Graphene field effect transistors

Dr Alex Porkovich, Archer Materials Senior Researcher in Biotechnology and Alumni of The University of Technology Sydney and former researcher at Okinawa Institute of Science and Technology, explains the concept of graphene field effect transistors (gFETs) and how the company is incorporating gFET devices as sensors for biochip applications.

Graphene’s exceptional sensitivity, electronic tunability, and fast device response times make it an excellent candidate for sensor materials in field effect transistors, enabling the development of highly sensitive and versatile sensing devices for a wide range of applications.

A gFET operates based on the manipulation of electrical conductivity through graphene. It comprises three main components: a graphene channel, source and drain electrodes connected to the graphene channel, and a gate electrode which is in contact with the graphene by a dielectric or electrically insulating material.

Graphene, a single layer of carbon atoms, acts as the conducting channel. When a voltage is applied to the gate electrode, it generates an electric field that influences the conductivity of the graphene channel.

Graphene is known for its unique properties, including being ambipolar. This means it can conduct both positive and negative charges, allowing it to carry electrons (negative charge) and holes (positive charge) simultaneously. This attribute results in the formation of a Dirac point, contributing to its potential for device integration in various electronic sensing applications.

The Dirac point is a crucial concept in gFETs for sensing because it represents the point where the valence and conduction bands of graphene meet. Changes in the environment, such as doping, adsorption of molecules, or interactions with other materials, can alter the position of the Dirac point. This sensitivity makes gFETs highly responsive to external stimuli.

Wettable chips

When a liquid is used to gate a gFET, it essentially serves as a dielectric for the gate electrode. By immersing or applying the liquid onto the graphene surface, the properties of the liquid can modulate the electrical behaviour of the graphene.

Liquid-gated gFETs carve a niche for themselves in highly sensitive, miniaturised, and adaptable sensing applications where the interaction with liquid environments or biological substances is crucial, for example in biosensing, environmental monitoring, and other analytical applications.

The choice of liquid can significantly impact the gFETs behaviour. Some liquids can introduce charge carriers into the graphene and even biological molecules like proteins or DNA, altering its conductivity. For instance, if the liquid contains ions, those ions can interact with the graphene and change its charge carrier concentration, affecting the current flow through the transistor.

gFET biosensors

The Archer team is working on getting gFETs to function effectively in biosensing applications. This involves advancements in graphene chemistry, improving device fabrication processes, and developing strategies for enhanced selectivity and sensitivity while maintaining stability and reproducibility.

There is a fundamental challenge that requires disrupting the electronic charge screening that attenuates biosensing signals in gFETs. At Archer, we addressed charge screening by electronically controlling the sensitivity of gFET devices.

As the screening layer is less than 1 nanometer (nm) in biologically relevant liquids, generally, electronic sensing beyond this distance is impossible. So, for sensing to work in Archer’s biochip, the analyte charge must not be screened as most of their charge is out of gFET sensitivity range in liquids.

Overcoming this technological challenge is critical for the selective detection of target molecules. As a result, one of Archer’s sensor design strategies currently involves the use of a range of dynamic electric fields to rid the gFET sensor of signal interference caused by the screening layer.

The Archer team has also designed and developed hardware and software systems that readout the signal from multiple gFET sensors at once on a single chip. This is a significant advance over reading one-sensor-at-a-time.

Foundry readiness

Producing gFETs in a semiconductor foundry is a current focus of Archer. The quality and consistency of foundry fabricated graphene is critical for device performance. Large-scale production can be a trade-off with uniform graphene quality across a wafer or batch, leading to variability in device characteristics.

Integrating graphene with conventional semiconductor fabrication processes requires significant engineering. Graphene’s unique properties require specialised processes that will need to align seamlessly with semiconductor manufacturing techniques.

Archer is currently developing various gFET design techniques through the engagement of several commercial semiconductor foundry partners. This has the potential to increase the applications of the Biochip, improve quality control, and bolster the company’s supply chain resilience.

 Early gFET chips at Archer designed for liquid gating and multiplexing have been validated through multi-project wafer and whole wafer runs with foundry partners in Europe. The semiconductor chip manufacturing processes and technology in each graphene foundry differ, including the characteristics of graphene within the devices. Performing wafer runs in several foundries is required as part of the gFET chip development process to optimise the gFET design and manufacturing for foundry readiness and compatibility.

Partnerships between Archer and semiconductor foundries are addressing these challenges and are paving the way for large-scale production of graphene-based devices for biosensing applications.

More information on liquid gated gFETs in biosensing applications: https://doi.org/10.1016/j.bios.2020.112419