Limit of Detection for Silicon BioFETS

Nitin Rajan


Tuesday, July 9, 2013 - 11:00am to 12:00pm


1601 Elings Hall


Over the past decade, silicon nanowire/nanoribbon field-effect transistors (NWFETs) have demonstrated great sensitivity to the detection of biomolecular species, with limits of detection (LOD) down to femtomolar concentrations. Several well known factors limit the LOD; among them, screening effects, efficiency of the biomolecule-specific surface functionalization, binding kinetics and equilibria, and the delivery of the analyte to the sensor surface. Recently, the noise properties of such biosensors have been receiving more attention, both as a factor that determines the LOD as well as a diagnostic tool to extract information about the electronic properties of the FET sensors. However, the signal-to-noise ratio (SNR) of these bioFET sensors, and the device parameters that determine the LOD, are not well understood.

In this talk, I will discuss our experiments on applying noise spectroscopy to silicon NWFETs with the goal of understanding and improving the detection limit of such devices. Using low frequency noise measurements and modeling, we are able to compare different devices/material systems and quantify the effect on device performance of different process parameters. We also consider the effects of temperature on the noise generating mechanism and investigate the fundamental origin of 1/f noise in these devices.

We then introduce SNR as a universal performance metric, which includes both the effects of noise and signal transduction, and we show that the SNR is maximized at peak transconductance due to the effects of 1/f noise, and not in the subthreshold regime where “sensitivity” is maximized. We also correlate the LOD predicted by the measurement of the SNR to pH sensing experiments, highlighting the relevance of this metric for ultra-sensitive detection of biomolecules. The effects on the SNR, of surface functionalization, gating scheme and device scaling are also considered and quantified, yielding interesting results.

The nanowire-based devices have shown a theoretical LOD of 4 electronic charges, ignoring the effects of screening. Using these devices, with very good performance in terms of SNR, we were able to measure and extract the binding kinetics of protein interactions, which have never been done with NWFETs. Binding constant determination is a critical parameter for biomolecular design, and has until now been primarily assessed by surface plasmon resonance (SPR). Utilizing the low LOD of these devices, we are able to extract binding constants into the sub-picomolar range.