Sensing with Advanced Computing Technology: Fin Field-Effect Transistors with High-k Gate Stack on Bulk Silicon

Potentiometric Chloride Ion Biosensor for Cystic Fibrosis Diagnosis and Management: Modeling and Design

The ion-sensitive field-effect transistor is a well-established electronic device typically used for pH sensing. The usability of the device for detecting other biomarkers in easily accessible biologic fluids, with dynamic range and resolution compliant with high-impact medical applications, is still an open research topic. Here, we report on an ion-sensitive field-effect transistor that is able to detect the presence of chloride ions in sweat with a limit-of-detection of 0.004 mol/m³. The device is intended for supporting the diagnosis of cystic fibrosis, and it has been designed considering two adjacent domains, namely the semiconductor and the electrolyte containing the ions of interest, by using the finite element method, which models the experimental reality with great accuracy. According to the literature explaining the chemical reactions that take place between the gate oxide and the electrolytic solution, we have concluded that anions directly interact with the hydroxyl surface groups and replace protons previously adsorbed from the surface. The achieved results confirm that such a device can be used to replace the traditional sweat test in the diagnosis and management of cystic fibrosis. In fact, the reported technology is easy-to-use, cost-effective, and non-invasive, leading to earlier and more accurate diagnoses.

Planar Junctionless Field-Effect Transistor for Detecting Biomolecular Interactions

Label-free field-effect transistor-based immunosensors are promising candidates for proteomics and peptidomics-based diagnostics and therapeutics due to their high multiplexing capability, fast response time, and ability to increase the sensor sensitivity due to the short length of peptides. In this work, planar junctionless field-effect transistor sensors (FETs) were fabricated and characterized for pH sensing. The device with SiO₂ gate oxide has shown voltage sensitivity of 41.8 ± 1.4, 39.9 ± 1.4, 39.0 ± 1.1, and 37.6 ± 1.0 mV/pH for constant drain currents of 5, 10, 20, and 50 nA, respectively, with a drain to source voltage of 0.05 V. The drift analysis shows a stability over time of −18 nA/h (pH 7.75), −3.5 nA/h (pH 6.84), −0.5 nA/h (pH 4.91), 0.5 nA/h (pH 3.43), corresponding to a pH drift of −0.45, −0.09, −0.01, and 0.01 per h. Theoretical modeling and simulation resulted in a mean value of the surface states of 3.8 × 10¹⁵/cm² with a standard deviation of 3.6 × 10¹⁵/cm². We have experimentally verified the number of surface sites due to APTES, peptide, and protein immobilization, which is in line with the theoretical calculations for FETs to be used for detecting peptide-protein interactions for future applications.

Comprehensive Analytical Modelling of an Absolute pH Sensor

In this work, we present a comprehensive analytical model and results for an absolute pH sensor. Our work aims to address critical scientific issues such as: (1) the impact of the oxide degradation (sensing interface deterioration) on the sensor’s performance and (2) how to achieve a measurement of the absolute ion activity. The methods described here are based on analytical equations which we have derived and implemented in MATLAB code to execute the numerical experiments. The main results of our work show that the depletion width of the sensors is strongly influenced by the pH and the variations of the same depletion width as a function of the pH is significantly smaller for hafnium dioxide in comparison to silicon dioxide. We propose a method to determine the absolute pH using a dual capacitance system, which can be mapped to unequivocally determine the acidity. We compare the impact of degradation in two materials: SiO2 and HfO2, and we illustrate the acidity determination with the functioning of a dual device with SiO2.

Process Variability in Top-Down Fabrication of Silicon Nanowire-Based Biosensor Arrays

