(Invited) Recent Advances in Ion Sensitive Graphene Field Effect Transistors
Authors: Szkopek Thomas 
Published: July 2021

I will summarize recent advances [1-3] in large-area graphene based ion sensitive field effect transistors (ISFETs), including the achievement of record potentiometric ion concentration resolution. Sensor resolution, the smallest measurable change in analyte concentration, is a critical parameter in the assessment of sensor performance. Notably, the absence of a bandgap in graphene is frequently perceived as prohibiting competitive graphene FET performance. A graphene FET cannot be driven into saturation, limiting intrinsic gain and high-frequency performance. The graphene FET cannot be turned off, severely limiting the on/off current ratio for switching applications. In contrast, in the case of graphene ISFETs, the graphene FET functions as an analog pre-amplifier directly integrated with a sensing layer, enabling record sensor resolution to be achieved.
Large-area graphene ISFETs combine high mobility charge transport, low flicker noise, facile integration with a variety of ion sensitive layers. The advent of wafer-scale graphene processing methods facilitates reproducible production of graphene ISFETs. Field effect mobilities in graphene ISFETs can reach μ = 5000 cm2 V-1 s-1, contributing to high gain gm
= ∂Id
/∂Vg
∝ μVDS. In particular, high mobility allows for the reduction of bias voltage VDS
, which improves potentiometric uniformity across the sensor channel. We further find that low-frequency (1/f) noise, which ultimately determines sensor resolution, decreases approximately inversely with graphene FET channel area, in accord with 1/f scaling in Si MOSFETs. In other words, large transistors are quiet transistors. Scaling graphene FETs up to a 5 mm x 5 mm channel area, we observe a normalized 1/f noise parameter K = f <Δv
2>/v0

2 = 5×10-13, the lowest reported to date for a graphene FET at room temperature. Facile integration of graphene FETs with ion sensitive layers, such as metal oxides and ionophore polymer membranes, enables sensitization to a wide range of ions including H+, K+, Na+, NH4
+, Cl-, NO3
-, HPO4
2- and SO4
2-. Combining the Nernstian limited sensitivity with low 1/f noise enables record ion concentration resolution as low as r ~ 0.003 log M.
Beyond sensor resolution, specificity is another important sensor parameter. Ionophores suffer cross-sensitivity to ionic species other than their target ion. The combined stability and resolution of graphene ISEFTs allows simultaneous, real-time measurement of multiple ionic analytes through the use of an array of graphene ISFETs, with each graphene ISFET element sensitized to a particular ion. Measurement of the cross-sensitivity within the array allows the application of Nikolskii-Eisenmann analysis to quantitatively account for cross-sensitivity, and achieve selective response with the graphene ISFET array that would otherwise not be possible with graphene ISFETs operated in isolation. I will conclude with a discussion of challenges and prospects for the future development of graphene FET based sensors.

REFERENCES

[1] Tran et al., Nanotechnology 32, 045502 (2020).
[2] Fakih et al., Nature Communications 11, 3226, (2020).
[3] Fakih et al., Sensors and Actuators B 291 89 (2019).