A compound semiconductor device is manufactured by forming an III-nitride compound semiconductor device structure on a silicon-containing semiconductor substrate, the III-nitride compound semiconductor device structure including a GaN alloy on GaN and a channel region arising near an interface between the GaN alloy and the GaN. One or more silicon-containing insulating layers are formed on a surface of the III-nitride compound semiconductor device structure adjacent the GaN alloy, and a contact opening is formed which extends through the one or more silicon-containing insulating layers to at least the GaN alloy. A region of GaN is regrown in the contact opening, and the regrown region of GaN is doped exclusively with Si out-diffused from the one or more silicon-containing insulating layers to form an ohmic contact which is doped only with the Si out-diffused from the one or more silicon-containing insulating layers.