Chapter 1: The Physics and Operating Principles of Field-effect Transistor-based Biosensors

Biosensors were first developed by Leland Clark Jr in 1956 for oxygen detection in the blood. This was followed by the demonstration of an amperometric enzyme electrode for the detection of glucose by Clark and Lyons in 1962,^1,2  following which the concept has been widely employed in various applications such as environmental monitoring, genetic screening, sports surveillance, agriculture, food pathogens, and medical diagnostics.^3–7  The current biosensors, especially those used in healthcare, provide more precise detection, are lightweight and easy to use, and work on low energy consumption to obtain information quickly compared to conventional assays. The commercialization of biosensors in the marketplace is evident from recent breakthroughs in this domain. This development aims to offer point-of-care testing (POC) capabilities, namely in the area of glucose monitoring, cholesterol, and Alzheimer’s disease biosensors,⁵  especially for remote areas. These commercial biosensor products offer label-free detection with better reliability, high signal-to-noise ratio (SNR), and low instrumentation. Biosensors typically consist of three components: specific bioreceptors that are able to recognize and capture a specific target molecule, such as nucleic acids, antibodies, aptamers, microorganisms, or enzymes; a transducer which will convert the reaction into an electrical signal that can be evaluated, quantified, and processed by hardware and software and displayed by the third component of the biosensor.^8,9  The primary component of a biosensor is depicted in a schematic diagram shown in Figure 1.1.

Amongst various types of biosensors, field effect transistor (FET)-based biosensors are the most promising due to their immense advantages such as high sensitivity, scalability, rapid real-time detection, inherent amplification, lower power requirements, direct electrical readout, and low-cost mass production compared to other established techniques.^9,10  FET-based biosensors are considered label-free electrochemical biosensors using electrochemical principles to detect biomolecules in a sample without the need for a labelling or tagging process. In other words, they do not require the use of a secondary molecule (such as a fluorescent or radioactive label) to detect the target molecule.^11,12  Label-free electrochemical biosensors have several advantages over traditionally labelled biosensors, including simplicity, speed, and cost-effectiveness.^13,14 

FETs are electronic devices that control the flow of electrical current through a semiconductor (acts as the channel) by varying an electric field. They consist of a gate electrode, a source electrode, and a drain electrode. FET-based biosensors operate by utilizing the gate region of the FET to detect the presence of biological molecules, such as antibodies, DNA probes, or enzymes. The operating principle of FET biosensors is based on the modulation of the gate voltage induced by the binding of target molecules to the surface (mostly the channel), which alters the electrical properties of the FET.^15–17  The schematic diagram of a field effect transistor (FET) is depicted in Figure 1.2.

The sensitivity of FET-based biosensors depends on the ability of the surface to capture target molecules selectively. This can be achieved by modifying the surface with specific receptor molecules, such as antibodies or peptides, that recognize and bind to the target molecules with high affinity and selectivity, allowing for the detection of even trace amounts of target molecules. The binding of the target molecules to the receptor molecules on the surface induces a local charge distribution that alters the gate potential and hence the FET electrical characteristics. The term biologically sensitive field-effect transistors (BioFETs) became familiar due to the binding of the bioreceptors to the channel/gate surface.^16,18–20  Furthermore, FET biosensors could make use of mature manufacturing techniques from the complementary metal–oxide semiconductor (CMOS) process to further enhance its advantages in terms of miniaturization, parallel sensing, and the integration with other solid-state platforms and systems for the development of biosensing devices.^10,21,22 

Further enhancement could be achieved by the incorporation of various nanomaterials to enhance its sensitivity and selectivity, and organic/inorganic materials such as polymeric and metal oxide materials could contribute to the overall improvement.²³  The properties of such nanomaterials provide excellent interfacing for biological identification, which improves electronic signal transduction and is useful for designing novel bioelectronic devices.^24,25  Different nanomaterials in various forms, such as magnetic nanoparticles, silicon nanowires, and different structures of titanium dioxide, zinc oxide, gallium nitride, and various carbon-based materials (graphene, graphite, and carbon nanostructures),^26,27  have been employed for the development of FET-based biosensors.^9,17,21,28  The incorporation of nanomaterials for the fabrication of FET-based biosensors has demonstrated a very high successful rate of binding the target-specific entities in the specific gate area for high functionality. This chapter is organized as follows: Section 1.2 will describe the structure, materials, bio-functionalization, and operating principle of FET-based biosensors. Section 1.3 will summarize the “performance criteria” of BioFET sensors. In Section 1.4, the most common configurations of FETs used as biosensors will be explained, and Section 1.5 will explore the current state of affairs in FET-based biosensor applications.

