Semiconductor device

A semiconductor device, comprising:

• a first semiconductor region;
• a second semiconductor region on the first semiconductor region;
• an active region between the second semiconductor region and the first semiconductor region;
• and
• a first electrode on the first semiconductor region;
• wherein first semiconductor region comprises a first layer and a second layer between the first layer and the active region, the second layer comprises a first portion and a second portion adjacent to the first portion, the first portion is overlapped with the active region in a stacking direction and has a first thickness, the second portion has a second thickness less than the first thickness, the first layer comprises a first material and a first dopant, the first material comprises multiple elements, the first dopant has a first concentration, the second layer comprises a second material and a second dopant, the second material comprises multiple elements, the second dopant has a second concentration, one of the elements of the first material of the first layer is different from the multiple elements of the second material of the second layer, the first concentration of the first dopant of the first layer is greater than the second concentration of the second dopant of the second layer, and the first electrode is on the second portion of the second layer.