Mechanism and control of current transport in GaN and AlGaN schottky barriers for chemical sensor applications

Abstract

There is a significant interest in concept of diagnosis approaches for Hydrogen exponent (Pondus Hydrogenii or pH) value that can be analyzed by a wide variety of sensors and biosensors. Detecting ion concentrations by semiconductor devices has stimulated a developing field in semiconductor-based ion sensors. Many semiconductor materials have been tested for their suitability as ion sensors; especially there is an emerging interest in the use of wide band gap semiconductors as sensitive chemical sensors. Group III-nitrides with wurtzite crystal structure are chemically stable semiconductors with high internal spontaneous and piezoelectric polarization, which make them highly suitable materials to create very sensitive but robust sensors for the detection of ions and polar liquids. Aluminum Gallium Nitride/Gallium Nitride (AlGaN/GaN) high-electron-mobility transistors (HEMTs) have been extremely useful for gas and liquid sensor due to primarily three reasons: 1) a high electron sheet carrier concentration channel induced by piezoelectric polarization of the strained AlGaN layer, 2) the carrier concentration which is strongly depends on the ambient 3) an opportunity of on-chip co-integration with signal processing and communication circuit. In addition, sensors fabricated from these wide band-gap semiconductors could be readily integrated with solar blind UV detectors or high temperature, high power electronics with wireless communication circuits on the same chip to provide high speed transmission of the data. For these reasons, GaN-based HEMT structures are versatile structures to be used for a variety of sensing applications. In this research we have investigated the feasibility of AlGaN /GaN HEMT strucure for pH sensing by fabricating an AlGaN/GaN HEMT structure pH sensor and investigate pH-sensing characteristics of the fabricated sensor. We have investigated the basic transistor characteristics and liquid-phase sensing capability of open-gate devices with bare undoped-AlGaN surfaces in aqueous solutions. The results show the typical current-voltage (I-V) characteristics of HEMTs with good gate controllability in aqueous solution. The potential of the AlGaN surface at the open-gate area is effectively controlled via aqueous solution by Silver/Silver Chloride (Ag/AgCl) reference gate electrode. The open-gate undoped AlGaN/GaN HEMT structure is capable of stable operation in aqueous electrolytes and exhibit linear sensitivity, and high sensitivity of 1.9 mA/pH or 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V is obtained. The Nerstian’s like characteristics is not observed since the occurrence of large leakage current. Suppression of leakage current should improve the sensing performance. The fabricated open-gate undoped-AlGaN/GaN structure is shown to be suitable for pH sensing application.