A gallium nitride (GaN) substrate, a semiconductor device, and methods for fabricating a GaN substrate and a semiconductor device are provided. The GaN substrate includes: a GaN base; an aluminum gallium nitride (AlGaN) layer, disposed on the GaN base; and a p-type conducting layer disposed on an active area of the AlGaN layer, and used to exhaust surface state negative electrons on the AlGaN layer and neutralize a dangling bond on the AlGaN layer. The p-type conducting layer is formed on the AlGaN layer, and a hole charge carrier in the p-type conducting layer can be used to exhaust the surface state negative electrons on an n-type AlGaN layer, neutralize the dangling bond on a section of the AlGaN layer, and prevent the forming of a virtual gate, so as to suppress a current collapse effect of the semiconductor device fabricated using the GaN substrate.