We discuss arrays of metal-semiconductor-metal cavities as electrically tunable terahertz metasurfaces. The operation of the considered device is based on reverse biasing the Schottky junction formed between top metal strips and the n-type semiconductor buried beneath. The effective Drude permittivity of the cavity array is tuned by changing the depletion layer thickness via a gate bias applied between the strips and a back metal reflector. Combining Maxwell equations for terahertz waves with a drift-diffusion model for the semiconductor carriers into a multiphysics framework, we show that the proposed modulation concept is promising for a large part of the terahertz spectrum.