To develop the high sensitive ion-sensitive field-effect-transistor (ISFET) urea biosensors, the AlGaN/GaN heterostructure with high electron sheet carrier concentration was utilized in this paper. Furthermore, the photoelectrochemical etching method and the oxidation method were, respectively, applied to recess the gate region and directly grow the GaxAlyO3 sensing membrane for enhancing sensing performances. Due to the progress of controlling ability from the gate-recessed region, the resulting characteristics of the gate-recessed AlGaN/GaN ISFET urea biosensors, including transconductance and sensing sensitivity, were obviously improved compared with the planar gate AlGaN/GaN ISFET urea biosensors. The maximum transconductance value and the urea sensitivity of the gate-recessed AlGaN/GaN ISFET urea biosensors were, respectively, improved to 75.9 mS/mm and 18.15 mA/pCurea compared with that of the planar gate ones. Compared with the planar gate structure, the maximum transconductance value and the urea sensitivity of the gate-recessed structure were improved 17.7% and 40.2%, respectively. Furthermore, the urea biosensors were, respectively, measured in the solution with various common interfering ions (glucose, K+, and Na+) to verify stability and reliability.
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