Silicon nanowire field-effect transistors (SiNW-FET) have been studied as ultra-high sensitive sensors for the detection of biomolecules, metal ions, gas molecules and as an interface for biological systems due to their remarkable electronic properties. "Bottom-up" or "top-down" approaches that are used for the fabrication of SiNW-FET sensors have their respective limitations in terms of technology development. The "bottom-up" approach allows the synthesis of silicon nanowires (SiNW) in the range from a few nm to hundreds of nm in diameter. However, it is technologically challenging to realize reproducible bottom-up devices on a large scale for clinical biosensing applications. The top-down approach involves state-of-the-art lithography and nanofabrication techniques to cast SiNW down to a few 10s of nanometers in diameter out of high-quality Silicon-on-Insulator (SOI) wafers in a controlled environment, enabling the large-scale fabrication of sensors for a myriad of applications. The possibility of their wafer-scale integration in standard semiconductor processes makes SiNW-FETs one of the most promising candidates for the next generation of biosensor platforms for applications in healthcare and medicine. Although advanced fabrication techniques are employed for fabricating SiNW, the sensor-to-sensor variation in the fabrication processes is one of the limiting factors for a large-scale production towards commercial applications. To provide a detailed overview of the technical aspects responsible for this sensor-to-sensor variation, we critically review and discuss the fundamental aspects that could lead to such a sensor-to-sensor variation, focusing on fabrication parameters and processes described in the state-of-the-art literature. Furthermore, we discuss the impact of functionalization aspects, surface modification, and system integration of the SiNW-FET biosensors on post-fabrication-induced sensor-to-sensor variations for biosensing experiments.

ANTIGONE: A Programmable Energy-Efficient Current Digitizer for an ISFET Wearable Sweat Sensing System

This article describes the design and the characterization of the ANTIGONE (ANalog To dIGital cONvErter) ASIC (Application Specific Integrated Circuit) built in AMS 0.35 m technology for low dc-current sensing. This energy-efficient ASIC was specifically designed to interface with multiple Ion-Sensitive Field-Effect Transistors (ISFETs) and detect biomarkers like pH, Na+, K+ and Ca2+ in human sweat. The ISFET-ASIC system can allow real-time noninvasive and continuous health monitoring. The ANTIGONE ASIC architecture is based on the current-to-frequency converter through the charge balancing principle. The same front-end can digitize multiple currents produced by four sweat ISFET sensors in time multiplexing. The front-end demonstrates good linearity over a dynamic range that spans from 1 pA up to 500 nA. The consumed energy per conversion is less than 1 J. The chip is programmable and works in eight different modes of operation. The system uses a standard Serial Peripheral Interface (SPI) to configure, control and read the digitally converted sensor data. The chip is controlled by a portable device over Bluetooth Low Energy (BLE) through a Microcontroller Unit (MCU). The sweat sensing system is part of a bigger wearable platform that exploits the convergence of multiparameter biosensors and environmental sensors for personalized and preventive healthcare.

Sensitivity, Noise and Resolution in a BEOL-Modified Foundry-Made ISFET with Miniaturized Reference Electrode for Wearable Point-of-Care Applications

Ion-sensitive field-effect transistors (ISFETs) form a high sensitivity and scalable class of sensors, compatible with advanced complementary metal-oxide semiconductor (CMOS) processes. Despite many previous demonstrations about their merits as low-power integrated sensors, very little is known about their noise characterization when being operated in a liquid gate configuration. The noise characteristics in various regimes of their operation are important to select the most suitable conditions for signal-to-noise ratio (SNR) and power consumption. This work reports systematic DC, transient, and noise characterizations and models of a back-end of line (BEOL)-modified foundry-made ISFET used as pH sensor. The aim is to determine the sensor sensitivity and resolution to pH changes and to calibrate numerical and lumped element models, capable of supporting the interpretation of the experimental findings. The experimental sensitivity is approximately 40 mV/pH with a normalized resolution of 5 mpH per µm², in agreement with the literature state of the art. Differences in the drain current noise spectra between the ISFET and MOSFET configurations of the same device at low currents (weak inversion) suggest that the chemical noise produced by the random binding/unbinding of the H⁺ ions on the sensor surface is likely the dominant noise contribution in this regime. In contrast, at high currents (strong inversion), the two configurations provide similar drain noise levels suggesting that the noise originates in the underlying FET rather than in the sensing region.

