Semiconductor device

A semiconductor device, comprising:

• a first electrode provided on a semiconductor multilayer structure;
• a second electrode provided on a substrate;
• and
• a bonding metal layer that bonds the first electrode and the second electrode together, wherein
• the bonding metal layer includes a gap inside,
• the gap is embedded in the bonding metal layer without contacting the first electrode and the second electrode,
• the gap has a shape which includes an angular portion including a corner portion,
• the bonding metal layer is contact with the first electrode,
• the bonding metal layer includes a first layer having a first average crystal grain size and a second layer having a second average crystal grain size, the second layer being located opposite to the first layer,
• the first average crystal grain size is larger than the second average crystal grain size, and
• the gap exists in the second layer.