ABSTRACTIt is reported that we demonstrated a large current operation AlGaN/GaN HFET with a low-on state resistance and a high breakdown voltage operation at room temperature and 500 K. We developed our unique ohmic electrode using Ti/AlSi/Mo. In addition, we investigated the dependence between the distance from the gate electrode to the drain electrode and the off-state breakdown voltage. As a result, the breakdown voltage of a unit HFET was over 1100 V. Furthermore, on the large scale HFET with the gate width of 240 mm, the maximum drain current of over 50 A was obtained at room temperature and also, that of over 25 A was obtained at 500 K. The off-state breakdown voltage was obtained about 800 V at room temperature and about 600 V at 500 K, although Si-based FETs can not operate in such a high temperature.