Carbon monoxide detection down to ppb-level realized by O2 plasma treated TiO2-gated AlGaN/GaN HEMT sensor

The AlGaN/GaN high electron mobility transistor (HEMT) based sensors with O plasma treated TiO film as the gate electrode were fabricated. The ultrafast ppb-level carbon monoxide detection was achieved under dry air atmosphere. It is revealed that the O plasma treatment on TiO film significantly increases the effective work function of TiO film due to the strong dipole layer formation which is induced by the adsorption of O bonding states. As a result, the HEMT drain current observably decreases. Thus, when CO gas is introduced, the reduction reaction between O bonding states and CO molecules weakens the strength of the formed dipole layer, resulting in the decrease of the effective work function of TiO electrode and the significant increase of HEMT conduction current. It is demonstrated that with O plasma treatment the gas sensor based on TiO gated AlGaN/GaN HEMT can detect CO concentration down to 10 ppb with a response time of ~100 s and 0.15 mA conduction current variation. Furthermore, the ultrafast response time of ~4 s for ≥ 500 ppb CO, high resolution of CO concentration, and long-term stability are also demonstrated.