In general, theoretical determination of attenuation of ultrasonic waves caused by acousto-electric interaction in any structure is cumbersome and difficult. However, by adopting a transmission line model and using the technique of capacitance perturbation, the analytical process can be greatly simplified. In this paper, we start to analyze a simple structure of an infinitesimally thin semiconductor film deposited on a SAW substrate without an isolation layer in-between. Results for a simplified case are obtained. This analysis is then extended to the general layer structure for both an arbitrary thickness semiconductor layer and an isolation layer. The device parameters are chosen such that the charge relaxation time is much smaller than the duration of the wavelength. Therefore, the electric potential accompanying the acoustic wave is thoroughly screened. Thus, the induced electric field beyond a Debye length or at the top of the semiconductor is negligible. Under this condition, the induced potential as well as the space charge density waves possess a simple form. By integrating the space charge across the thickness of the semiconductor, an effective surface charge density is obtained. With the surface charge density known, one can readily calculate the value of the perturbed capacitance and determine the attenuation constant. The results are in agreement with other analyses.