In this section, we will briefly go through the structure of FETs, the material that is typically utilized in the production of FETs, as well as the operating principle of these devices.

Field effect transistors are manufactured according to the standard semiconductor manufacturing process. However, due to the emergence of new applications such as wearable and flexible electronics, materials such as paper and polymers have been employed to fulfill the requirements for such applications. Given its abundance in nature and the established manufacturing process, silicon is expected to remain the predominant element in conventional semiconductor technology.²⁹  Silicon is used as the substrate of most semiconductor devices. Initially, silicon is covered with an insulator layer by oxidizing its surface with silicon dioxide, which will play the role of a gate dielectric to create the device known as a MOSFET (metal oxide semiconductor field effect transistor). As previously mentioned, the device has three terminals: a source, a drain, and a gate. The space between the drain and source terminals is known as the channel through which the current flows.^10,30  Usually, it is functionalized with a biological recognition element (bioreceptors) that interacts with the analyte and senses its presence and concentration.^31,32  The third electrode “the gate” is separated from both the source and the drain electrode by a dielectric layer. This electrode is the control terminal of the device. It controls the flow of the current between the drain and source terminals by applying a bias voltage to the gate terminal.³³  Biasing the gate terminal will generate an electric field that modulates the current flow through the channel. If there is no bias or low bias voltage at the gate terminal, the FET is in the ‘off’ state.⁸  In fact, the FET operates by controlling the bias voltages over its both (drain and the gate) source terminals. There are several FET configurations according to the position of the gate terminal towards the drain–source terminals, which can be back, top, both sides (dual gate) and solution/electrolyte gated FET biosensors,^15,34  which are known as electrolyte-gated field effect transistors³⁵  (due to the importance of electrolyte-gated FET biosensors, further discussion will be undertaken in the subsequent chapter). However, most FET biosensors are based on an architecture known as an ion sensitive field effect transistor (ISFET), where the metal gate is replaced by a reference electrode in contact with the gate oxide via the electrolytic solution.^16,31,33,36  The reference electrode will maintain the performance stability of a biosensing system. Figure 1.3 shows the schematics of both devices: a MOSFET and an ISFET.

Silicon dioxide was the first material to be tested for biosensing applications.³⁷  Due to the low stability and drift issues in aqueous solutions,³⁸  several alternative materials were introduced to achieve better stability and lower the drifting issue. Here, we will summarize some of these materials that display better performance. Oxides such as ZnO, TiO₂, Al₂O₃, SnO₂, ZrO₂, HfO₂, GO_x and others have been heavily investigated.^39,40  There are several reasons why these oxide-based materials are preferable as sensing-based materials, such as a cost-effective growth process, the ability to grow over different types of substrates with good quality, biocompatibility, and easy integration in the conventional semiconductor CMOS process.^22,41–43  Nitride-based materials such as Si₃N₄ and TiN were tested as BioFET sensors,^44,45  and both reveal a better performance than that reported for SiO₂ due to their thermodynamic stability.⁴⁶ 

