Nitride semiconductor device

A nitride semiconductor device comprising:

• a first semiconductor layer including a nitride semiconductor;
• a second semiconductor layer including a nitride semiconductor;
• a third semiconductor layer including a nitride semiconductor provided between the first semiconductor layer and the second semiconductor layer, the third semiconductor layer being contact with the first semiconductor layer and the second semiconductor layer, and a concentration of acceptor type impurity in the third semiconductor layer being higher than that in each of the first semiconductor layer and the second semiconductor layer;
• a source electrode and a drain electrode provided on the opposite side of the first semiconductor layer from the third semiconductor layer;
• agate electrode provided on the opposite side of the first semiconductor layer from the third semiconductor layer and provided between the source electrode and the drain electrode;
• a first insulating layer provided between the gate electrode and the first semiconductor layer;
• and
• a fourth semiconductor layer and a fifth semiconductor layer interposing the gate electrode therebetween, the fourth semiconductor layer is provided between the source electrode and the first semiconductor layer, the fifth semiconductor layer is provided between the drain electrode and the first semiconductor layer,
• wherein the nitride semiconductor device satisfying the following formula
• 0<d<2[(2∈Eg)/(qNa)]1/2
• when d is thickness of the third semiconductor layer, Na is concentration per unit volume of the acceptor type impurity in the third semiconductor layer, Eg is band gap energy of the third semiconductor layer, ∈ is permittivity of the third semiconductor layer and q is elementary charge.