Electric field management techniques in GaN based semiconductors that utilize patterned regions of differing conductivity under the active GaN device, such as a GaN high electron mobility transistor (HEMT), are described. As an example, a patterned layer of oxidized silicon can be formed superjacent a layer of silicon dioxide during or prior to the heteroepitaxy of GaN or another semiconductor material. These techniques can be useful for back-side electric field management because a silicon layer, for example, can be made conductive to act as a back-side field plate.