Graphene Field-Effect Transistors
Revolutionizing the Biomedical Industry
“Nanosensors — they may be tiny, but they have a huge impact.” Nanotechnology is an incredibly fascinating technology that is already disrupting thousands of important industries around the world. An essential technology within this field is the use of nanosensors. Advancements in nanosensors can open unprecedented perspectives for the application in various fields such as molecular-level diagnostic and treatment instruments in medicine. Graphene field-effect transistors (GFET) are a very exciting type of nanosensor that can allow us to understand our bodies on a molecular level and detect biomarkers at a level that has never been seen before, revolutionizing personalized healthcare.
Content Outline
- Graphene
- Nanosensors
- Defining a GFET Sensor
- Inner Workings of GFET Sensors
- Types of Gated GFETs
- GFET Advantages Over Regular FETs
- Fabricating GFETs
- Applications of GFETs in Healthcare
- Challenges of GFETs
- The Future of GFETs
- TL;DR
Graphene
Graphene is one of the most versatile and strongest metals on earth. It is an allotrope of carbon, which consists of a singular layer of carbon atoms that all lie in a plain together, arranged in a tightly-packed 2D hexagonal lattice structure.
Some of graphene’s superior properties that help improve nanosensor performance include:
- light absorption: graphene is an extremely thin and transparent material that has the ability to absorb about 2.3% of white light, which is quite a lot for a 2D material.
- extremely light weight: graphene is lighter than a feather and extremely thin, a property that makes it excellent for wearable devices, along with its extreme flexibility.
- high electrical conductivity: graphene can theoretically transfer electric current with 100% efficiency.
- high thermal conductivity: graphene is an isotropic conductor that has the ability to dissipate heat in all directions, and has better thermal conductivity than any other material, including diamond.
- excellent chemical properties: under certain conditions, graphene can absorb different molecules and atoms that alter its properties, making it suitable for applications such as chemical and biological sensors.
To learn more about the properties of graphene, read my article Graphene: The Material That Is Changing The World.
Nanosensors
Nanosensors are nanoscale devices that detect and measure small particles or miniscule physical quantities of something. Nanosensors then convert these to signals that can be detected and analyzed. They work by identifying changes in the electrical conductivity of active sensing materials. To put it simply, nanosensors are extremely small sensing devices that collect information and data at a nanoscale.
There are many different types of nanosensors. The main two types of nanosensors are chemical and mechanical sensors. Nanosensors have numerous applications in almost every field, including defense, environmental and healthcare industries.
Biosensors
A graphene field-effect transistor (GFET) sensor is a type of biosensor known as bioanalytical sensors. Biosensors are analytical devices that detect changes in biological processes (any biological element or material like enzymes, tissues, microorganisms, cells, acids, etc.) and converts them into an electrical signal. Bioanalytical sensors are designed for quantitative analyses of biologically relevant molecules such as nucleic acids, proteins, metabolites, drugs, etc. Graphene biosensors are designed to provide a fusion of electronics and biology that essentially transforms biological testing capabilities, and sensor speed and size.
Defining a GFET Sensor
A graphene field effect transistor (GFET) is composed of a graphene channel between two electrodes, along with a gate contact to modulate the electronic response of the channel.
When a target molecule binds to the receptor (i.e. glucose, hemoglobin, cholesterol, hydrogen peroxide, etc) on the graphene channel, an electronic charge redistributes and generates a change in the electric field of the channel region. This changes the electronic conductivity of the channel and overall device response. While graphene does not generally react or bind with most materials, it is composed of carbon, and there are several procedures that allow functionalization through the formation of binding sites on the surface.
Bioreceptors such as amino acids, antibodies or enzymes can be added through adsorption, or a linker molecule attached to the channel surface. Molecules can then attach to these bioreceptors through covalent bonding, electrostatic forces or Van der Waals forces, which will create an electronic transfer throughout the sensor.
With the correct functionalization, a GFET sensor enables the detection of targeted analytes with an all-electronic device control and readout that is:
- highly sensitive
- highly selective
- direct
- label-free
Inner Workings of GFET Sensors
A graphene sheet connects the source and drain electrodes, under which the solid gate electrode is embedded, usually on a silicon substrate.
A GFET typically consists of:
- A graphene channel
- Two electrodes: a source and a drain
- A gate: either a top gate, back gate, or dual-gate
Graphene Channel
The graphene channel of the GFET is only one atom thick, meaning that the entire channel is effectively on the surface. Any molecule attached to the surface of the channel impacts electronic transfer through the entire device. This channel runs between the source and the drain electrodes.
