Ion-sensitive Field-effect Transistor pH Sensor Research Articles

Micro-heater embedded ISFET pH sensor with high-k gate dielectrics for enhanced sensitivity

The measurement of pH is an important routine practice in many chemical and biomedical applications. This work reports the design of a pH sensor based on the Ion Sensitive Field Effect Transistor (ISFET). The COMSOL Multiphysics® platform has been used to model and simulate the pH sensor in three dimensions (3D) while combining heat transport, electrostatic, and semiconductor modules. The binding of ions in gate dielectrics results in induced charge carriers in the conducting channel of 3D ISFET, which is controlled by the applied gate voltage for determining ion concentration. Here, the pH of water as the bulk electrolyte is measured by attaining the required gate voltage to achieve a certain drain current in 3D ISFET. The pH sensitivity of 3D ISFET with different high-k gate dielectrics such as Ta2O5, Al2O3, HfO2, ZrO2, and SiO2 is measured and compared. The 3D ISFET with Ta2O5 exhibited excellent sensitivity of 59.0 mV pH−1 with a wide linear detection range of pH from 1 to 13 at room temperature compared to other high-k gate dielectrics. Furthermore, the sensitivity was further enhanced to 66.0 mV pH−1 at 60 °C due to the incorporation of a micro-heater into the ISFET. The excellent sensitivity and wide linear detection range can be attributed to the high concentration of surface sites in the Ta2O5 sensing film and improved disassociation constants in the presence of the gate dielectric in contact with the electrolyte. Finally, this sensor demonstrates its potential for real applications.

Net community production in the northwestern Mediterranean Sea from glider and buoy measurements

Abstract. The Mediterranean Sea comprises just 0.8 % of the global oceanic surface, yet considering its size, it is regarded as a disproportionately large sink for anthropogenic carbon due to its physical and biogeochemical characteristics. An underwater glider mission was carried out in March–April 2016 close to the BOUSSOLE and DyFAMed time series moorings in the northwestern Mediterranean Sea. The glider deployment served as a test of a prototype ion-sensitive field-effect transistor pH sensor. Dissolved oxygen (O2) concentrations and optical backscatter were also observed by the glider and increased between 19 March and 1 April, along with pH. These changes indicated the start of a phytoplankton spring bloom, following a period of intense mixing. Concurrent measurements of CO2 fugacity and O2 concentrations at the BOUSSOLE mooring buoy showed fluctuations, in qualitative agreement with the pattern of glider measurements. Mean net community production rates (N) were estimated from glider and buoy measurements of dissolved O2 and inorganic carbon (DIC) concentrations, based on their mass budgets. Glider and buoy DIC concentrations were derived from a salinity-based total alkalinity parameterisation, glider pH and buoy CO2 fugacity. The spatial coverage of glider data allowed the calculation of advective O2 and DIC fluxes. Mean N estimates for the euphotic zone between 10 March and 3 April were (-17±36) for glider O2, (44±94) for glider DIC, (17±37) for buoy O2 and (49±86) mmolm-2d-1 for buoy DIC, all indicating net metabolic balance over these 25 d. However, these 25 d were actually split into a period of net DIC increase and O2 decrease between 10 and 19 March and a period of net DIC decrease and O2 increase between 19 March and 3 April. The latter period is interpreted as the onset of the spring bloom. The regression coefficients between O2 and DIC-based N estimates were 0.25 ± 0.08 for the glider data and 0.54 ± 0.06 for the buoy, significantly lower than the canonical metabolic quotient of 1.45±0.15. This study shows the added value of co-locating a profiling glider with moored time series buoys, but also demonstrates the difficulty in estimating N, and the limitations in achievable precision.