Recently, there has been significant interest in advanced materials centered around carbon and its derivatives, notably carbon nanotubes (CNTs) along with single- and multi-walled CNTs. CNTs are unique one-dimensional nanostructures formed through C–C sp² hybridization,⁴⁷  composed of one or more layers of graphite arranged in the shape of hollow cylindrical tubes. Furthermore, owing to their remarkable characteristics, such as high conductivity, relatively high carrier mobility,⁴⁸  and robust chemical stability, with an impressive surface-to-volume ratio that enhances detection sensitivity, CNTs and related structures have found widespread utility in various biosensor applications. They serve as channels in field-effect transistor FET biosensors, proficiently detecting a spectrum of bioanalytes such as nucleic acids, viruses, and structural proteins of various pathogens.²⁶  Furthermore, graphene and its derivatives, graphene oxide (GO) and reduced graphene oxide (RGO), are also part of the carbon family. Graphene, a single-atom-thick layer of carbon, boasts exceptional properties that position it as a prime candidate for cutting-edge technologies and electronic devices. Its characteristics parallel those of CNTs, making it a prominent player in FET platforms. Researchers have extensively explored its applications, particularly in pathogen detection encompassing bacteria like Escherichia coli and viruses including HIV, avian influenza, Ebola, and coronaviruses, among others. This functionality is achieved through the binding of analyte targets to their corresponding receptors.^35,49–52  The resultant alteration in conductivity is attributed to changes in charge carriers on the graphene, GO, and RGO surfaces, induced by the presence of the analyte. For instance, Kakatkar et al.⁵³  demonstrated a graphene-based FET biosensor capable of detecting double-stranded DNA and poly-l-lysine. The sensor registered shifts in the Dirac voltage in response to the binding or non-binding of charged biomolecules to the graphene surface. Reports on RGO biosensors for the label-free detection of peptide-nucleic acid (PNA)-DNA hybridization⁵⁴  achieved a detection limit of less than 100 femtomolar (fM). Furthermore, RGO-FET was reported to detect Ebola virus⁵¹  in the concentration range from 2.4 × 10⁻¹² g mL⁻¹ to 1.2 × 10⁻⁷ g mL⁻¹, with a limit of detection as low as 2.4 pg mL⁻¹.

In spite of the fact that carbon-based materials have great electron mobility, off-state current leakage in graphene-based biosensors may increase due to its near-zero band gap, leading to misleading indications.^10,55  Two-dimensional (2D) transition metal dichalcogenides such as WSe₂ and MoS₂ monolayers have recently emerged as potential materials for biosensing applications. They have been under extensive investigation as channels in BioFET devices, indicating a promising biosensing performance under different bioanalyte contents.^56,57  In addition to the above inorganic-based materials, organic field-effect transistors (OFETs) have emerged as one of the most advanced biosensor platforms owing to their unique signal amplification capabilities, high sensitivity, rapid analysis, label-free detection, small size and simple operation,^58,59  and they have become potential candidates for biosensing applications where they have been extensively applied in the detection of DNA, proteins, antibodies, antigens, and various biological substances.^60–62 

The preparation methods of materials used as channels in BioFETs play a crucial role in their performance. Different methods were employed for this purpose, such as sol–gel, hydrothermal, RF/DC sputtering, chemical vapor deposition, metal organic chemical vapor deposition, and spin coating, among others.^63–65  The crucial step in preparing BioFETs is surface immobilization with bioreceptors (biomolecules), such as antibodies, nucleic acids, or enzymes that can recognize and bind with the target analyte. The selection of bioreceptors is primarily based on the specific analyte that needs to be detected. This layer will enhance the sensitivity, selectivity, reliability, and stability of the biosensors in general. The bioreceptors are normally immobilized on the channel surface of the FET.^57,64,66  For a stable sensing performance of biosensors, bioreceptors must be capable of preserving their structure, function, and biological activity.³²  There are two main immobilization methods; one is the physical immobilization method, in which the bioreceptors are attached to the surface of the FET gate or the channel without the creation of chemical bonds. The most common methods for the physical immobilization process are adsorption, bioaffinity binding, and chelation or metal binding, where bonding occurs through non-covalent interactions. In this process, the bioreceptors can be detached from the transducer material without destroying either the biological activity or the transducer. The process is also known as the reversible immobilization method. On the other hand, the chemical immobilization method can be covalent binding, covalent cross-linking, or entrapment/micro-encapsulation. Here, the process is more complicated and it can be referred to as a functionalization process, which requires a more complex process than immobilizing the probes only.²¹  These processes are based on chemical bonding. Here, the bioreceptors cannot be detached without destroying either their biological activity or the transducer. The process is also known as the irreversible immobilization method.^32,67  Figure 1.4 depicts various approaches that have been used widely for bioreceptor immobilization.

