preparing a semiconductor wafer including a substrate, a buffer layer, a first crystalline layer, and a second layer, the substrate, the buffer layer, the first crystalline layer, and the second layer being positioned in order of the substrate, the buffer layer, the first crystalline layer and the second layer, the buffer layer and the first crystalline layer being made of a group III nitride layer, a bandgap of the first crystalline layer being smaller than a bandgap of the second layer, when the semiconductor wafer is formed as a transistor wafer, a channel of a transistor being formed at or near an interface between the first crystalline layer and the second layer;

providing a first electrode and a second electrode, electrically connected to the channel, to be closer to a front surface than the channel is and providing a third electrode, at which an electric field is applicable to a spatial region positioned between the channel and the substrate, to be closer to a back surface than the channel is;

applying negative voltage to the third electrode or applying positive voltage to the second electrode with the first electrode serving as a reference to achieve a space charge redistribution for emitting electrons and holes from a bandgap of a crystal positioned in the spatial region; and

determining that the semiconductor wafer has passed the inspection when an electron emission speed in space charge redistribution is higher than a hole emission speed, and determining that the semiconductor wafer has failed to pass the inspection when the electron emission speed in space charge redistribution is not higher than the hole emission speed.