This paper deals with a scalable and distributed electro-thermal model of AlGaN/GaN HEMT. This model has been especially developed for multi-fingers and large periphery transistors. Quasi isothermal measurements by means of pulsed I(V) and pulsed [S] have been performed at different temperatures to extract the model and its thermal dependencies. Then load-pull measurements have been performed to final validations. In terms of accuracy, the developed model has been compared to an usual compact model. Three devices with different sizes have been verified to check the validity of the scaling rules implemented.