A biosensor converts the biological information into a measurable signal that can be displayed, processed, or sent for further analysis.^1,10,22,57  The operation of a biosensor involves a change in analyte concentration, which alters the charge near the sensor interface, inducing a change in effective gate voltage and leading to changes in drain current that can be evaluated from the I–V characteristics. The main parameters that are usually monitored in BioFETs is the drain–source current (I_DS) (that represents the current following through the channel) or the shift in the threshold voltage (V_TH).^10,22,35,40  Both parameters are related to the analyte contents in a solution. In fact, the FET operates depending on both the gate–source and drain–source voltages. By controlling these two parameters, the FET is said to be operating in one of the three main regions: sub-threshold, linear, and saturation.^10,68  Figure 1.5 depicts the three regions of the FET operation depending on the V_DS and V_GS.

In the linear region of a BioFET, the output of the BioFET will reflect how the analyte content modifies the V_TH (at constant I_DS and V_DS). This can be achieved by sweeping the gate voltage at constant V_DS and recording the I_DS output. The relation is known as transfer characteristics, where the measurements are performed to find the relationship between I_DS and V_GS as a function of the analyte contents. The variation of the analyte contents will result in changes of the threshold voltage V_TH, where a horizontal shift in the transfer curve is obtained, which might be attributed to either the transfer of charge between the analyte and the FET channel or a result of electrostatic gating.^10,35,69  In the context of two-dimensional materials, such as graphene-based field-effect transistors (FETs), a significant transition voltage exists that separates the regions of hole-dominant and electron-dominant conduction. This transition voltage corresponds to the point at which I_DS achieves its minimal value. This point is known as the Dirac point, and the voltage at this point is known as the Dirac voltage (V_Dirac). At the Dirac voltage, there are very few charge carriers in the device. Consequently, it is appropriate for monitoring changes in the device’s environment, making it an attractive feature for biosensing applications.^9,57  In the saturation region of the FET operation, I_DS can be related to the analytes. Further, the dynamic behavior of I_DS vs. time can be recorded with the variation of analyte content at fixed values of both V_GS and V_DS. The I_DS vs. time measurement is useful for studying the kinetics of analyte–BioFET interactions. However, it does not reveal the physical mechanism of the reactions due to the fixed bias voltages.³³  Typical transfer, the Dirac voltage point, and the I_DS curve obtained from a BioFET in both operating regions are depicted in Figures 1.6 and 1.7, respectively. The voltage sensitivity towards the analyte contents can be found in the transfer curve.

For practical reasons, it is common practice to utilize an electrolyte as the gate dielectric in the BioFET structure since most of the bioanalytes are found in physiological media (such as blood, serum, plasma, saliva, and urine). The gate is usually replaced by reference electrodes such as Ag/AgCl (silver/silver chloride).⁷⁰  However, a conventional reference electrode is unsuitable for miniaturized biosensors, so a pseudo-reference electrode is utilized. In this configuration, the electrolyte will be in direct contact with both the gate and the channel (insulator/semiconductor) layer. The operating principle relies on tuning the electrical current traveling through the channel by applying a gate voltage, which results in the formation of two electrical double layers (EDLs) at the insulator/electrolyte and electrolyte/gate interfaces.^35,71  Since the reaction between the analytes with the bioreceptors (mostly the reactions are in the channel part) will result in charge release, the potential at the gate/electrolyte interface will be modified, thus affecting the current flowing between the drain and source (I_DS) due to the V_TH shift.⁷² 

Alternatively, the change in V_TH due to analyte binding can be measured directly from the shift in the I_DS−V_GS curve.¹⁰  The change in charges at the gate surface caused by the presence of bioanalytes is analogous to applying a voltage to the gate of a MOSFET device. It is known that almost all biomolecule reactions are leading to a local change of the pH value, which can bind or release protons (protonation/deprotonation events).^73–75  Accordingly, the sensing mechanism of biomolecules can be dealt with as a pH sensing mechanism.^16,34,75 

