Semiconductor device and method of manufacturing semiconductor device

A semiconductor device comprising:

• a first nitride semiconductor layer;
• a second nitride semiconductor layer provided above the first nitride semiconductor layer and forming a two-dimensional electron gas between the first nitride semiconductor layer and thereof;
• a source electrode provided above the second nitride semiconductor layer and electrically connected to the two-dimensional electron gas;
• a drain electrode provided above the second nitride semiconductor layer and electrically connected to the two-dimensional electron gas;
• a gate electrode provided above the second nitride semiconductor layer and arranged between the source electrode and the drain electrode;
• a protective film provided above the second nitride semiconductor layer and arranged between the gate electrode and the drain electrode;
• a first oxide layer provided above the second nitride semiconductor layer and arranged only between the gate electrode and the source electrode;
• and
• a second oxide layer provided above the first oxide layer, wherein,
• at a junction interface between the first oxide layer and the second oxide layer, an oxygen area density of the first oxide layer is lower than an oxygen area density of the second oxide layer.