NH3 Plasma-Treated Magnesium Doped Zinc Oxide in Biomedical Sensors with Electrolyte-Insulator-Semiconductor (EIS) Structure for Urea and Glucose Applications

This study compared the sensing characteristics of ZnO (ZO) treated with ammonia (NH3) plasma for 1 min, 3 min, and 6 min, under the EIS structure. The measurement results revealed that, after 3 min of NH3 plasma treatment, the Mg-doped ZnO (MZO) sensing film had a high hydrogen ion sensitivity, linearity, hysteresis, and drift rate of 53.82 mV/pH, 99.04%, 2.52 mV, and 1.75 mV/h, respectively. The sensing film was used with sodium and potassium ion solutions, and it performed satisfactorily in sensing hydrogen ions. Additionally, we investigated the biomedical sensing properties of Mg-doped ZnO (MZO) sensing film with regard to urea, creatinine, and glucose solutions and found that the Mg-doped ZnO (MZO) sensing film treated with NH3 plasma for 3 min had the best properties for sensing urea, creatinine, and glucose. Specifically, with glucose, the sensing film achieved the best linearity and sensitivity and of 97.87% and 10.73 mV/mM, respectively. The results revealed that the sensing characteristics varied with the processing environment and are useful in the developing biomedical sensing applications with different sensing elements.

Size-dependent optical properties of periodic arrays of semiconducting nanolines

We study the size-dependent optical properties of periodic arrays of semiconducting nanolines in the near-infrared to near-ultraviolet spectral range, where the absorption of the semiconductor increases. Using band structure calculations, we demonstrate that specific dimensions allow the slow down of the light, resulting in an enhanced absorption as compared to bulk material once the extinction coefficient of the semiconductor becomes comparable to its refractive index. Further, the refractive properties of the arrays can be tailored beyond the values of the constituting materials when the extinction coefficient of the semiconductor exceeds its refractive index. To confirm our theoretical findings, we propose a simple semi-analytical model for the light interactions with such structures and validate it with experimental reflectance spectra collected on arrays for the next-generation transistors.

Emerging Designs of Electronic Devices in Biomedicine

A long-standing goal of nanoelectronics is the development of integrated systems to be used in medicine as sensor, therapeutic, or theranostic devices. In this review, we examine the phenomena of transport and the interaction between electro-active charges and the material at the nanoscale. We then demonstrate how these mechanisms can be exploited to design and fabricate devices for applications in biomedicine and bioengineering. Specifically, we present and discuss electrochemical devices based on the interaction between ions and conductive polymers, such as organic electrochemical transistors (OFETs), electrolyte gated field-effect transistors (FETs), fin field-effect transistor (FinFETs), tunnelling field-effect transistors (TFETs), electrochemical lab-on-chips (LOCs). For these systems, we comment on their use in medicine.

Sweat Biomarker Sensor Incorporating Picowatt, Three-Dimensional Extended Metal Gate Ion Sensitive Field Effect Transistors

Ion sensitive field effect transistors (ISFETs) form a very attractive solution for wearable sensors due to their capacity for ultra-miniaturization, low power operation, and very high sensitivity, supported by complementary metal oxide semiconductor (CMOS) integration. This paper reports for the first time, a multianalyte sensing platform that incorporates high performance, high yield, high robustness, three-dimensional-extended-metal-gate ISFETs (3D-EMG-ISFETs) realized by the postprocessing of a conventional 0.18 μm CMOS technology node. The detection of four analytes (pH, Na⁺, K⁺, and Ca²⁺) is reported with excellent sensitivities (58 mV/pH, -57 mV/dec(Na⁺), -48 mV/dec(K⁺), and -26 mV/dec(Ca²⁺)) close to the Nernstian limit, and high selectivity, achieved by the use of highly selective ion selective membranes based on postprocessing integration steps aimed at eliminating any significant sensor hysteresis and parasitics. We are reporting simultaneous time-dependent recording of multiple analytes, with high selectivities. In vitro real sweat tests are carried out to prove the validity of our sensors. The reported sensors have the lowest reported power consumption, being capable of operation down to 2 pW/sensor. Due to the ultralow power consumption of our ISFETs, we achieve and report a final four-analyte passive system demonstrator including the readout interface and the remote powering of the ISFET sensors, all powered by an radio frequency (RF) signal.

