The use of Ag in the gate region of AlGaN/GaN heterostructure diodes is shown to provide stable, reversible changes in barrier height and thus current during exposure to ethanol at 250 • C. The exposed ethanol molecules are adsorbed on the silver and oxidized, resulting in the increase of Schottky barrier height. The detection limit of ethanol at this temperature was 58 ppm, and the sensor response was linear over the range 58-58700 ppm. For the high end of this concentration range (5.87%), ethanol exposure at 250 • C caused the Schottky barrier height to change from 0.604 eV to 0.656 eV, which produced a forward current relative change of 45.4% at 0.9V forward bias. There are many applications in which detection of the common aliphatic alcohol, ethanol, is needed, including the chemical, biomedical, and food industries, where the rate of ethanol production during fermentation needs to be monitored. 1 As an example, the change in alcohol content during fermentation of grapes is of great importance in the winemaking industry.2 Ethanol gas sensors can detect early spoilage of carbohydrate-rich foods, and these sensors are now employed in consumer electronics including refrigerators and mobile devices.1 Another common application is monitoring blood alcohol content related to consumption of alcoholic beverages. Most of the research into solid state sensors has focused on metal oxide semiconductors such as SnO 2 , Fe 2 O 3 , CuO, and ZnO.3-31 The attractive features of semiconductor-based sensors include that they can be operated with fast response at low power consumption and with compact size.
32-36The AlGaN/GaN heterostructure materials system is an attractive one for sensors since it allows robust high temperature operation in harsh environments. [37][38][39][40][41] Many types of GaN based devices have been reported for gas and chemical sensing, including Schottky diodes, metal oxide semiconductor (MOS) diodes, and AlGaN/GaN high electron mobility transistors (HEMTs). The specificity of the sensors can be tailored by the choice of the gate material, which might include antibody layers for biological species detection or catalytic metals for gas sensing. 42,43 In particular, the AlGaN/GaN HEMT structure with its two dimensional electron gas (2-DEG) channel induced by piezoelectric and spontaneous polarization at the interface between the AlGaN and GaN layers shows highly sensitive current changes to surface charges created by catalytic reaction of target gases on the specific active sensing layer. With 30% Al concentration in the AlGaN layer, 5∼10 times higher channel sheet electron densities are obtained compared to GaAs or InP HEMTs.In this study, AlGaN/GaN HEMT based Schottky diode ethanol sensors using silver as a sensing material were fabricated, and the response of the devices to ethanol gas was investigated as a function of temperature and ethanol concentration. The diodes showed a current reduction in ethanol-containing ambients due to the increase of Schottky barrier height. This process was most effici...