High-temperature sensing of low-concentration CO2 using La2O3-gated AlGaN/GaN HEMT sensor

This work presents a gallium nitride (GaN) heterojunction field-effect carbon dioxide (CO2) sensor utilizing lanthanum oxide (La2O3) as the gas sensitive material, which was prepared using RF magnetron sputtering 20 nm lanthanum (La) metal followed by rapid thermal annealing (RTA). The La2O3 film was patterned onto the gate region of pre-fabricated AlGaN/GaN high electron mobility transistor structures. This La2O3 gate GaN-based sensor demonstrated effective detection of CO2 in the concentration range of 1-500 ppm in air. The trends of response/recovery time and sensing response impacted by temperature and injected concentration were charaterized. The results showed the best response/recovery time as 12.9/6.4 min and the best sensing response of approximately 3 % for 500 ppm CO2 injection at 330 degrees C. To enhance the capability of the sensor in detecting CO2 at ultra-low concentrations down to 1 ppm, a process involving first-derivative transformation and low-pass filtering was implemented. Additionally, the sensing reaction process that involves O2 and transfer of electrons participating in the redox reaction on the La2O3 surface, was discussed. Finally, the transient response of the sensor to nitric oxide (NO) and ammonia (NH3) were also shown.