The modification of I_DS depends on the charge–exchange properties of the analyte and the sensing membrane. The charges from analytes affect the conductivity of the FET channel due to attractive or repulse electrostatic force.^63,80  Figure 1.7 depicts the working principle of an n-type channel FET. As the BioFET faces the neutral biofluid, the I_DS shows a stable output (Figure 1.7(a)). For bioanalytes with negative charges (such as DNA molecules), this will lead to repulsive electrostatic force between the negatively charged analytes and the n-type charge carriers of the channel (Figure 1.7(b)); consequently, a depletion of charge carriers inside the channel leads to a decrease in the electrical conductance and a drop will be noticed on the output signal (I_DS). In the other case, where the bioanalytes release a positive charge (such as small-size protein) (Figure 1.7(c), positive charges accumulate on the surface of the n-type semiconductor, resulting in electrostatic attraction resulting in a higher output signal (I_DS).^10,33,35 

BioFET performance must be characterized using a variety of criteria, both static and dynamic. For a biosensor system to be effective and powerful, the standards below will be used to optimize the functionality of biosensors in practical settings.⁸¹ 

• Sensitivity is defined as the smallest amount of analyte that can be accurately detected or identified in the fewest steps at low concentrations (ng mL⁻¹ or fg mL⁻¹) to confirm the presence of analyte traces in the sample.

• Linearity contributes to the precision of the measured outcomes. The higher the detected analyte concentration, the greater the linearity. Furthermore, it is closely linked to the biosensor's resolution, which is defined as the smallest change in the target analyte concentration required for the biosensor to respond.

• Selectivity is an essential characteristic to consider when choosing a bioreceptor for a biosensor. A bioreceptor is capable of detecting a specific target analyte molecule in a mixture of spices and contaminants.

• The biosensor response time is when the device produces a measurable signal or response after introducing the target analyte to the sensor surface. It is the time necessary to reach 90% of the steady-state response.

• Reproducibility is the ability of a sensor to generate a mean value closer to the actual value when the sample is measured repeatedly. It also reflects the accuracy of the sensor.

• Stability is one of the most important characteristics in biosensor applications that require continuous monitoring. Stability is the susceptibility of a biosensor to internal and external environmental disturbances. Furthermore, it is affected by the affinity of the bioreceptor (how well the analyte binds to the bioreceptor) and the degradation of the bioreceptor over time.

This section provides an overview of different field-effect transistor sensor architectures that have been explored in the field of biosensors. The electrolyte gated FET is the most potential BioFET platform that detects a variety of pathogens.⁹  For its importance, Chapter 2 will cover its mechanism and applications in detecting pathogens. For this reason, this platform will be avoided in the current chapter. Here, we will summarize the other types of FET platforms that have been used for pathogens (BioFETs).

The first work on FET biosensors is initiated by Bergveld in the early 1970s through the introduction of ion-sensitive field effect transistors (ISFETs) in order to measure the value of pH in solution. In an ISFET, the current is produced through the channel surface of the ISFET depending on the ion activities, which could be used to evaluate the acidity of solutions. As shown in Figure 1.3, its configuration is slightly different compared to conventional MOSFETs where the metallic gate is separated and the underlying gate oxide is inserted in an aqueous solution along with a reference electrode (e.g., Ag/AgCl electrode) through which the gate voltage is applied, and the gate dielectric layer is replaced with an electrolyte. Several gate modifications have been proposed to enhance biomolecular interaction detection, such as enzyme-modified field-effect transistors,⁸²  electrolyte gated FETs (EG-FETs)³⁵  and others.²²  The gate dielectric could be made from oxide, nitride, graphene, or organic-based membranes, usually deposited between the two electrodes (drain and source). The interaction with biomolecules would occur through the capture of the bioanalytes by the receptors in the electrolyte, thus releasing charges and consequently directly changing the potential of the FET channel surface; carriers will accumulate or deplete, increasing or decreasing the source–drain current.⁸³ 

