The performances of a digital optical switch, based on the thermo-optic effect in amorphous silicon, are investigated for the first time to our knowledge. Numerical simulations shows that the strong thermo-optic effect of amorphous silicon combined with the possibility of realizing micrometric integrated structures, allow the realization of efficient waveguide devices capable of microsecond switching times. The switch, operating at the infrared communications wavelength of 1550 nm, can be easily integrated in silicon optoelectronic circuits and is thought for low-cost photonic applications.