This paper reports a novel acoustic sensor with a frequency output based on AlAs/InxGa1−xAs/GaAs resonant tunneling diode (RTD). The RTD is incorporated in a 1um thick membrane and the fabrication technology of the membrane is based upon the selective etch of GaAs with AlAs an etch stop layer. A relaxation oscillator is obtained with the RTD biased in the negative differential resistance (NDR) region. Acoustic pressure applied to the RTD changes the frequency of oscillation due to the shift in current–voltage characteristics. The main feature of this sensor type the direct frequency output, which is linearly dependent on pressure, and the linear sensitivity can be up to 21kHz/kPa.