AlGaN/GaN HEMT based gas sensor characterisation and measurement

Abstract

In past decades, the need for gas sensors has risen as the detection of hazardous gases in the environment is imperative for the safety of human health. Superior properties of GaN such as ability to withstand harsh and corrosive environment, chemical and mechanical stability, high electron mobility, good thermal stability and high sheet carrier concentration in the order of 1013 cm-2 or higher, in two-dimensional electron gas (2DEG) channel at AlGaN/GaN interface is highly beneficial for sensing applications. 2DEG channel is formed due to the spontaneous polarization and piezoelectric polarization and is extremely sensitive to any surface changes on the device. The variation of concentration of free carrier charge in 2DEG channel caused by changes in environmental condition inducing overall resistance change is the principle behind AlGaN/GaN based gas sensing. In this report, the epilayer of AlGaN/GaN epistructure was grown on Si substrate by MOCVD. Hall measurement was used to measure the 2DEG layer properties such as sheet carrier density of 1.27x1013cm-2 and electron mobility of 1286cm2/V.s. Gas sensor device was fabricated on the epistructure whose steps are briefly explained in this report. Pt was used as the functionalization layer for improved sensing of NO2 and NH3. Fabricated device was tested for NO2 first at various temperature range from room temperature to 300°C at different concentration. It was found that 275°C was the optimum temperature for sensing NO2 as it gives the best recovery on device after NO2 exposure. Sensitivity of 4.4% for 2 ppm NO2 at 275°C is reported in this project. Subsequently, device was tested on NH3 under same conditions. Device showed a sensitivity of 0.3% for 50 ppm of NH3 at room temperature.