It is necessary to detect promptly amounts of carbon dioxide for the safety of workplace and the health of workers because concentrations of 7 to 10% may cause an acute symptom such as unconsciousness even in the presence of high concentration of oxygen. It is colorless and odorless. Also, its density is higher than common air, so that it is easily accumulated on the ground in the enclosed construction. Carbon dioxide is used in chemical, biomedical, energy and food industries as a reactant, an intermediate, and a final product. Recently, there are growing market-driven demands for the carbon dioxide gas sensors to detect early risk of high concentration of carbon dioxide exposure and to monitor quality of air in daily lives. In this applications, low power consumption, compact size, as well as reliability and sensitivity of the sensor are required to be integrated with IoT (internet of things) devices. Semiconductor type gas sensors satisfy these requirements, and GaN based gas sensors shows many superior performances including reliability and sensitivity to other semiconductors.

ZnO is a direct bandgap II-V compound semiconducting material with wide bandgap energy of 3.37 eV and large exciton binding energy of 60 meV. One of the advantages of ZnO is facile nanorod growth via hydrothermal method, which is low cost, non-toxic, low temperature, and scalable process. In hydrothermal growth, growth rate of c-plane is faster than m-plane in wurtzite crystal structure due to the higher surface energy of c-plane, hence ZnO nanowires or or bundle of nanorods along c-axis are generally formed. These ZnO nanorods with high surface to volume ratio can be employed in the gas sensor device as a sensing material to obtain high sensitivity for target gas.

In this study, AlGaN/GaN HEMT (high electron mobility transistor) based carbon dioxide sensor using ZnO nanorods as sensing material was fabricated, and the response of the device to carbon dioxide gas was investigated. The device showed current increase in carbon dioxide ambient due to the increase enhanced channel conductivity from 150^oC, and fast reliable repeatability to cyclic exposures of various concentrations of carbon dioxide gas. Also, effect of humidity on sensing behavior was discussed.