In this study, we analyze a 3rd type of FBAR's fundamental mode by finite element method. We show the importance of the substrate's wave velocity and conductivity on the electromechanical coupling efficiency. Then we demonstrate that etching traditional SAW sensors in between its electrodes is an effective solution to increase its performances. Finally, the finite device simulations of optimized geometries demonstrate the full potential of AlSc(0.4)N based SAW/BAW hybrid resonators: an impedance response free of spurious modes and large figure of merit (k(t)(FD, 1.35GHz)(2) = 2.48%).