Nitrate-selective gallium nitride transistor-based ion sensors with low detection limit M. Myers 1,2 , A. Podolska 2 , T. Pope 2 , F.M.L. Khir 2 , U.K. Mishra 3 , B.D. Nener 2 , M.V. Baker 2 , G. Parish 2 1 CSIRO Earth Science and Resource Engineering, Australia 2 The University of Western Australia, 35 Stirling Hwy, Crawley WA 6009, Australia 3 University of California, Santa Barbara CA 93106, USA M018, 35 Stirling Hwy, Crawley, WA 6009, Australia giacinta.parish@uwa.edu.au Abstract: AlGaN/GaN heterostructures functionalised with ion-selective membranes have been used to form field effect transistor (FET)-like nitrate sensors which operate without the requirement for a reference electrode. These sensors demonstrate a wide range of sensitivity of 1x10 -6 – 1x10 -2 M (62 ppb – 620ppm) and a low detection limit below 1x10 -7 M (6.2 ppb). The low detection limit for this device is superior to the original published results for standard ion selective electrodes (ISEs) using a similar membrane chemistry. The membrane chemistry for these sensors was modified from the original recipe in order to optimise application to small area devices. Key words: AlGaN/GaN, nitrate sensor, transistor, ion-selective membrane Intrroduction Ion-sensitive field effect transistors (ISFETs) are a solid-state solution to potentiometric sensing that offer the advantages of compact size, mass-producibility, on-chip integration, speed, and sensitivity [1]. To date the majority of investigations use Si-based ISFETs. While they have shown promise, the gate dielectrics are highly sensitive to charging effects and can be chemically unstable in aqueous solutions, and the devices require a reference electrode to bias the gate of the transistor beyond threshold (so as to allow majority carriers to travel through the conductive channel) [2]. Also, Si itself is not chemically or thermally robust. AlGaN/GaN high electron mobility transistors (HEMTs), which have recently come to dominate the high-power electronics market, have the unique benefits of high sensitivity of device conductivity to surface charge, fast response, and excellent chemical and thermal stability [1]. AlGaN and GaN are members of the III-nitride alloy family, which has become extremely technologically important for a variety of applications such as wireless base stations, solid-state lighting, automotive electronics, power conditioning and information storage. The properties of AlGaN/GaN HEMTs are also very attractive for sensors. Importantly, in contrast to Si-based transistors, AlGaN/GaN HEMTs do not require a gate bias to be turned on; when used as ion sensing transistor devices no reference electrode is therefore required for aqueous operation [3,4], as shown in Fig. 1. This is a significant advantage over electro- chemical cell techniques. Fig. 1 Schematic of AlGaN/GaN HEMT showing ON and OFF operation. In all transistors positive current flows source (S) to drain (D) when device is ON and a positive voltage is applied to the gate (G). However, unlike silicon-based devices, the channel is turned OFF by application of a negative gate voltage. IMCS 2012 – The 14th International Meeting on Chemical Sensors 671 DOI 10.5162/IMCS2012/8.1.5