A nitride semiconductor device is provide, the nitride semiconductor device including: an epitaxial layer; and an ion implantation layer that is provided on the epitaxial layer over a continuous depth range that extends over 100 nm or longer, and has a P type doping concentration equal to or higher than 110.sup.17 cm.sup.3, wherein the ion implantation layer has a region with a crystal defect density equal to or lower than 110.sup.16 cm.sup.3, the region being located in a range which is on an upper-surface-side of an interface between the epitaxial layer and the ion implantation layer, and is within 100 nm from the interface.

A method for forming a transistor device includes: implanting dopant atoms of a first doping type and dopant atoms of a second doping type into opposite sidewalls of each of a plurality of trenches of a first semiconductor layer having a basic doping of the first doping type, the dopant atoms of the first doping type having a smaller diffusion coefficient than the dopant atoms of the second doping type; filling each trench with a second semiconductor layer of the first doping type; and diffusing the dopant atoms of the first doping type and the dopant atoms of the second doping type such that a plurality of first regions of the first doping type and a plurality of second regions of the second doping type are formed. The second regions are spaced apart from each other. Each first region is at least partially arranged within a respective second region.

A vertical semiconductor apparatus includes: a gallium nitride substrate; a gallium nitride semiconductor layer on the gallium nitride substrate; a p-type impurity region in the gallium nitride semiconductor layer and having an element to function as an acceptor for gallium nitride; an n-type impurity region in the p-type impurity region and having an element to function as a donor for gallium nitride; and an electrode provided contacting a rear surface of the gallium nitride substrate. The element to function as the donor in the n-type impurity region includes: a first impurity element to enter sites of gallium atoms in the gallium nitride semiconductor layer; and a second impurity element different from the first impurity element and to enter sites of nitrogen atoms in the gallium nitride semiconductor layer. In the n-type impurity region, a concentration of the first impurity element is higher than that of the second impurity element.

A group III nitride semiconductor substrate may include: a p-type conduction region into which a group II element has been implanted in a depth direction of the group III nitride semiconductor substrate from a surface of the group III nitride semiconductor substrate, the p-type conduction region having p-type conductivity, wherein hydrogen has been implanted from the p-type conduction region across an n-type conduction region adjacent to the p-type conduction region in the depth direction of the group III nitride semiconductor substrate.

A vertical semiconductor apparatus includes: a gallium nitride substrate; a gallium nitride semiconductor layer on the gallium nitride substrate; a p-type impurity region in the gallium nitride semiconductor layer and having an element to function as an acceptor for gallium nitride; an n-type impurity region in the p-type impurity region and having an element to function as a donor for gallium nitride; and an electrode provided contacting a rear surface of the gallium nitride substrate. The element to function as the donor in the n-type impurity region includes: a first impurity element to enter sites of gallium atoms in the gallium nitride semiconductor layer; and a second impurity element different from the first impurity element and to enter sites of nitrogen atoms in the gallium nitride semiconductor layer. In the n-type impurity region, a concentration of the first impurity element is higher than that of the second impurity element.

A method of obtaining a doped semiconductor layer, including the successive steps of: a) performing, in a first single-crystal layer made of a semiconductor alloy of at least a first element A1 and a second element A2, an ion implantation of a first element B which is a dopant for the alloy and of a second element C which is not a dopant for the alloy, to make an upper portion of the first layer amorphous and to preserve the crystal structure of a lower portion of the first layer; and b) performing a solid phase recrystallization anneal of the upper portion of the first layer, resulting in transforming the upper portion of the first layer into a doped single-crystal layer of the alloy.