Pixel-based biosensor for enhanced control: silicon nanowires monolithically integrated with field-effect transistors in fully depleted silicon on insulator technology

Silicon nanowires (SiNWs) are a widely used technology for sensing applications. Complementary metal-oxide-semiconductor (CMOS) integration of SiNWs advances lab-on-chip (LOC) technology and offers opportunities for read-out circuit integration, selective and multiplexed detection. In this work, we propose novel scalable pixel-based biosensors exploiting the integration of SiNWs with CMOS in fully-depleted silicon-on-insulator technology. A detailed description of the wafer-scale fabrication of SiNW pixels using the CMOS compatible sidewall-transfer-lithography as an alternative to widely investigated time inefficient e-beam lithography is presented. Each 60 nm wide SiNWs sensor is monolithically connected to a control transistor and novel on-chip fluid-gate forming an individual pixel that can be operated in two modes: biasing transistor frontgate (V _G) or substrate backgate (V _BG). We also present the first electrical results of single N and P-type SiNW pixels. In frontgate mode, N and P-type SiNW pixels exhibit subthreshold slope (SS) ≈ 70-80 mV/dec and I _on/I _off ≈ 10⁵. The N-type and P-type pixels have an average threshold voltage, Vth of -1.7 V and 0.85 V respectively. In the backgate mode, N and P-type SiNW pixels exhibit SS ≈ 100-150 mV/dec and I _on/I _off ≈ 10⁶. The N and P-type pixels have an average V _th of 5 V and -2.5 V respectively. Further, the influence of the backgate and frontgate voltage on the switching characteristics of the SiNW pixels is also studied. In the frontgate mode, the V _th of the SiNW pixels can be tuned at 0.2 V for 1 V change in V _BG for N-type or at -0.2 V for -1 V change in V _BG for P-type pixels. In the backgate mode, it is found that for stable operation of the pixels, the V _G of the N and P-type transistors must be in the range 0.5-2.5 V and 0 V to -2.5 V respectively.

A Doping-Less Tunnel Field-Effect Transistor with Si0.6Ge0.4 Heterojunction for the Improvement of the On–Off Current Ratio and Analog/RF Performance

In this paper, a novel doping-less tunneling field-effect transistor with Si0.6Ge0.4 heterojunction (H-DLTFET) is proposed using TCAD simulation. Unlike conventional doping-less tunneling field-effect transistors (DLTFETs), in H-DLTFETs, germanium and Si0.6Ge0.4 are used as source and channel materials, respectively, to provide higher carrier mobility and smaller tunneling barrier width. The energy band and charge carrier tunneling efficiency of the tunneling junction become steeper and higher as a result of the Si0.6Ge0.4 heterojunction. In addition, the effects of the source work function, gate oxide dielectric thickness, and germanium content on the performance of the H-DLTFET are analyzed systematically, and the below optimal device parameters are obtained. The simulation results show that the performance parameters of the H-DLTFET, such as the on-state current, on/off current ratio, output current, subthreshold swing, total gate capacitance, cutoff frequency, and gain bandwidth (GBW) product when Vd = 1 V and Vg = 2 V, are better than those of conventional silicon-based DLTFETs. Therefore, the H-DLTFET has better potential for use in ultra-low power devices.

Recent Progress in Electrochemical pH-Sensing Materials and Configurations for Biomedical Applications

pH-sensing materials and configurations are rapidly evolving toward exciting new applications, especially those in biomedical applications. In this review, we highlight rapid progress in electrochemical pH sensors over the past decade (2008-2018) with an emphasis on key considerations, such as materials selection, system configurations, and testing protocols. In addition to recent progress in optical pH sensors, our main focus in this review is on electromechanical pH sensors due to their significant advances, especially in biomedical applications. We summarize developments of electrochemical pH sensors that by virtue of their optimized material chemistries (from metal oxides to polymers) and geometrical features (from thin films to quantum dots) enable their adoption in biomedical applications. We further present an overview of necessary sensing standards and protocols. Standards ensure the establishment of consistent protocols, facilitating collective understanding of results and building on the current state. Furthermore, they enable objective benchmarking of various pH-sensing reports, materials, and systems, which is critical for the overall progression and development of the field. Additionally, we list critical issues in recent literary reporting and suggest various methods for objective benchmarking. pH regulation in the human body and state-of-the-art pH sensors (from ex vivo to in vivo) are compared for suitability in biomedical applications. We conclude our review by (i) identifying challenges that need to be overcome in electrochemical pH sensing and (ii) providing an outlook on future research along with insights, in which the integration of various pH sensors with advanced electronics can provide a new platform for the development of novel technologies for disease diagnostics and prevention.

