High speed high power nitride semiconductor device

A method for manufacturing a nitride semiconductor device comprising the steps of:

• (a) epitaxially growing a semiconductor lamination layer including a nitride semiconductor layer, on a substrate;
• (b) forming a source electrode and a drain electrode respectively in ohmic contact with said semiconductor lamination layer;
• (c) forming an insulating protective layer on said semiconductor lamination layer;
• (d) etching said insulating protective layer by using a resist mask to form an opening portion, the insulating protective layer has a steeply raising sidewall at an opening edge to a partial thickness of a total thickness of said insulating protective film and reaching a total thickness portion via a transition portion which has monotonously increasing thickness with a distance from the opening edge;
• and
• (e) forming a gate electrode contacting said semiconductor lamination layer in said opening and extending on said insulating protective layer to portions of increased thickness thicker than said partial thickness.