oa Improved Gas and Water Pollution Sensors Based on AlGaN/GaN HEMTs for Air and Water Treatment Systems

At the present time, serious threats are being posed to the environment due to pollutants in both the air (e.g. automotive exhaust emissions) and water (e.g. phosphates, nitrates, heavy metals). In an effort to improve these environmental hazards, automotive emissions standards and regulations on water pollutants are becoming more and more strict each year. One of the major limitations in meeting these regulations is the ability to quickly and accurately detect low concentrations of the target molecules.

We report the fabrication and experimental testing of a new generation of microsensor technology that can detect a wide range of gases in the air as well as pollutants in a liquid environment. These sensors are based on AlGaN/GaN high electron mobility transistors (HEMT) with functionalized gate contacts acting as the sensing layer. The use of high bandgap, group-III nitride semiconductor materials gives the sensors a high thermal and chemical stability, making them suitable for high temperature applications (above 600°C) and in harsh chemical conditions. Depending on the type and structure of functional layer, the adsorption of the target molecules leads to change the surface depletion layer which has a direct affect on the sensitivity and the selectivity of the device. Extensive modeling and design of the AlGaN/GaN HEMTs has been performed to optimize the devices for detection of exhaust gases and water pollutants. This includes the design of each of the material parameters (e.g. AlGaN thickness, Al incorporation) and the functionalized contacts (e.g. material, dimensions, morphology).

Experimental results using a Pt sensing layer show detection of 10–1000 ppm NO and NO2 gas with changes in current of 0.8 and 2.8 mA, respectively. Detection of NH3 gas in a range of 150 ppb–15 ppm was also demonstrated, and can be compared to detection limits of only 35 ppm for similar devices. This is the first time that detection of NO has been demonstrated using a HEMT sensor, and the first time selective detection of NO, NO2, and NH3 has been demonstrated using a single transistor device. Furthermore, the sensitivity to NO2 and NH3 were significantly higher in our optimized device compared to other HEMT sensors. Dynamic response times were shown to be between 1–3 seconds for each gas, showing that these sensors can perform in real-time applications.

In addition to exhaust gas detection, an open-gate HEMT sensor was designed and fabricated as a water pollution sensor and showed detection of both phosphates and nitrates in water. This is the first time that phosphates have been detected using a HEMT sensor, and shows that HEMT sensors can be useful in the prevention of eutrophication of fresh water bodies. We are currently exploring HEMT sensor designs for the detection other water pollutants such as heavy metals, inorganic and organic molecules that damage or contaminate water supplies.

The experimental results presented in this abstract demonstrate that the functionalized AlGaN/GaN HEMT sensors we have developed are promising for real-time air and water treatment applications due to their low detection limits, high sensitivities, fast response times, and selectivity by functionalizing the sensing layer for specific target molecules. Therefore, this technology has the potential to drastically improve the reduction of pollution in the air caused by automotive exhaust and other gases, as well as pollution in lakes and other viable sources of water.