High-speed high-power semiconductor devices are disclosed. In an exemplary design, a high-speed high-power semiconductor device includes a source, a drain to provide an output signal, and an active gate to receive an input signal. The semiconductor device further includes at least one field gate located between the active gate and the drain, at least one shallow trench isolation (STI) strip formed transverse to the at least one field gate, and at least one drain active strip formed parallel to, and alternating with, the at least one STI strip. The semiconductor device may be modeled by a combination of an active FET and a MOS varactor. The active gate controls the active FET, and the at least one field gate controls the MOS varactor. The semiconductor device has a low on resistance and can handle a high voltage.