Abstract: Semi-insulating gallium arsenide wafers manufactured with varying silicon nsity shallow donors are copper compensated by heating to temperature of at least 550.degree. C. to thermally diffuse the copper into the wafers and thereby provide deep copper acceptors in the wafer. Higher annealing temperatures are employed for higher concentrations of silicon in the wafers and the thermal diffusion is accomplished in the presence of copper, and in some instances, in the presence of varying quantities of arsenic. The copper compensated, silicon doped, gallium arsenide wafers obtained have the electrical property characteristic capability of being used as photoconductive switching components. In one aspect of the invention the silicon doped gallium arsenide wafer is sealed in a quartz ampoule in the presence of solid copper and solid arsenic and heated to the annealing temperature.