Low frequency electrochemical noise in AlGaN/GaN field effect transistor biosensors

Abstract

Little has been studied on how the electrochemical noise impacts the limit of detection of field effect transistor (FET) biosensors. Herein, we investigate low frequency noise associated with phosphate-buffered saline (PBS) solutions at varying ionic strengths (Ni) under both weak and strong gate biases corresponding to saturation and sub-threshold regimes, respectively, in AlGaN/GaN heterojunction FET biosensors. We show that the electrochemical noise is strongly dependent on the ionic strength and gate biasing conditions. In the saturation regime (low bias), varying the ionic strength (a range of 10−6× PBS to PBS 1 × stock solutions used for testing) has little to no effect on the characteristic frequency exponent β(β=1), indicating a predominately diffusion-based process. Conversely, under higher biases (sub-threshold regime), the β parameter varies from 1 to 2 with ionic strength exhibiting both diffusion and drift characteristics, with a “cut point” at approximately 10−5× PBS (Ni≈9×1014/mL). Under a high bias, once the PBS concentration reaches 10−3×, the behavior is then drift dominant. This indicates that the higher bias likely triggers electrochemical reactions and by extension, faradaic effects at most physiologically relevant ionic strengths. The signal-to-noise ratio (SNR) of the device has an inverse linear relationship with the low frequency current noise. The device exhibits a higher SNR in the sub-threshold regime than in the saturation regime. Specifically, within the saturation regime, an inversely proportional relationship between the SNR and the ionic concentration is observed. The electrochemical noise induced from ionic activities is roughly proportional to Ni−1/2.

Development of a novel immunoFET technology‐based POC assay for detection of Leishmania donovani and Leishmania major

“…Depending on choosing V g and V d s parameters, the device might be operating in one of three regimes: sub‐threshold, linear and saturation which have been well described previously 23 . Throughout the detection of Leishmania parasite hybridization, the AlGaN/GaN immunoFET biosensor was biased under 0.5 V, at 1.5 mA with approximately 95 mV/decade as described previously 21 . We obtained the baseline electrical measurements (PBS only), and the S12 protein derived from different concentrations of Leishmania parasites in 1× PBS for different concentrations of L. donovani and L. major .…”

“…Purified antibody against L. donovani S12 ribosomal proteins was labelled with FITC conjugation Kit according to the manufacturer's protocol for sandwich ELISA 20 . The sample was processed using the new protocol 21 …”

“…The AlGaN/GaN immunoFET biosensor serves as an ideal platform for binding of analytes to a receptor (antigen–antibody interaction) due to its innate chemical stability under variable treatments. A previously described detailed cross‐sectional AlGaN/GaN structure of FET was used 21,22 (Figure 1). All the devices were fabricated with an AlGaN/GaN wafer with a 23 nm AlGaN barrier layer and 2 μm GaN on the SiC substrate, as previously described 21 …”

“…A previously described detailed cross‐sectional AlGaN/GaN structure of FET was used 21,22 (Figure 1). All the devices were fabricated with an AlGaN/GaN wafer with a 23 nm AlGaN barrier layer and 2 μm GaN on the SiC substrate, as previously described 21 …”

“…The threshold voltage was controlled by adjusting the back‐gate bias thanks to inter‐gate coupling, as previously described 24 . The measured key device parameters include I dss (drain current at V g 0 V) = 6.88 mA at V d = 5 V, the off‐state current I off = 0.86 nA, the current on/off ratio = 2 × 10 6 , the sub‐threshold slope SS = d V g /d log ( I d ) (where I d is drain current) 95 mV/decade, the maximum transconductance G m _MAX = 0.53 mS, and the threshold voltage V th = −4,2 V 21 …”