NITRIDE SEMICONDUCTOR SUBSTRATE AND NITRIDE SEMICONDUCTOR DEVICE

Provided is a nitride semiconductor structure capable of preventing deterioration of transistor characteristics caused by diffusion of a P-type conductive element by using an extremely simple configuration, instead of introducing a diffusion suppression layer. A nitride semiconductor substrate comprises at least a layered structure made of group 13 nitride semiconductors, wherein a first layer, a second layer having a wider band gap than the first layer, and a third layer containing a P-type conductive impurity at a concentration of 5E+18 atoms/cc or more are stacked in this order in the layered structure, and a maximum concentration of P-type conductive impurity in the first layer is 10% or less of the concentration of P-type conductive impurity in the third layer.