NOVELTY - A field effect transistor (FET)-based chemical sensor (10) comprises a semiconductor material (preferably gallium nitride material) region (18) on a substrate (20); a source electrode (12), a drain electrode (14), and a gate electrode (16) formed on the semiconductor material region; and a sensing region (23), separated from the gate electrode, and capable of interacting with the chemical species to change a measurable property of the sensor. The sensing region comprises a sensing layer (24), and a surface region (25) of the semiconductor material region positioned under sensing layer.    USE - As a field effect transistor (FET)-based chemical sensor for detecting chemical species (claimed); useful in a wide variety of applications including engine emission monitoring, carbon dioxide detection and flue gas monitoring, and also used to detect a wide variety of chemical species including, but not limited to, carbon dioxide, carbon monoxide, hydrogen, oxygen, nitrogen, nitrous oxide, hydrocarbons, alcohols and ionic species.    ADVANTAGE - Because the sensing layer is separated from the gate electrode, the dimensions and design of the sensing layer (e.g., when functioning as a sensing electrode) are not subject to any gate electrode design requirements. This increases the ability to tailor the dimensions of the sensing layer (e.g., when functioning as a sensing electrode) to improve sensor performance. Thus, because of the added dimensional freedom, sensors have sensing layers that can be tailored to have improved sensor performance (i.e., sensitivity, responsivity, and recovery) compared to such conventional sensors. The sensing layer may be thinner than typically possible in conventional sensors in which the gate electrode also functions as a sensing electrode. Thin sensing electrodes permit relatively rapid diffusion of chemical species through the electrode to surface region which can improve sensitivity and responsivity of the sensor. Also, thin sensing layers enable rapid out diffusion of chemical species which may lead to improved sensor recovery times. The direct modification of the surface energy of the semiconductor material surface, particularly when the semiconductor material is a gallium nitride material, can lead to increased sensitivity, responsivity and recovery of the sensor. These improved properties arise because the surface potential of the semiconductor material (particularly, when the semiconductor material is a gallium nitride material) can be strongly dependent on the concentration of adsorbed chemical species and also the drain current can be strongly dependent on the surface potential of the semiconductor material region. Thus, adsorption of even a small concentration of chemical species can change the surface potential of the semiconductor region which causes measurable changes in drain current.    DETAILED DESCRIPTION - A field effect transistor (FET)-based chemical sensor (10) comprises a semiconductor material (preferably gallium nitride material) region (18) on a substrate (20); a source electrode (12) formed on the semiconductor material region; a drain electrode (14) formed on the semiconductor material region; a gate electrode (16) formed on the semiconductor material region; and a sensing region (23), separated from the gate electrode, and capable of interacting with the chemical species to change a measurable property of the chemical sensor. The sensing region comprises a sensing layer (24), and a surface region (25) of the semiconductor material region positioned under the sensing layer. The sensing layer has a thickness of less than 500 nm.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of a field effect transistor (FET)-based chemical sensor.    FET-based chemical sensor (10)    Source electrode (12)    drain electrode (14)    Gate electrode (16)    Semiconductor material region (18)    Sensing region (23)    Sensing layer (24)    Surface region. (25)