Early work in acoustic charge transport (ACT) and heterostructure acoustic charge transport (HACT) devices employed simple delay lines in which a single input electrode and a single output electrode were set for some desired time delay.
More complex electrode configurations for more advanced signal processing were contemplated but the art was aware of potential drawbacks. Even though there is no physical transport of electrons from the charge transport layer into the sensing electrodes, there is still potential for modification of the SAW itself and consequent adverse impact on the signals being carried by the electron packets. In particular, a spacing of electrodes at one SAW wavelength would be analogous to a Bragg diffraction grating used in optical devices. The art would expect a considerable amount of reflection of the SAW and other perturbations of the SAW and the electron packets. It would be expected that integral multiples of the SAW wavelength would also give rise to relatively high reflectivity.
Prior work employed electrodes spaced a considerable distance from the surface, which reduced electron perturbation, but decreased the signal coupling to the tap electrodes and decreased the frequency response.
The need for ground electrodes alternating with signal electrodes also was not appreciated by the art.