Sensor device 10 for and method of sensing ammonia (NH3) in a gas sample. The device comprising a sensing element with a first region 14 comprising a p-type metal oxide semiconductor (MOS) material, and second region 12 comprising an n-type MOS material, the regions being adjacent and in contact. The p-type MOS material may comprise or may consist of NiO and the n-type MOS may comprise or consist of In2O3. The device may comprise first and second electrodes 17a, 17b, within the first and second regions respectively and wiring interconnecting the electrodes, where a measured resistance along the wiring is indicative of NH3 in a gas interfacing with the sensing element. Also claimed is a system for sensing NH3 in a breath sample from a patient, comprising the sensor device, a mouthpiece, and a controller for assigning a score for the progression of a H. pylori infection based on an estimated concentration of NH3 in the sample.
