A tunable breakdown voltage RF MESFET and/or MOSFET and methods of manufacture are disclosed. The method includes forming a first line and a second line on an underlying gate dielectric material. The second line has a width tuned to a breakdown voltage. The method further includes forming sidewall spacers on sidewalls of the first and second line such that the space between first and second line is pinched-off by the dielectric spacers. The method further includes forming source and drain regions adjacent outer edges of the first line and the second line, and removing at least the second line to form an opening between the sidewall spacers of the second line and to expose the underlying gate dielectric material. The method further includes depositing a layer of material on the underlying gate dielectric material within the opening, and forming contacts to a gate structure and the source and drain regions.