In this paper, the mechanism of the open-gate AlGaN/GaN HEMT based sensors were discussed and the effect of the ratio of gate length (L<sub>G</sub>) to source-drain distance (L<sub>SD</sub>) on the transconductance (g<sub>m</sub>) of the sensors was investigated. It was shown that the smaller L<sub>G</sub>/L<sub>SD</sub> of the devices would get a higher maximum g<sub>m</sub> (g<sub>m-max</sub>). However, when the gate voltage (V<sub>G</sub>) increased to a certain extent, the g<sub>m</sub> of the larger L<sub>G</sub>/L<sub>SD</sub> devices would be higher. The experimental results were demonstrated by further theoretical calculation and analysis which is beneficial to enhance the sensitivity of the AlGaN/GaN HEMT based chemical sensors and biosensors by improving the g<sub>m</sub> of them.
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 × 10<sup>5</sup> μA/mM·cm<sup>2</sup>. The performance of the fabricated biosensor demonstrates the possibility of using AlGaN/GaN HEMTs for high sensitivity glucose detection in biochemical application.