Under ISFET architectures, the detection of analyte contents is achieved by direct contact with the gate dielectric membrane. This will potentially deteriorate and reduce the operational lifetime of the device due to direct contact of the channel with the analytes. An alternative approach was introduced by Spiegel et al.⁸⁴  In their configuration, an external electrode was attached to the gate of a separated MOSFET. The configuration is known as separative extended-gate FET (SEG-FET), also known as an extended-gate field-effect transistor (EG-FET).⁸⁵  It is the most straightforward FET structure that could be used for general (bio)chemical sensing.⁸⁶  The SEG-FET consists of two separate parts: a sensing (the electrode) unit and a transducer unit (the MOSFET). The sensing part is immersed into the test solution, whereby the MOSFET is kept in a dry environment. Several MOSFET models have been used for this task, and one of the most commercial ICs used is CD4007UB.⁸⁷  This configuration in which the sensing part is separated from the transducer becomes suitable for POC applications.⁸⁸  It has been used for label-free bacterial detection such as E. coli.⁸⁹  Here, the sensing area of the external electrode was functionalized with porphyrins and immersed in liquid containing bacteria. The negative charge on the surface induced by E. coli cells will change the surface potential of the electrode reflected directly in the electrical characteristics of the SEG-FET. The SARS-CoV-2 genome⁸⁸  was detected in wastewater samples with the same platform. The limit of detection was 0.31 × 10⁻³ ng µL⁻¹, and the device provides its results within a short measurement time. Furthermore, SEG-FETs have also been employed to detect DNA,^90–92  proteins,⁹³  and viruses.^94,95  The SEG-FET structure possesses the advantages of a straightforward operation and cost-effectiveness in the context of personal healthcare devices. Figure 1.8 depicts a typical measurement system employing an extended electrode for the SEG-FET platform.

Floating gate field effect transistors (FG-FETs)⁶⁹  are another interesting FET platform for bio- and chemical sensing. The platform has two gates and one gate serves as a sensing gate, which will yield a response from the desired target, and the other as a control gate (used for biasing). Both gates operate similarly and are capacitively coupled to a shared floating gate.²²  With this configuration, the floating electrode protects the oxide sensing region on top of the channel.³⁴  A floating-gate FET sensor can detect changes in electric charge caused by various stimuli, such as the presence of certain biomolecules.^96,97  The magnitude of the current flowing through the transistor is dependent on the control voltage applied and the charges present on the sensing surface.⁹⁸  Any variations directly influence the electrical characteristics of the field-effect transistor (FET) in charge. In contrast to the ISFET, the reference electrode is not required. A floating-gate FET sensor can be used to detect and quantify biological molecules, such as DNA,⁹⁷  proteins, and antibodies.⁹⁶  The use of a floating gate allows for highly sensitive detection of these molecules with high specificity and selectivity.⁹⁹ 

The sensitivity of a floating-gate FET biosensor depends on the amount of target molecule that binds to the bioreceptor, as well as the sensitivity of the floating gate to change with the accumulating charges. One advantage of floating-gate FET biosensors is that they can be easily integrated into electronic devices for signal processing and data analysis.^102,103  This allows for real-time monitoring of biological processes and rapid detection of target molecules in complex biological samples, such as blood or saliva.⁹⁶  In terms of applications, they can be used to detect pathogens in food samples and in patients, screen for genetic mutations in DNA samples^102,103  and viruses such as the coronavirus.⁹⁶  Figure 1.9 depicts the schematics of both the ISFET and FG-FET for comparison.

While traditional biosensors have limitations, such as low sensitivity and difficulty in detecting neutral biomolecules, DM-FET devices offer a more efficient solution. These devices are able to detect both charged and neutral biomolecules in small sample volumes with high sensitivity and fast screening.^104,105  A DM-FET biosensor (Figure 1.10(a) and (b)) consists of a nanogap positioned at the gate dielectric edge where it formed between the gate and gate oxide on both sides (source and drain junction) to accommodate the target analytes.

Due to these effects in the n-type channel, the integration of the sensor surface and an amplifier in the shift in V_TH is not collaborative, but it is in the same direction for a p-type channel. Due to its great sensitivity, a p-channel DM-FET is preferred over an n-channel DM-FET. DM-FETs have been successfully applied for various pathogen sensing applications, such as SARS-CoV-2,¹⁰⁷  DNA¹¹⁰  and avian influenza.¹¹² 

