A method of manufacturing a High-Electron-Mobility Transistor (HEMT) includes: preparing a substrate; forming a first buffer over the substrate; forming a second buffer over the first buffer, wherein forming the second buffer includes doping a first thickness of a material such as gallium nitride (GaN) with a first concentration of a dopant such as carbon, and doping a second thickness of the material with a second concentration of the dopant such that the second concentration of dopant has a gradient though the second thickness which progressively decreases in a direction away from the first thickness; forming a channel layer such as a GaN channel over the second buffer; forming a barrier layer such as aluminum gallium nitride (AlGaN) over the channel layer; and forming drain, source and gate terminals for the HEMT.

A GaN device with N.sub.2 pre-treatment is provided in the present invention, including a GaN substrate, an AlGaN layer covering the GaN substrate, a p-GaN gate on the AlGaN layer, a TiN electrode on the p-GaN gate, a first dielectric layer on the AlGaN layer surrounding the p-GaN gate, wherein a horizontal spacing is between the first dielectric layer and the p-GaN gate, and an interface between the AlGaN layer and the GaN substrate not covered by the first dielectric layer is subject to N.sub.2 pre-treatment, and a second dielectric layer covering on and directly contacting the exposed first dielectric layer, AlGaN layer, p-GaN gate and TiN electrode.

A method of manufacturing a High-Electron-Mobility Transistor (HEMT) includes: preparing a substrate; forming a first buffer over the substrate; forming a second buffer over the first buffer, wherein forming the second buffer includes doping a first thickness of a material such as gallium nitride (GaN) with a first concentration of a dopant such as carbon, and doping a second thickness of the material with a second concentration of the dopant such that the second concentration of dopant has a gradient though the second thickness which progressively decreases in a direction away from the first thickness; forming a channel layer such as a GaN channel over the second buffer; forming a barrier layer such as aluminum gallium nitride (AlGaN) over the channel layer; and forming drain, source and gate terminals for the HEMT.

A semiconductor device includes a semiconductor substrate, an Aluminum Gallium-Nitride (AlGaN) back-barrier layer formed above the semiconductor substrate, and a GaN channel layer formed on the AlGaN back-barrier layer. A two-dimensional hole gas (2DHG) is formed at an interface between the GaN channel layer and the AlGaN back-barrier layer, and a p-type doped region is formed above the semiconductor substrate and next to the GaN channel layer and the AlGaN back-barrier layer. The p-type doped region is configured to provide an ohmic contact for the 2DHG. The p-type doped region comprises Magnesium as a p-type dopant. The p-type doped region comprises one or more hollow chambers extending from the top face of the p-type doped region. The hollow chambers are configured to form an escape path for Hydrogen atoms which are formed during a dopant activation of the p-type doped region during fabrication of the semiconductor device.