Semiconductor device

A semiconductor device comprising:

• a superlattice buffer layer formed on a substrate;
• a first semiconductor layer formed by a nitride semiconductor on said superlattice buffer layer;
• a second semiconductor layer formed by a nitride semiconductor on said first semiconductor layer;
• and
• a gate electrode, a source electrode and a drain electrode formed on said second semiconductor layer,
• wherein said superlattice buffer layer is formed by periodically laminating a third superlattice formation layer, a second superlattice formation layer and a first superlattice formation layer in that order from said substrate,
• said first superlattice formation layer is formed by AlxGa1-xN, said second superlattice formation layer is formed by AlyGa1-yN, and said third superlattice formation layer is formed by AlzGa1-zN, where a relationship x>y>z is satisfied, and
• a concentration of an impurity element serving as an acceptor doped into said second superlattice formation layer is higher than a concentration of the impurity element serving as an acceptor doped into said first superlattice formation layer and said third superlattice formation layer.

A semiconductor device comprising:

• a superlattice buffer layer formed on a substrate;
• a first semiconductor layer formed by a nitride semiconductor on said superlattice buffer layer;
• a second semiconductor layer formed by a nitride semiconductor on said first semiconductor layer;
• and
• a gate electrode a source electrode and a drain electrode formed on said second semiconductor layer,
• wherein said superlattice buffer layer is formed by periodically laminating a second superlattice formation layer, a first superlattice formation layer and a third superlattice formation layer in that order from said substrate,
• said first superlattice formation layer is formed by AlxGa1-xN and said second superlattice formation layer is formed by AlyGa1-yN, where a relationship x>y is satisfied,
• said third superlattice formation layer is formed by a material containing InGaN, and
• a concentration of an impurity element serving as an acceptor doped into said second superlattice formation layer is higher than a concentration of the impurity element serving as an acceptor doped into said first superlattice formation layer and said third superlattice formation layer.