AlGaN/GaN high electron mobility transistors (HEMTs) have more remarkable properties in application of microwave transistors for high power and high frequency. A less widely studied application is high sensitivity to detect a wide range concentration of glucose. In this work, a photo-electrochemically treated open-gate AlGaN/GaN HEMT biosensor for glucose detection was developed. Through photo-electrochemical treatment, a smooth and thin gallium oxide can be formed on the sensing region. The threshold voltage was changed from -3.3 V to -1.3 V at a swept gain voltage. And a maximum value of transconductance was obtained at the gate voltage of 0 V. Effective functionalization of 3- aminopropyltriethoxysilane (APTES) and immobilization of glucose oxidase (GOx) can be realized on the oxidized sensing region. The proposed sensor exhibited good current response to glucose concentration over a wide linear range with high sensitivity above 8.61 × 105 μA/mM·cm2. The performance of the fabricated biosensor demonstrates the possibility of using AlGaN/GaN HEMTs for high sensitivity glucose detection in biochemical application.