Devices and methods for enhancing insertion loss performance of an antenna switch are disclosed. In one example, a semiconductor device formed to serve as an antenna switch is disclosed. The semiconductor device includes: a substrate, a dielectric layer and a polysilicon region. The substrate includes: an intrinsic substrate; a metal-oxide-semiconductor device extending into the intrinsic substrate; and at least one isolation feature extending into and in contact with the intrinsic substrate. The at least one isolation feature is disposed adjacent to the metal-oxide-semiconductor device.