In situations wherein the conductivity ?? is not constant but has a distribution, the layer conductivity ?? and the layer thickness W preferably satisfy a relationship given in equation (2) ??? 0 W ??? J 0 ??? ( W - x ) W ??? ??? ?? ??? ??? ( x ) ??? ??? x ??? ?? ??? ??? E ?? - L q ( 2 ) where ??E??-L is ??-valley/L-valley energy separation, q is an electron charge, and x is a position in the firs