Abstract: A composition of matter comprising a bulk material of uniform composition having first and second spaced apart surface regions and a dopant in the bulk material of progressively increasing concentration in a direction from the first to said second surface regions providing an interface intermediate the first and second surface regions wherein the portion of the bulk material on one side of the interface is electrically conductive and the portion of the bulk material on the other side of the interface is relatively electrically insulative. The bulk material is one of Ge, Si, group II-VI compounds and group III-V compounds and preferably GaAs or GaP. The dopant is a shallow donor for the bulk material involved and for GaAs and GaP is Se, Te or S. The ratio of the resistivity of the portion of the bulk material on one side of the interface to the portion of the bulk material on the other side of the interface is at least about 1:10.sup.7.