffer layers are employed. By comparing the measured temperature rise with that computed as a function of the known transistor geometry and unknown buffer-layer thermal conductivity, we estimate that the thermal conductivity of a uniform InP buffer layer is 35±5 W/k-m, while a linearly graded InAlP buffer layer with 0.2 μm InP upper layer has 8±3 W/k-m effective thermal conductivity. These results strongly suggest the use of InP metamorphic buffer layers in metamorphic InP-based DHBTs grown on GaAs substrates.