A semiconductor substrate in includes a buffer layer and a first crystalline layer. A bandgap of the first crystalline layer is smaller than a bandgap of a second layer. When a semiconductor wafer is formed as a transistor wafer, a channel of a transistor is formed at or near an interface between the first crystalline layer and the second layer. With a first electrode and a second electrode provided and a third electrode provided, when space charge redistribution, for emitting electrons and holes from a bandgap of a crystal positioned in the spatial region, is achieved by applying negative voltage to the third electrode or by applying positive voltage to the second electrode with the first electrode serving as a reference, an electron emission speed in the space charge redistribution is higher than a hole emission speed.