Field-effect transistor and method for designing same

A field-effect transistor includes a Ga2O3-based semiconductor layer, a source region and a drain region that are formed inside the Ga2O3-based semiconductor layer, a gate electrode that is formed, via a gate insulating film, on a channel region as the Ga2O3-based semiconductor layer between the source region and the drain region, a source electrode connected to the source region, and a drain electrode connected to the drain region. An interface charge including a negative charge is formed between the gate electrode and the channel region, and a gate threshold voltage is not less than 4.5V.