Device Noise Reduction for Silicon Nanowire Field-Effect-Transistor Based Sensors by Using a Schottky Junction Gate

The sensitivity of metal oxide semiconductor field-effect transistor (MOSFET) based nanoscale sensors is ultimately limited by noise induced by carrier trapping/detrapping processes at the gate oxide/semiconductor interfaces. We have designed a Schottky junction gated silicon nanowire field-effect transistor (SiNW-SJGFET) sensor, where the Schottky junction replaces the noisy oxide/semiconductor interface. Our sensor exhibits significantly reduced device noise, 2.1 × 10^-9 V² μm²/Hz at 1 Hz, compared to reference devices with the oxide/semiconductor interface operated at both inversion and depletion modes. Further improvement can be anticipated by wrapping the nanowire by such a Schottky junction, thereby eliminating all oxide/semiconductor interfaces. Hence, a combination of the low-noise SiNW-SJGFET device with a sensing surface of the Nernstian response limit holds promises for future high signal-to-noise ratio sensor applications.

Three-Dimensional Integrated Ultra-Low-Volume Passive Microfluidics with Ion-Sensitive Field-Effect Transistors for Multiparameter Wearable Sweat Analyzers

Wearable systems could offer noninvasive and real-time solutions for monitoring of biomarkers in human sweat as an alternative to blood testing. Recent studies have demonstrated that the concentration of certain biomarkers in sweat can be directly correlated to their concentrations in blood, making sweat a trusted biofluid candidate for noninvasive diagnostics. We introduce a fully on-chip integrated wearable sweat sensing system to track biochemical information at the surface of the skin in real time. This system heterogeneously integrates, on a single silicon chip, state-of-the-art ultrathin body (UTB) fully depleted silicon-on-insulator (FD-SOI) ISFET sensors with a biocompatible microfluidic interface, to deliver a "lab-on-skin" sensing platform. A full process for the fabrication of this system is proposed in this work and is demonstrated by standard semiconductor fabrication procedures. The system is capable of collecting small volumes of sweat from the skin of a human and posteriorly passively driving the biofluid, by capillary action, to a set of functionalized ISFETs for analysis of pH level and Na⁺ and K⁺ concentrations. Drop-casted ion-sensing membranes on different sets of sensors on the same substrate enable multiparameter analysis on the same chip, with small and controlled cross-sensitivities, whereas a miniaturized quasireference electrodes set a stable analyte potential, avoiding the use of a cumbersome external reference electrode. The progress of lab-on-skin technology reported here can lead to autonomous wearable systems enabling real-time continuous monitoring of sweat composition, with applications ranging from medicine to lifestyle behavioral engineering and sports.

Silicon Nanowire Field Effect Transistor Sensors with Minimal Sensor-to-Sensor Variations and Enhanced Sensing Characteristics

Silicon nanowire field effect transistor (FET) sensors have demonstrated their ability for rapid and label-free detection of proteins, nucleotide sequences, and viruses at ultralow concentrations with the potential to be a transformative diagnostic technology. Their nanoscale size gives them their ultralow detection ability but also makes their fabrication challenging with large sensor-to-sensor variations, thus limiting their commercial applications. In this work, a combined approach of nanofabrication, device simulation, materials, and electrical characterization is applied toward identifying and improving fabrication steps that induce sensor-to-sensor variations. An enhanced complementary metal-oxide-semiconductor-compatible process for fabricating silicon nanowire FET sensors on 8 in. silicon-on-insulator wafers is demonstrated. The fabricated nanowire (30 nm width) FETs with solution gates have a Nernst limit subthreshold swing (SS) of 60 ± 1 mV/decade with ∼1.7% variations, whereas literature values for SS are ≥80 mV/decade with larger (>10 times) variations. Also, their threshold voltage variations are significantly (∼3 times) reduced, compared to literature values. Furthermore, these improved FETs have significantly reduced drain current hysteresis (∼0.6 mV) and enhanced on-current to off-current ratios (∼10⁶). These improvements resulted in nanowire FET sensors with the lowest (∼3%) reported sensor-to-sensor variations, compared to literature studies. Also, these improved nanowire sensors have the highest reported sensitivity and enhanced signal-to-noise ratio with the lowest reported defect density of 2.1 × 10¹⁸ eV^-1 cm^-3, in comparison to literature data. In summary, this work brings the nanowire sensor technology a step closer to commercial products for early diagnosis and monitoring of diseases.

