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ABSTRACT — GaN and its related alloys constitute a family of wide bandgap semiconductors suitable to optoelectronics and power microwave applications. For the latter applications, their high breakdown fields in the 3MV/cm range and their high peak electron velocity above 107cm/s are crucial. The high electron mobility transistor (HEMT) based on GaN is suitable to high frequencies and power applications. Moreover, those materials show excellent chemical and metallurgical stability. One peculiarity of GaN is stemming from the fact that the crystal growth is mostly achieved by heteroepitaxy since no commercial GaN substrates are yet available. The substrates currently chosen are sapphire, silicon carbide and silicon. The high power RF device performance decreases as operation temperature increases (e.g. fall of electron mobility impacting the cut-off frequencies and degradation of device reliability) so it is very important to understand the thermal effect in the device. This work present an analytical model of electron transport based on, at one hand, experimental characterisation such as I-V pulsed measurement, thermal characterisation and, at the other hand, thermal simulation and physical analysis. We were able to derive the variation of the electron velocity model as a function of temperature thanks to the thermal characterisation of parameters such electron mobility, ohmic contact, carrier density and gate Schottky barrier. I