High-electronic-quality thin films of mercuric iodide have been deposited on SnO2 and other transparent contacts. The crystallites exhibit some preference for c-axis orientation and have lengths comparable to the overall film thickness of 200-400 mum. With visible photons of 584 nm, quantum efficiencies up to 0.5 are observed at an applied voltage of 50 V. AMPs((C)), a first-principles simulation code, is used to model device performance. Characterization is complicated by complex electric field profiles. Using representative values for fundamental parameters, current-voltage and quantum efficiency spectral response plots are generated that are in agreement with experimental results from single-crystal reference devices. Comparisons with thin-film devices suggest that relatively small reductions in the values of carrier transport parameters result from the thin-film growth process.