Selective layer-free blood serum ionogram based on ion-specific interactions with a nanotransistor

Despite being ubiquitous in the fields of chemistry and biology, the ion-specific effects of electrolytes pose major challenges for researchers. A lack of understanding about ion-specific surface interactions has hampered the development and application of materials for (bio-)chemical sensor applications. Here, we show that scaling a silicon nanotransistor sensor down to ~25 nm provides a unique opportunity to understand and exploit ion-specific surface interactions, yielding a surface that is highly sensitive to cations and inert to pH. The unprecedented sensitivity of these devices to Na⁺ and divalent ions can be attributed to an overscreening effect via molecular dynamics. The surface potential of multi-ion solutions is well described by the sum of the electrochemical potentials of each cation, enabling selective measurements of a target ion concentration without requiring a selective organic layer. We use these features to construct a blood serum ionogram for Na⁺, K⁺, Ca²⁺ and Mg²⁺, in an important step towards the development of a versatile, durable and mobile chemical or blood diagnostic tool.

Control of the Intrinsic Sensor Response to Volatile Organic Compounds with Fringing Electric Fields

The ability to control surface-analyte interaction allows tailoring chemical sensor sensitivity to specific target molecules. By adjusting the bias of the shallow p-n junctions in the electrostatically formed nanowire (EFN) chemical sensor, a multiple gate transistor with an exposed top dielectric layer allows tuning of the fringing electric field strength (from 0.5 × 10⁷ to 2.5 × 10⁷ V/m) above the EFN surface. Herein, we report that the magnitude and distribution of this fringing electric field correlate with the intrinsic sensor response to volatile organic compounds. The local variations of the surface electric field influence the analyte-surface interaction affecting the work function of the sensor surface, assessed by Kelvin probe force microscopy on the nanometer scale. We show that the sensitivity to fixed vapor analyte concentrations can be nullified and even reversed by varying the fringing field strength, and demonstrate selectivity between ethanol and n-butylamine at room temperature using a single transistor without any extrinsic chemical modification of the exposed SiO₂ surface. The results imply an electric-field-controlled analyte reaction with a dielectric surface extremely compelling for sensitivity and selectivity enhancement in chemical sensors.

Multi-Wire Tri-Gate Silicon Nanowires Reaching Milli-pH Unit Resolution in One Micron Square Footprint

The signal-to-noise ratio of planar ISFET pH sensors deteriorates when reducing the area occupied by the device, thus hampering the scalability of on-chip analytical systems which detect the DNA polymerase through pH measurements. Top-down nano-sized tri-gate transistors, such as silicon nanowires, are designed for high performance solid-state circuits thanks to their superior properties of voltage-to-current transduction, which can be advantageously exploited for pH sensing. A systematic study is carried out on rectangular-shaped nanowires developed in a complementary metal-oxide-semiconductor (CMOS)-compatible technology, showing that reducing the width of the devices below a few hundreds of nanometers leads to higher charge sensitivity. Moreover, devices composed of several wires in parallel further increase the exposed surface per unit footprint area, thus maximizing the signal-to-noise ratio. This technology allows a sub milli-pH unit resolution with a sensor footprint of about 1 µm², exceeding the performance of previously reported studies on silicon nanowires by two orders of magnitude.

Microfabricated electrochemical pH and free chlorine sensors for water quality monitoring: recent advances and research challenges

Recent advances of micro-electrochemical ph and free chlorine sensors are reviewed and their technological challenges and perspectives are provided.