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Thermal properties of metamorphic InP-InGaAs heterojunction bipolar transistors (HBTs) on GaAs substrates using a linearly graded InGaP buffer have been investigated. Compared to the widely used InAlAs metamorphic buffer, InGaP offers better thermal properties resulting in a much smaller thermal resistance for the metamorphic HBTs (MHBTs). Theoretical calculations of the thermal resistance of devices have been made based on a simple constant heat-spreading model, and the results are shown to be consistent with experimental results. It has been made clear that the smaller thermal resistance measured from the MHBTs using a linearly graded InGaP buffer is due to the small bowing parameters and high thermal conductivity of the binary endpoints. Although the use of InGaP as a buffer may slightly degrade the devices thermal properties compared to one using InP directly on GaAs substrate, it gives more freedom to the growth optimization of metamorphic buffer by using compositional grading. With regards to the thermal conductivity and flexible growth optimization, InGaP metamorphic buffer could be considered as an important alternative to the existing InAlAs and InP schemes.