Electrodes: Source and Drain
When a voltage is applied at the gate, the device changes the conductivity between source and drain to control the flow of current between them. The source is where the molecules enter the channel. The drain is where the molecules leave the channel.
Back Gate GFET
The gate is the terminal that modulates the channel conductivity. The back gate GFET is the most common of the GFET sensor configurations. In this type of GFET, the gate is fabricated first and then stacked with another layer of drain and source electrode.
Top Gate GFET
The gate controls how electrons respond and hence the channel’s behavior. The top gate GFET sensors use a bilayer gate dielectric to propose a new compensation mechanism which leads to high operational stability. Using a thin, top-gate insulator improves GFET parameters including open-circuit gain, forward transmission coefficient, and cutoff frequency.
Dual-Gate GFET
In typical applications, the dual gated GFET uses the two gate biases to control the channel’s charge concentration. The dual-gate GFET enables biasing the channel with two different voltages.
Types of Gated GFETs
Graphene is used to form a conducting channel in a field effect transistor, which allows highly sensitive electric detection of analytes. GFET sensors, when operating in liquid media, are generally constructed in a solution-gated or solid-gated configuration.
Solution-gated GFET
In a solution-gated GFET sensor (also known as a liquid-gated GFET), a wire (typically Ag/AgCl) is inserted into the electrolyte solution and comes into contact with the graphene. This serves as the gate electrode, while the electric double layer (EDL) formed at the solution-graphene interface provides the gate dielectric. Using various compositions of an electrolyte solution, these nanosensors have been able to detect physicochemical parameters including pH and biochemical analytes like DNA.
Solution-gated sensors can hinder the integration and miniaturization of the sensor because they typically require an external electrode inserted into the electrolyte solution. The EDL dielectric layer or gate capacitance is susceptible to disturbances to liquid media. This can result in fluctuations in electrical measurements of properties of graphene including the conductance and the location of the Dirac point.
Solid-Gated GFET
In a typical solid-gated GFET, a SiO2 dielectric layer between the graphene and underlying silicon substrate serves as the gate electrode, providing the gate capacitance. By removing the need for any external wire insertion into the electrolyte solution, solid-gated sensors are open to integration and miniaturization. Due to the naturally low capacitance of the SiO2 layer, however, the solid-gated GFETs usually require unwanted high gate voltages (40~50 V). This can impede the sensor’s application for biosensing in liquid media.
GFET Advantages Over Regular FETs
GFET have several advantages over bulk semiconductor devices. These advantages include:
- Unprecedented sensitivity
- Enhanced performance and efficiency
- Fewer molecular defects
- Fabrication advantages over 1D materials
- Extreme surface-to-volume ratio
Unprecedented Sensitivity
In traditional FET sensors, silicon acts as a thin conducting channel. When the silicon is replaced with graphene in GFET sensors, the sensor yields a much thinner, more sensitive channel region. The response sensitivity of the normal semiconductor devices are limited because local electric field changes on the channel surface have little effect deeper in the device channel. Most semiconductor transistor sensors are three-dimensional, which means that the electric charge changes on the surface of the channel, however it doesn’t always penetrate deeper into the device.
Enhanced Performance and Efficiency
Graphene transistors have the potential to provide enhanced performance and efficiency due to it’s superior electrical and thermal conductivity that results in low resistance losses and better heat dissipation than silicon. GFETs have a higher carrier mobility than the regular FET sensors. If often reaches levels greater than 100,000 cm2V-1s-1 for hBN-encapsulated single-crystal CVD graphene. The GFET sensors also have a residual charge carrier density of the order of 5 x 1011 cm-2.
Fewer Molecular Defects
Using silicon (or other bulk semiconductors) as thin as graphene is also not effective because it causes the surface defects to dominate the material characteristics. These bonds form additional defects in the semiconductor channel, and make non-specific binding possible. This can give rise to false positives. Graphene is a two-dimensional material, and as such, it does not have surface dangling bonds to form defects. This results in high conductivity and sensitivity to surface defects, and since the material has no dangling bonds, it eliminates nonspecific binding and false positives. The GFET sensor is so sensitive that it can detect individual molecules detaching from or attaching to a surface.
Fabrication Advantages Over 1D Materials
Graphene field-effect transistor sensors have distinct fabrication advantages over devices fabricated with one-dimensional materials like carbon nanotubes and nanowires. 1D materials cannot currently be fabricated with the same consistency, while graphene can be produced in uniform films with material characteristics. 2D materials provide the path to achieving device-to-device consistency. Uniform graphene films can also be produced through chemical vapor deposition, and these films are responsive to the photolithographic fabrication techniques developed for integrated circuit fabrication processes.
