We demonstrate a 20-mum-long tunable optical resonator integrated on a silicon-on-insulator waveguide. The microresonator consists of a planar Fabry-Perot microcavity defined by deep Si/SiO2 Bragg reflectors with a high finesse of 11.2. The device is electrically driven and shows a modulation depth as high as 53% for a power consumption of only 20 mW.