The use of expensive catalysts (e.g. platinum) and high operation temperature ( > 300 degrees C) has plagued the cost-effectiveness and thereby the commercialization of III-Nitride semiconductors based gas sensing technology. Inadequate research on the development of catalyst-free room-temperature CO sensing using GaN based structures is the critical reason behind the subjugation of this area. Therefore, in the present article, we aimed the development of GaN & AlGaN/GaN heterostructure based gas sensors for catalyst-free low-temperature CO sensing (at 100 ppm). To explore the underlying science behind such mechanism, the morphological, electronic and electrical properties of the devices were thoroughly investigated. The analysis revealed that CO sensing on GaN (and AlGaN/GaN heterostructure) is governed via the chemical nature of ambient-oxidation induced amorphous oxide layer (O-2(-), O2- or OH(- )species), which acts as donor/acceptor state at the surface. Besides, the critical device parameters like Schottky barrier height and electron accumulation associated with series resistance and leakage current (forward/reverse) displayed significant variation with temperature (27-250 degrees C) and perturbed the effective carrier transport/collection and ultimately the device efficiency. The study demonstrates that nanostructured surfaces can open avenues for the development of catalyst-free room temperature operating III-Nitride semiconductor based CO sensors.