Extreme Surface-to-Volume Ratio
With graphene’s broad electrochemical potential and its ability to be functionalized, GFETs are an appealing device for biomolecules to attach to. Due to graphene’s extreme surface-to-volume ratio, changes in the channel conductivity can easily occur with even the smallest concentration of attached molecules.
Fabricating GFETs
There are many different ways to fabricate nanosensors and graphene, which include these main methods:
- top-down nanofabrication
- bottom-up nanofabrication
- chemical vapor deposition (graphene)
Top-down Nanofabrication
An analogy that best describes top-down approaches to fabricating nanosensors is to think of it as a sculptor carving out a statue from a template and thus removing material. A piece of the base material is gradually eroded until the desired shape is achieved. With the top-down approach, you start at the top of the blank piece and work your way down, removing any unnecessary material.
A top-down method that is important in the semiconductor industry is photolithography. With this method, a short wavelength light (electrons in e-beam lithography) is used to create the desired pattern in a photoresist and etching to form nanostructures by removing material underneath. Different types of etching methods can include chemical, plasma, or reactive ion etching. Other types of top-down methods used are chemical or electropolishing to smoothen a surface, or nano-imprint techniques that involve using a miniature stamp pressed down into a material to form the desired nanostructure.
Bottom-up Nanofabrication
A good analogy for bottom-up fabrication is building a brick house, as you place bricks on top of each other one at a time to create a house. Instead of just placing bricks on top of each other one at a time to produce a house, bottom-up fabrication methods place atoms or molecules on each other one at a time to build the desired nanostructure. Since these processes are time consuming, self assembly techniques are used, a method where the atoms arrange themselves as required.
Chemical Vapor Deposition
GFET sensors are typically fabricated on silicon wafers/SiO2 substrates with metal contacts to take advantage of the low-cost, highly reliable lithography, deposition, and integration processes of the integrated circuit industry. Graphene films are formed by atmospheric pressure and deposited onto the wafer through chemical vapor deposition (CVD).
The main steps to CVD are:
- Decomposition of the carbon source at high temperatures. The careful use of a catalyst such as copper, nickel, or iron is required to lower the effective temperatures required from over 2 500°C to 1 000°C. Care must be taken so that the catalyst doesn’t create impurities within the graphene.
- After this, the carbon atoms are laid down on the deposition substrate where a continuous single layer of hexagonal lattice, graphene, will form (all within five minutes, depending on the gas flow ratio and the size of the layer required).
The graphene layer is then transferred from the deposition substrate and overlaid onto a wafer which is typically made of silicon. Next, metal electrodes are deposited on graphene lithographically, which is further used to change the graphene channels into the desired shape and size.
Applications of GFETs in Healthcare
GFET Nucleic Acid Sensors
Nucleic acids such as DNA, RNA, and miRNA play a major role in many diseases, which means that rapid and highly sensitive detection methods of nucleic acid abnormalities/expression are extremely important for disease diagnosis. When nucleic acids come into close proximity with the graphene channel surface, they considerably change the graphene’s electronic properties by doping the graphene. This results in a direct change in the properties of the graphene to allow the sensor to detect when these nucleic acid abnormalities or expressions are present.
DNA Sensor
DNA contains an individual’s entire genetic code, therefore being able to assess a person’s genetic makeup will not only aid in the diagnosis of many diseases, but will also reveal information regarding a person’s predisposition to genetic diseases and cancers. Many GFET DNA biosensors have already been developed using different sensing methods such as back-gated GFETs and liquid-gated GFETs.
One of the most common mutations in the genetic sequence that GFETs have been proven to detect is single nucleotide polymorphism (SNP), also known as a single nucleotide mutation in the DNA sequence. SNPs have been linked to the development of cancers and genetic disorders. These kinds of mutations can have a dramatic effect on an individual’s health. Such tests will aid considerably in disease diagnosis, genetic screening, molecular diagnostics, pharmacogenomics, drug discovery, and even prevention by enabling early treatment.
Immunosensors
Immunoassays are biomolecular recognition tests that analyze biorecognition by antibody-antigen interactions. Immunosensors work by the immobilizing an antibody on the GFET sensor’s channel surface. The sensor detects when the target analyte binds to the antigen binding fragment of the antibody, as shown in the photo above. Certain GFET immunosensors are even capable of distinguishing BNP (the binding of brain natriuretic peptide to anti-BNP) from other proteins within the complicated sample matrix of whole blood.
Monitoring Glucose Levels
Certain GFET sensors have been developed recently that are capable of measuring glucose levels non-invasively. Developments such as these are revolutionary as GFET sensors continue to improve in accuracy and efficiency with practically endless applications.