Recently, dual-gate field-effect transistors (DG-FETs) emerged as advanced biosensors. The dual-gate system utilizes two independent gate dielectrics: the top gate and the bottom gate. This will result in the amplification of the sensor signals and greatly improve the performance of sensor systems by enhancing the sensitivity and detection capabilities.²²  One of the gates is the sweeping gate that measures the voltage signal, and the second gate is the biasing gate (supporting gate). The biasing gate will ensure the sensor operates in a sensitive region.¹¹³  Figure 1.11 depicts the schematic of a DG-FET. With this platform, it was reported that the sensitivity of the DG-FET for pH has exceeded the Nernstian limit from 59.16 mV pH⁻¹ up to 304.12 mV pH⁻¹.¹¹⁴  This reflects the enhancement of the device by adding the extra gate. The DG-FET has been further enhanced through adding a gap at the gate dielectric edge, which will behave as a dual-gate dielectric modulated FET.^115,116  The platform has been successfully used for the detection of SARS-CoV-2,^117,118  nucleic acid such as miRNA,¹¹⁹  DNA¹¹⁶  and proteins.¹²⁰ 

Field effect transistor-based biosensors have garnered considerable attention and have been the subject of intensive research in recent years. The field of FET-based biosensing continues to advance rapidly. As researchers strive to improve the performance, sensitivity, and specificity of these devices, new opportunities for their use in different applications are emerging. The utilization of BioFETs presents numerous benefits, including label-free biomolecule sensing, enhanced sensitivity, rapid detection, ultra-low limit of detection, compatibility with the conventional CMOS manufacturing process, adaptability to read-out circuits for on-chip integration, design flexibility, and compatibility with microfluidic systems. The latter has been a breakthrough, which allows for precise sample handling and control, enabling real-time and continuous monitoring of analytes. This integration not only enhances the performance of FET-based biosensors but also reduces the sample volume and minimizes the risk of cross-contamination. The advancements in the field of BioFETs lead to the development of a variety of biosensors that can overcome the limitations of other types of biosensors, such as optical and electrochemical-based biosensors. Research articles highlight the progress made in the field of BioFETs, specifically focusing on their applications in various areas such as infectious diseases, biomarker detection, and biomedical applications. Further studies depict the improvements in the detection sensitivity of BioFETs by utilizing different bioreceptors such as aptamers, making them well-suited for varieties of diagnostics. An important development in the field is the substitution of conventional insulating layer materials with electrolytes, specifically electrolyte-gated FETs, which has facilitated the broadening of applications for BioFETs. With ongoing research and development, they are expected to play a crucial role in the future of biosensing technologies.

Several BioFET biosensors have a wide range of structural designs and sensing materials, resulting in numerous combinations of biosensors. These design variations have improved sensor performance and device parameters, expanding the field of application. Sensing materials with exceptional properties, such as high charge mobility or mechanical strength, such as graphene FETs, have added diversity to the field of FET biosensors. Nanowire FETs offer a broad limit of detection and high sensitivity. Research is focused on incorporating nanotechnology by exploiting different nanomaterials and nanostructures for miniature and highly efficient BioFET devices. Multidisciplinary approaches, combining knowledge from different fields, have opened up opportunities for novel sensor architectures. Due to their advantages, such as electronic output signals, high sensitivity, integration flexibility, low-cost fabrication, and disposable strip-type sensing systems, these devices became suitable for point-of-care (POC) testing applications. Consequently, BioFETs have become competitive candidates for POC applications compared to bulky optical-based biosensor instruments.

Despite the improvements in the technology, there are challenges in the practical implementation of BioFETs due to variations in the FET structures, target analyte properties, sensing components, and testing environments. The lack of a common design rule for BioFETs hinders their technical maturation and commercial acceptance. Once the device performance parameters are optimized, highly efficient and reliable biosensors can be designed.

In conclusion, the field of FET biosensors has undergone tremendous development in recent years, thanks to their scalability, high sensitivity, real-time detection, inherent amplification, and low power requirements. Researchers have successfully detected a wide range of biomolecular targets of varying sizes, such as nucleotides (RNA and DNA), amino acids, antibodies and antigens, to diagnose various diseases. Different types of FET-based biosensors, including ISFETs, SEG-FETs, and FG-FETs, have been used to achieve these results. Despite facing some challenges, FET-based biosensors hold great promise for healthcare applications, and they could become the next generation of biomedical detection devices due to their robust chemical properties and customizable biosensing functions. Continued research and development could improve the versatility and effectiveness of these devices.

This work was supported by the Universiti Kebangsaan Malaysia (UKM) through matching short-term grant number GPS-DPK-2023-002 (UKM).