Traveling-wave method of Ader et. al. and Fritzsche for determining the drift mobility and the sign of the majority carriers in low-mobility is analyzed. It is found that in homogeneous semiconductor films the acoustoelectric voltage originates from two surface currents which in turn are a consequence of the boundary conditions at film surfaces. The acoustoelectric effect vanishes in the bulk when diffusion of carriers is negligible in transport equation. We report the measurements of the drift mobility of n-type heavily doped grow-discharge amorphous semiconductors. The results agree well with time-of-flight measurements even though the latter was carried out on nearly intrinsic samples.
