Abstract: A gas sensor includes first and second electrodes (107, 109) and a first semiconductor layer (113) comprising a semiconducting transition halide or pseudohalide, for example copper thiocyanate, in electrical contact with the first and second electrodes. The semiconducting transition metal halide or pseudohalide provide that a semiconductor based gas sensor is sensitive to alkenes and can detect low concentrations of alkenes. Furthermore, the gas sensor may comprise a second semiconductor layer (111), different from the first semiconductor layer. The second semiconductor layer is preferably an organic semiconductor. The gas sensor may be a top-gate or bottom-gate thin film transistor (103: gate electrode; 105: gate dielectric) or a horizontal or vertical chemiresistor. The gas sensor may be used for detection of alkenes, for example ethylene or 1-MCP.