Field effect transistor and semiconductor device

A field effect transistor comprising:

• a substrate including a main surface and a back surface;
• a semiconductor region on the main surface, the semiconductor region including a first inactive region, an active region, and a second inactive region arranged side by side in a direction;
• a gate electrode, a source electrode, and a drain electrode on the active region;
• a gate pad on the first inactive region and electrically connected to the gate electrode;
• a gate guard on and in contact with the semiconductor region, the gate guard being apart from the gate pad and located between the gate pad and an edge on the first inactive region side of a pair of edges of the semiconductor region arranged side by side in the direction;
• a drain pad on the second inactive region and electrically connected to the drain electrode;
• a drain guard on and in contact with the semiconductor region, the drain guard being apart from the drain pad and located between the drain pad and an edge on the second inactive region side of the pair of edges of the semiconductor region;
• and
• a metal film on the back surface and electrically connected to the gate guard,
• wherein the drain guard is in a non-conductive state with respect to the metal film, the gate electrode, the source electrode and the drain electrode.