Toward single molecule detection in physiological buffer using planar FET biosensors

Among a variety of sensing mechanisms and device architectures, field effect transistors (FETs) based devices are ideal biosensors that can directly convert interactions of biomolecules to electrical signals for low cost, rapid, label free, and sensitive detection. Si-based MOSFETs (planar or nanowire) have the great advantage of compatibility to the current microelectronic manufacturing processes, but suffer from current drift due to ion diffusion into the gate oxide layer, making them difficult to detect target analytes at low concentrations in physiological buffers or fluids with high ionic strengths. III-nitride semiconductors have attracted considerable interests for biosensor applications due to their unique properties, such as chemical inertness, non-toxicity, and thermal stability. Especially, III-nitrides FETs are ion-impermeable and highly stable in electrolytic solutions, making them ideal for detection at ultralow concentration in fluids with high ionic strengths. Theoretical calculations suggest that nanowire and quantum dot FETs should have higher sensitivity than planar FETs due to the large surface to volume ratio. Single molecule detection has been demonstrated using single wall carbon nanotube devices
¹
. Previously we demonstrated pM sensitivity of streptavidin (SA) protein detection in 0.25×PBS with an ionic strength of 40 mM using an AlGaN/GaN FET biosensor with a control gate electrode when the device was biased in the sub-threshold regime
²
. Here we show a recessed gate AlGaN/GaN heterojunction FET (HFET, Fig. 1) for detection of proteins at a concentration of 16 aM in PBS buffer (163 mM).