A Composition-Dependent Unified Analytical Model for Quaternary InAlGaN/GaN HEMTs for pH Sensing

Abstract

Group III–V material alloys such as gallium nitride (GaN) with piezoelectric and pyroelectric properties are used for designing advanced devices suitable for harsh surroundings such as high temperature and acidic ambience. In the present paper, we introduce a unified model using InAlGaN quaternary alloy in the barrier layer to analyze the performance of pH sensors based on high electron-mobility transistors (HEMTs) Our model is used to accurately calculate the threshold voltage, the sheet charge concentration and thus the drain current, in response to changes in the pH values of the electrolyte put in the gate area of the InAlGaN/GaN heterostructure based sensor. We have taken three devices for our studies with varying In mole fraction from 0% to 16% and Al mole fraction from 23% to 74%. The sensitivity of a HEMT based pH sensor depends on its transconductance. The maximum transconductance values of the InAlGaN/GaN devices were found to be much higher than those of the AlGaN/GaN HEMT devices. A theoretical sensitivity of 1.3 mA/pH was achieved for the quaternary structures. Our model shows good agreement with the experimental data available in literature, presenting less than 1.2% root mean square error in almost all the devices. We observe that HEMTs based on the InAlGaN/GaN structures have better sensitivity than the AlGaN/GaN structures in pH sensing applications. Our findings may be used in designing quaternary HEMT based novel pH sensors.