Graphene, once called tomorrow’s material, extended into so many areas that its future-like applications in material science such as Graphene Field Effect Transistors (GFETs) puts everything from biology to biosensing into a height.
Where there is a weak electronic signal or electrical power, transistors rise as a solution to amplify and switch them. Field-effect transistors (FETs) and the latest generation of them, graphene field-effect transistors (GFETs), are a class of transistors that take the advantage of an electric field to control the electrical current. These semiconductor devices contain three terminals known as gate, drain and source. FETs alter the conductivity between the source and drain through applying a voltage to the gate which in turn maintains the application of an FET as a switching device to conduct low current when it is off and high electrical while it is on. In fact, this ON/OFF gate controlling operation is carried out by modulating the gate from a higher value in the ON mode to the lowest OFF value. The ON/OFF ratio of the current is a significant measure of digital switches performance as the larger the ratio, the better the performance 1. Also known as unipolar transistors, field-effect transistors carry electrical charge using either electron or electron holes (lack of an electron at a given positing causing a positive charge) but not simultaneously.
2. Graphene Field-Effect Transistors
Graphene, the highly conductive, flexible and transparent super material, has left its considerable impact on the field-effect transistors too and given birth to the new generation of transistors called graphene field-effect transistors (GFETs). Considering the unique electronic property, the two-dimensional (2D) structure of graphene leads to a zero band-gap semiconductor making it act like a semimetal with its chiral low energy quasi-particles as well as Dirac fermions (with no mass) with large Fermi velocities. This property of graphene maintains an interestingly fast charge carrying behavior making it a highly potential semiconductor material attractive enough for applications in high frequency circuits. Silicon-based semiconductors, however, seem to have reached to their scaling limit as the industry has been more and more attracted to graphene field-effect transistors and therefore, the age of silicon field-effect transistors is coming to its end. Even though carbon nanotubes (CNT), as a substructure of graphene, with similar electronic and physical properties to those of graphene, are the other highly potential alternative to replace silicon, technology and industry are attracted to graphene more as it can be modified with more complementary metal oxide semiconductors (CMOS) taking the advantage of its planar geometry.
Regarding graphene’s high current and large mobility carrying qualities, it is also a serious and efficient candidate to be applied in radio frequency circuits. The problem with the conventional technology is their truly high expenses and price while graphene application is proved to be a compatible modification with CMOS to overcome the expensive processing. Additionally, the graphene’s planar geometry makes it an ultra- thin structure less likely to face poor performance. What’s more, the compatibility of graphene with CMOS and the possibility to modify it, makes it available for any graphene-based and graphene/silicon hybrids to be used in computer chips as well 1. In addition to graphene’s mechanical and optoelectronic properties, it can engage in proper combination/interaction with some organic compounds to produce high-performance organic field-effect transistors2.
Bioelectronics as the interface between electronics and biology has also started to flourish after the appearance of graphene and graphene field-effect transistors. Conducting studies on developing bioelectronics paves a way to interpret biological signals into electronic ones which has absolute health monitoring benefits in modeling health disorders and pharmaceutics. Because of graphene’s unique properties, graphene-based devices can be employed for biosensing applications in vivo and in vitro. Among these biosensing applications, graphene field-effect transistors exhibit practically higher sensitivity with a little amount of energy. GFETs can transduce biomolecular charges and cellular voltage signals into compatible data through changes in the current and voltage.
Read More on Graphene Transistors: Graphene Transistors
3. Graphene Field-Effect Transistors for Molecular/Cellular Biology
Graphene field-effect transistors have widely been used in molecular biology. Single-GFETs for cellular biology have been designed to study the real-time recording of cell activities in vitro and in vivo. GFETs array for cellular biology have applications in accessing the single cell dynamics in neurocircuitry closely packed cell networks. Single-GFETs for molecular biology used to investigate molecular biology through the change biomolecule concentrations adjacent to GFET channels or the addition of biomolecules to the functionalized GFETs. GFET arrays for molecular biology applied to build GFETs with larger sizes via photolithography aimed to develop high throughput assays to study miRNA and human genotype 1.
4. Graphene Field-Effect Transistors for Biosensing
Graphene field-effect transistors show high sensitivity based on their limit of detection (LOD) of at picomolar ranges. This very low LOD is mainly due to (1) the exposure of their channel to an analyte under study and (2) graphene’s resistance to oxidization when an analyte is in aqueous solutions. This undesired condition usually comes up in the case of using FETs based on other nanomaterials (Nano-FETs) and consequently, limits their sensitivity. It should be noted that the oxidization layer restricts the direct doping to channels of FET. In addition to GFETs’ higher sensitivity, they are known for their higher compatibility and specificity in fabrication of chips since they are chemically inert and physically planar. Added to these, GFETs have transistor gain with the consequent biomolecules’ current changes and cellular voltage signals.
Recently in 2020, a strategy was suggested to build an acetylcholine sensor based on graphene field-effect transistors. Through this study, an amino moiety bearing polymer layer is synthesized on the graphene channel. This copolymer provides adequate electrostatic charge along with non-denaturing environment to immobilize enzyme and improves the pH sensitivity of GFETs. Acetylcholine is identified as a neurotransmitter with a significant role in the cholinergic system. Several brain disorders like Parkinson’s disease, depression, Alzheimer’s disease, addiction, epilepsy and schizophrenia are linked to any inappropriate alteration of transmission in central cholinergic system. Since inadmissible levels of acetylcholine are proved to be related to the disorders mentioned above, the need to develop a technique with higher sensitivity to detect acetylcholine seems crucial 3.
5. Graphene Field-Effect Transistors Structure/Fabrication
So far, graphene field-effect transistors have been fabricated out of graphene nano mesh (GNM-FET) which is an array of nano-sized holes punched on graphene 4, graphene sheets (GS-FET) 5 and graphene nanoribbons (GNR-FET) 6. Unlike GS-FETs with a zero bandgap, GNR-FETs and GNM-FETs adopt bandgaps with considerable energy depending on the width of ribbon and neck and chirality. In details, these types of graphene field-effect transistors convert charges and extracellular voltage signals of biomolecules at the FETs’ gate into altered current-voltage characteristics. These GFETs can be set for label-free cell or biosensing recording in a minimized probe or platform in order to act as a potential alternative instead of the common electrochemical and optical techniques 1. A thin layer of graphene field-effect transistor has been fabricated on a doped Si substrate. This GFET exhibits transporting behavior which is either a vertical or horizontal carrier used to increase the density of transistor on chip through integrating three dimensional technologies 7.
As a promise of graphene and its broad applications, graphene field-effect transistors (GFET) are going to bring about unprecedent changes in electronics, biosensors, chemical sensors, gas sensors, pH sensors, metal ionic sensors, thermal transistors and phototransistors with higher sensitivity and selectivity.