Enhancing the sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensors by controlling the threshold voltage

The threshold voltage (V-T) of the AlGaN/GaN HEMT based pH sensor was adjusted by the method of the photoelectrochemical (PEC) oxidation on the GaN cap layer surface. After the PEC oxidation treatments, the V-T of the device shifted from -3.46 V to -1.15 V and the gate voltage (V-G) corresponding to the maximum transconductance (g(mMAx)) position (V-G vertical bar g(mMAX)) of the device shifted from -2.6 V to -0.1 V. The drain current (I-D) variation per pH of the AlGaN/GaN HEMT based pH sensor without reference electrode increased from 0.7 mu A to 14 mu A when the drain voltage (V-D) was 0.5 V. The sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensor can be significantly increased by regulating the V-T to make V-G vertical bar g(mMAX) approached the equivalent V-G when liquid droplet on the sensing window surface (V-G-(EQU)), which is beneficial to the miniaturization and integration of the AlGaN/GaN HEMT based sensors in the future.