In this study, a theoretical relationship between two-dimensional electron gas (2DEG) properties of AlGaN/GaN high-electron-mobility transistor (HEMT) hetero- structure and various gas-sensing characteristics using this structure was established. Based on the analytical study, it is proposed that a thinner barrier layer in AlGaN/GaN HEMT heterostructure should result in lower detection limits, higher sensing response, and a faster response time. To prove the analytical study, a thin-barrier (~8 nm) AlGaN/GaN HEMT structure was designed and grown to study its sensing characteristics. The results are compared with the thick-barrier (~18 nm) gas sensor structure. For the thin-barrier NO2 sensor, the sensing response increased by almost ten times when was changed by more than 25%. The sensor also showed a faster response time of 10 s when compared to the thick barrier sensor. Similarly, the thin-barrier NH3 sensor showed a remarkable improvement in sensing response by 17 times and by two times. Based on the response time, the thin-barrier NH3 sensor was also found to be 40% more faster compared to the thick-barrier AlGaN/GaN HEMT sensor.