We present a new design of a mass sensor capable of detecting mass values in the zeptogram range. The sensor comprises a resonant beam and two gate electrodes used to oscillate the beam at its resonance frequency. The detection method is based on the tunnelling current between the vibrating beam and the gate electrodes. When a small mass is added to the beam, the latter sees a shift in its resonant frequency. The added mass thus leads to a decrease in the oscillation's amplitude and hence a drop in the tunnelling current.