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Abstract Vapor sensors with both high sensitivity and broad detection range are technically challenging yet highly desirable for widespread chemical sensing applications in diverse environments. Generally, an increased surface‐to‐volume ratio can effectively enhance the sensitivity to low concentrations, but often with the trade‐off of a constrained sensing range. Here, an approach is demonstrated for NH₃sensor arrays with an unprecedentedly broad sensing range by introducing controllable steps on the surface of an n‐type single crystal. Step edges, serving as adsorption sites with electron‐deficient properties, are well‐defined, discrete, and electronically active. NH₃molecules selectively adsorb at the step edges and nearly eliminate known trap‐like character, which is demonstrated by surface potential imaging. Consequently, the strategy can significantly boost the sensitivity of two‐terminal NH₃resistance sensors on thin crystals with a few steps while simultaneously enhancing the tolerance on thick crystals with dense steps. Incorporation of these crystals into parallel sensor arrays results in ppb–to–% level detection range and a convenient linear relation between sheet conductance and semi‐log NH₃concentration, allowing for the precise localization of vapor leakage. In general, the results suggest new opportunities for defect engineering of organic semiconductor crystal surfaces for purposeful vapor or chemical sensing.