Abstract: This work demonstrates the improvement of mass detection sensitivity and time response using a quite simple structure of sensor. Indeed, complicated technological processes are often required to reach high sensitivity when we want to detect specific molecules in biological fields. These developments constitute an obstacle to the early diagnosis of diseases. An alternative is the design of coupled structures. The device is based on the piezoelectric excitation and detection of two GaAs micro structures vibrating on antisymmetric modes. GaAs is a piezoelectric material which has the advantage to be micromachined easily using clean room processes. Moreover, we showed its high potential in direct biofunctionalisation to be used in biological field¹. A specific design of electrodes (three electrodes) was performed to improve the detection at low mass and an original detection method has been developed. The principle is to exploit the variation in amplitude at a fixed frequency when we are concerned by a weak frequency shift of the resonance peak. The three electrodes are geometrically identical. We excited the device at the resonance frequency, corresponding to maximum voltage of the initialization electrode. Thus we noted the voltage on the measuring electrode, which had, in the vicinity of weak added mass, the greatest slope. Therefore, we get a very good resolution for an infinitely weak mass: relative voltage variation of 8%/1fg. The analysis given in this paper is based on results obtained by finite element modeling.     ¹A. Bienaime et al, Materials 2013, 6, 4946-4966