Semiconductor device

A semiconductor device comprising:

• a substrate;
• a first nitride semiconductor layer provided above the substrate, the first nitride semiconductor layer having a first lattice period in a first direction parallel to a substrate plane, the first nitride semiconductor layer including nitrogen and aluminum;
• a second nitride semiconductor layer provided between the substrate and the first nitride semiconductor layer, at least a portion of the second nitride semiconductor layer having a second lattice period in the first direction parallel to the substrate plane, the second lattice period being three times the first lattice period, the second nitride semiconductor layer including nitrogen and aluminum;
• a third nitride semiconductor layer provided above the first nitride semiconductor layer;
• a fourth nitride semiconductor layer provided on the third nitride semiconductor layer, the fourth nitride semiconductor layer having a larger bandgap than the third nitride semiconductor layer;
• at least one main electrode provided on the fourth nitride semiconductor layer;
• and
• a control electrode provided above the third nitride semiconductor layer, the control electrode being configured to control a current of the semiconductor device.