A low leakage current switch device (110) is provided which includes a GaN-on-Si substrate (11, 13) with one or more device mesas (41) in which isolation regions (92, 93) are formed using an implant mask (81) to implant ions (91) into an upper portion of the mesa sidewalls and the peripheral region around each elevated surface of the mesa structures exposed by the implant mask, thereby preventing the subsequently formed gate electrode (111) from contacting the peripheral edge and sidewalls of the mesa structures.
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