Inherently n-type conductivity of ZnO has been exploited for sensing applications. The high sensitivity of ZnO, towards the exposure to NH3, H2, O3, CO, NO2, and ethanol etc., makes it viable for gas sensing applications. Similarly, the same sensors should respond to any oxidizing or reducing chemical environment when it is placed in a solution or exposed to a vapour or jet containing chemical analytes. We report AlGaN/GaN based HEMT chemical sensors which show a reasonably fast response on exposure to chemical analyte in gas or solution phase. State-of-the-art GaN-HEMT process was used to fabricate bare-gated FET-like sensor structures, which was later covered by a dense layer of nanorods. While the most of the reported studies on similar topics exploit the conductance modulation in ZnO nanostructures, this work exploits surface and interface effects at the ZnO/GaN heterointerface coupled with signal modulation capabilities of HEMT structure. A two step growth process was used to grow ZnO nanorod arrays using a solution method that resulted in a matrix of nanorods over the entire exposed gate of proposed sensor structure. Typically, nanorods posses’ micrometer-scale length and submicrometer-scale width, and act as integrated nanoscale gate arrays of the GaN HEMT. Hall measurements revealed a noticeable decay of charge density in the AlGaN/GaN heterostructure after the nanorod growth. Low-frequency noise measurements revealed the role of interface traps at the ZnO/GaN heterointerface. Analytical techniques indicated a high crystalline quality of the nanorods. A dense array of nanorods over the gate area played important role in the presented devices by means of facilitating quick absorption of chemical analytes, and in a much larger volume in comparison to a control device with no ZnO decoration on the gate area. A very large surface-to-volume ratio offered by the ZnO nanorod arrays could facilitate quick adsorption of large volume of chemical analyte. Sensor devices were installed in a chamber where gasses could be introduced in a controlled environment. Electrical response of the devices was recorded by means of acquiring current-voltage data at a fixed temperature and without any gate-bias. A device with dimensions identical to test devices and without any surface modification was used as control. Sensor response to Hydrogen gas was recorded at different temperatures. Further, transient response of the sensor was also recorded when it was exposed to human breath and it showed a significant and quick response. Sensing results suggest the potential of studied structure for chemical sensing. This work was supported by the City University of Hong Kong Strategic Research Grant (Project No. 7008102).