Open-Gated GaN HEMT-Based pH Detectors Using Patterned Sensing Area

This article presents a pioneering study on the pH sensing performance of open-gated gallium nitride (GaN) high-electron mobility transistors (HEMTs) with five different device variants, introducing novel approach to patterning the open gate area. The device variants include normal HEMTs with open gate (O-HEMT) used as a reference, with horizontal lines etched in the open gate area (H-HEMTs), with vertical lines etched in the open gate area (V-HEMTs), fully blank etched (B-HEMTs), and with horizontal and vertical lines or grids etched (G-HEMTs). The fabrication of the GaN HEMTs was achieved through metal-organic chemical vapor deposition (MOCVD), while the source and drain ohmics were fabricated using e-beam evaporation. The sensitivity analysis involved drop casting samples of varying pH onto the open gate area and measuring the device response for acidic (pH =4), basic (pH =10), and neutral (pH =7) solutions. The pH values of 4, 7, and 10 for our analysis have been chosen to showcase the sensor’s functionality within a range pertinent to common practical applications, such as biological systems and environmental monitoring—environments, where these pH levels are typically found. The normalized output drain current has been employed as the sensing metric. Our findings reveal the remarkable novelty of this study, as it demonstrates that G-HEMTs exhibit the highest sensitivity among the variants, with an average sensing current of 1.355 mA/pH. The V-HEMTs displayed a sensitivity of 2.15%, followed by H-HEMTs at 1.98%, and normal HEMTs at 1.16%. Notably, the B-HEMT devices initially showcased good sensitivities; however, their performance declined below that of H-HEMTs beyond 15 V. These results underscore the significance of patterning the open gate area in optimizing the pH sensing capabilities of GaN HEMTs.