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In this paper, we propose to use a lateral metal-semiconductor-metal (MSM) structure with a thick amorphous selenium (a-Se) layer intended for direct-conversion X-ray detection. For the purposes of demonstration, a variety of single-pixel detectors with electrode spacing ranging from 2 to 10 μm were fabricated and characterized. Compared with the vertical structure, the MSM structure avoids the usage of high voltage, therefore eliminating a safety concern. However, the simulation results indicate that the electric field in such a structure is not uniformly distributed and only confined into a region near the bottom electrodes up to a thickness of ~20 μm. The charge collection is therefore undertaken in the bottom layer and the top layer where a majority of energy deposits instead plays a dominant role in charge generation and diffusion. We believe that the lateral MSM detector with thick a-Se will be feasible for direct-conversion X-ray detection.