AlGaN/GaN HEMT Based pH Detection Using Atomic Layer Deposition of Al2O3 as Sensing Membrane and Passivation

Herein, we report the design and implementation of planar and circular aluminium gallium nitride/Gallium nitride high electron mobility transistor (AlGaN/GaN HEMT) designs for pH sensing applications along with the insights into pH sensing to elucidate the underlying mechanism. The planar and circular devices were fabricated on the same wafer where the sensing membrane and passivation is realized by
$\rm Al_{2}O_{3}$
. The devices were encapsulated with photoresist and small openings are created for source/drain electrode contact pads and the sensing region where pH solution is confined to test the device response. The response of sensors is compared after normalizing circular HEMT output for a width of 100
$\mu{\rm m}$
. The output current sensitivity is in the range of 75 (0.5 V) to 222.5 (2 V)
$\mu {\rm A}$
/pH for planar HEMT device, whereas sensitivity values for circular HEMT is in the range of 0.152 (0.5 V) to 0.5 (2 V) mA/pH, respectively between pH 2 to 10. The circular HEMT device design depicted higher sensitivity and better linearity in response to pH due to the conduction throughout its circumference. The transient response is also studied and the results suggest a stable pH resolution and good linearity for both acidic and basic phosphate buffer saline (PBS). The higher current values are obtained, which is an advantage in terms of signal processing and conditioning unit for proper measurement that will help in reducing complexities in the integrated circuit (IC) and, therefore ease in point-of-care diagnosis.