This paper presents a flexible and reusable GFET nanosensor that detects glucose using pyrene-1-boronic acid (PBA) as the receptor. Simply and accurately monitoring a person’s blood glucose level is essential for effective treatment of diabetes, a chronic metabolic disease that has effect on blood sugar level and affects millions of people around the world.
This paper presents a GFET nanosensor for affinity-based detection of low-charge and low-molecular-weight molecules, using glucose as a representative. The sensor is capable of measuring glucose concentration in a relevant range of 2 μMto 25 mM. The paper states that this GFET sensor can potentially be used in noninvasive glucose monitoring.
Challenges of GFETs
Lack of Bandgap
While the GFET sensor is a fast and efficient transistor, it does not have a bandgap. The gapless structure of the sensor means that the valence and conduction bands both meet at zero volts, thereby making graphene behave like a metal. In typical semiconductor materials like silicon, two bands are separated by a gap, behaving like an insulator under normal conditions.
The electrons usually require additional energy to jump from a valence band to a conduction band. FETs have a bias voltage that enables a current to flow through the band. This acts as an insulator when there is no bias. The lack of a band gap in GFET sensors makes it hard to turn off the transistor, as it cannot behave as an insulator. Since it cannot be completely switch off, there is an on/off current ration of around 5 (quite low for logic operations). This means that utilizing GFET sensors in digital circuits us a challenge. With analog circuits, however, this is not a problem therefore making GFETs suitable for mixed-signal circuits, amplifiers, and different analog applications.
Scalability
Mass producing graphene without defects or impurities is one of the main difficulties in creating GFET sensors. However, steps are being taken towards higher quality CVD graphene growth and transfer. These steps will help prevent the graphene from being corrupted by any metallic contaminants, cracks, holes, folds, residues, etc. Work is continuously being done to move the production of GFET sensors from the lab to the industry, despite the scalability problems that may still be an issue depending on the technique used to fabricate them.
The EU’s Graphene Flagship is one of the initiatives conducting research in this area who are aiming to develop graphene consumer products by 2025–2030. Researchers from the University of Pennsylvania’s Department of Physics and Astronomy are also discovering a way to mass produce label-free GFET DNA biosensors through a CVD fabrication process that they found offers greater than 90% yield.
Saturation in Analog Circuits
One of the other main challenges that stands in the way of widespread GFET sensor adoption is the insufficient current saturation. This prevents the transistor from reaching the maximum voltage gain and oscillation frequency in RF applications.
Manufacturers are learning to overcome this challenge by optimizing the dielectric material insulating the top gate of a GFET sensor. Good dielectric gate material can usually provide better control of the carriers in the graphene channel to improve the performance of the overall device.
The Future of GFETs
The exceptional electronic properties of graphene continue to hold great promise for sensing applications and advancements in nanosensors. GFET sensors can enable rapid, sensitive, specific, low-cost, and all-electronic detection and analysis for biological and chemical applications. Since GFETs can also be multiplexed, it is possible for them to rapidly test thousands of targets with high sensitivity on just one tiny chip. The possibilities are endless as GFET sensors have the potential to disrupt healthcare, drug discovery, chemical detection markets, and many more industries around the world.
TL;DR
- Graphene is a single layer of carbon atoms that all lie in a plain together, arranged in a tightly-packed 2D hexagonal lattice structure. Properties of graphene include light absorption, extremely light weight, high electrical conductivity, high thermal conductivity and excellent chemical properties.
- A GFET sensor is a type of biosensor known as bioanalytical sensors. Biosensors are analytical devices that detect changes in biological processes and converts them into an electrical signal.
- A graphene field effect transistor (GFET) is composed of a graphene channel between two electrodes, along with a gate contact to modulate the electronic response of the channel.
- Three different gate configurations of GFET sensors: back gate, top gate, and dual gate. Back gate is the most common configuration.
- The advantages of GFETs over bulk semiconductor devices include unprecedented sensitivity, enhanced performance and efficiency, fewer molecular defects, and extreme surface-to-volume ratio.
- GFET sensors are typically fabricated on silicon wafers/SiO2 substrates with metal contacts using lithography, deposition, and integration processes of the integrated circuit industry. Graphene films are formed by atmospheric pressure and deposited onto the wafer through chemical vapor deposition.
- There are many incredible applications of GFET sensors in healthcare and the biomedical industry, such as being used as nucleic acid sensors, DNA sensors, miRNA sensors, and immunosensors.
- Some challenges of GFET sensors include lack of bandgap, scalability, and saturation in analog circuits.
