(Invited) Chemical Sensors Based on AlGaN/GaN Transistors
Authors: Parish Giacinta 
Published: February 2020

Reliable, economically accessible technology for in situ monitoring of contaminants in water has the power to transform health, industry, and society. Applications of such monitoring range from process control monitoring and optimisation for industry, to water supply quality and wastewater monitoring, to environmental monitoring for resource extraction, and beyond. Semiconductor-based technology offers high performance for in situ, real-time contaminant monitoring and can also be mass produced at low-cost with flexible functionalisation allowing for a variety of analytes. Furthermore, it offers the ability to integrate multiple sensors into one chip, along with wireless communication technology for maximum benefit of the in situ monitoring capability.
AlGaN/GaN transistors have the unique combined benefits of high sensitivity to surface charge, fast response, and excellent chemical and thermal stability. AlGaN and GaN are members of the III-nitride alloy family (AlN, GaN and InN), which has become extremely technologically important for a variety of applications such as wireless base stations, solid-state lighting, automotive electronics, power conditioning and DVD information storage. The properties of AlGaN/GaN transistors are also very attractive for sensors; however, this application niche has not yet been widely explored. The strained AlGaN layer induces a region of high sheet carrier concentration, electron mobility and high saturation velocity induced by piezoelectric polarization known as a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The 2DEG responds to changes in surface charge, so ion selective field effect transistors (ISFETS) can be developed into robust and sensitive ion sensors. Importantly, in contrast to many other potentiometric sensors, including Si-based ISFETS, high sensitivity can be obtained without the need for a reference electrode. This is a significant advantage over electrochemical cell techniques, for which a reference electrode is a major impediment to in situ and robust operation.
This paper will report on development of various AlGaN/GaN ISFETs for chemical sensing – in particular pH sensors, nitrate ion, mercury (II) ion and calcium ion sensors. The ability to fabricate different sensors from the same underlying technology paves the way for array-based multi-analyte sensing solutions. The ion sensors were all obtained by functionalising the transistors with a plasticised PVC membrane containing a dissolved ionophore selective to the ion of interest. Using transistor theory, we show that the apparent gate response for these sensors is near-Nenrstian under a variety of testing conditions. In each case, devices without a PVC membrane coating were tested under identical conditions and exhibited negligible response to ion exposure.
Some other key results are summarised below.

pH sensors

AlGaN/GaN transistor-based pH sensors have been investigated by several research groups. When operating without a reference electrode, researchers have reported two types of pH sensor response: linear (related to pH selectivity) and U-shape (related to anion selectivity). Previously, this distinct difference in behaviour had not been well explained. We have compared the pH response of a reference electrode-free AlGaN/GaN device with and without a GaN capping layer. From our investigations, in the absence of a reference electrode, a linear response towards pH requires a GaN capping layer. Apparently the difference in surface chemistry for GaN versus AlGaN surface affects pH/anion selectivity when there is no reference electrode controlling the surface potential.

Nitrate ion sensors

In a 0.1 M KH2PO4 ion buffer, a detection limit of less than 10−
6M and a linear response range of 10-6-10−
3M were achieved. The detection limit remains consistently low over multiple runs/days. In addition, there was minimal change in sensor response upon addition of KOH increasing the pH from approximately 4–11 [1].

Mercury (II) ion sensors

At pH 2.8 in a 10-2 M KNO3 ion buffer, a detection limit below 10−8 M in and a linear response range between 10-8 M-10−4 M were achieved. X-ray photoelectron spectroscopy (XPS) experiments confirmed that the sensing membrane is reversible after being exposed to Hg2+ solution and rinsed with deionised water.

Calcium ion sensors

In 0.1M KOH ion, a buffer detection limit of less than 10-8 M and a linear response range between 10-8M - 10-3M were achieved. In addition, there was negligible change in sensor response upon the addition of KOH, which increased pH level from approximately 4 to 11.
[1] M. Myers, F.L.M. Khir, A. Podolska, G.A. Umana-Membreno, B. Nener, M. Baker, G. Parish, “Nitrate ion detection using AlGaN/GaN heterostructure-based devices without a reference electrode”, Sens. Actuators, B
181 301-5 (2013).