Credit: © 2007 AIP

AlGaN/GaN heterostructures are used for producing high-power field-effect transistors, where a gate voltage modulates the source–drain current flowing through a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. This 2DEG is formed because of polarization effects, and counter positive charges are induced below the AlGaN surface. These positive charges are extremely sensitive to environmental conditions, which leads to fluctuations in the source–drain current.

Now, Fan Ren and colleagues1 from the University of Florida, Gainesville, and Nitronex Corporation in North Carolina have exploited the environmental dependence of the source–drain current in AlGaN/GaN structures to detect a substance known as kidney injury molecule-1 (KIM-1), an important factor for early diagnosis of kidney disease. KIM-1 antibodies were immobilized onto a 5-nm-thick gold gate electrode on top of the AlGaN layer of an AlGaN/GaN heterostructure by thiol-group-mediated self assembly. When the Au region is exposed to KIM-1, the carrier concentration of the 2DEG is altered owing to surface charge accumulation, resulting in a decrease in the conductance of the device.

The detection limit of 1 ng ml-1 KIM-1 for a 20 × 50 µm2 sensing area and the compact size of the sensor should enable the use of AlGaN/GaN heterostructures as arrays for multi-analyte detection, thus overcoming limitations of conventional enzyme-linked immunsorbent assay protocols.