Analysis of output characteristics of ion sensitive field effect transistor based biosensor for measurement of pH in biochemical solutions

The need for improved system’s response time, sensitivity, selectivity and miniaturization has continued to form the premise for focusing research attention on innovative design and development of biochemical sensors. Ion sensitive field effect transistor (ISFET) can be used to innovatively drive biochemical sensors for the measurement of concentration of ions and molecules in solutions. This simulation study focuses on the determination of output characteristics of ISFET pH sensor as applied in the measurement of concentration of hydrogen potential (pH) which is a function of the proton concentration in an analyte solution. The pH of a target analyte interacts with the ion sensitive membrane of ISFET to produce charges which in turn changes the potential of the working electrode of the sensor. The output characteristic of interest is the gate voltage of ISFET which is a function of pH concentration of analyte under study. The relationship between the sensor’s input pH concentration and its output gate voltage describes the sensitivity of the device. From the results obtained from the study, as the input pH concentration was increased from 5 to 11, the output gate voltage ISFET pH sensor increased from 2.65 V to 2.9 V.

Sensitivity optimization of a Double-Gated ISFET pH-sensor with HfO2/SiO2 gate dielectric stack

This paper investigates the sensitivity enhancement of Ion-Sensitive Field Effect Transistor (ISFET) pH-sensor using double-gate operation and illustrates how different structure modifications between the top and back oxides give different sensitivity gain factors. The gain factor is controlled through top and back oxide thickness, channel thickness and SiGe channel material. The sensitivity at the back gate is 59.52 mV/pH (27 °C) and the amplified sensitivity at the front gate reaches 476.54 ±33 mV/pH (27 °C). Furthermore, the issues related to high sensitivity-amplification such as sensitivity variation and dielectric breakdown are discussed, followed by optimization of the device structure in the linear regime to overcome them. Stacking a thin HfO2 layer over the top SiO2 layer helps in preventing dielectric breakdown at high sensitivity analysis. Synopsys TCAD is utilized for this proposed work with the electrolyte material and site-binding reactions being modeled through user-defined features of TCAD with explicit references from experimental data.

Design, fabrication and characterization of TiN sensing film-based ISFET pH sensor

In this work, we report the development of Titanium Nitride (TiN) as a sensing film for Ion-Sensitive Field-Effect Transistor (ISFET)-based pH sensor, which has been deposited using pulsed-DC (Direct Current) magnetron-assisted reactive sputtering process. The material characterization was carried out using various techniques. TiN-gate ISFETs have been fabricated using self-aligned fabrication process by utilizing five levels of lithography steps. The packaging and encapsulation of the sensor has been done by thick film technology and dam-and-fill approach, respectively. The sensor has been characterized and a high sensitivity of nearly 53.98 mV/pH has been obtained. Drift and hysteresis studies were also carried out. The drift rates for different pH buffer solutions 4, 7, 10 are measured to be 0.368 μA/min, 0.296 μA/min, 0.237 μA/min, respectively. A hysteresis value of 0.50 μA has been obtained for pH cycle pH 7 → pH 10 → pH 7 → pH 4 → pH 7.

Modeling and simulation of temperature drift for ISFET‐based pH sensor and its compensation through machine learning techniques

SummaryThe paper presents modeling and simulation of ion‐sensitive field‐effect transistor (ISFET)‐based pH sensor with temperature‐dependent behavioral macromodel and proposes to compensate the temperature drift in the sensor using intelligent machine learning (ML) models. The macromodel is built using SPICE by introducing electrochemical parameters in a metal‐oxide‐semiconductor field‐effect transistor (MOSFET) model to simulate ISFET characteristics. We account for the temperature dependence of electrochemical and semiconductor parameters in our macromodel to increase its robustness. The macromodel is then exported as a subcircuit element, which is used to design the readout interface circuit. A simple constant‐voltage, constant‐current (CVCC) topology is utilized to generate the data for temperature drift in ISFET pH sensor, which is used to train and test state‐of‐the‐art ML‐based regression models in order to compensate the drift behavior. The experimental results demonstrate that the random forest (RF) technique achieves the best performance with very high correlation and low error rate. Corresponding curves for output signal using the trained models show highly temperature‐independent characteristics when tested for pH 2, 4, 7, 10, and 12, and we obtained a root mean squared error (RMS) variation of ΔpH ≤ 0.024 over a temperature range of 15°C to 55°C in comparison with ΔpH ≤ 1.346 for uncompensated output signal. This work establishes the framework for integration of ML techniques for drift compensation of ISFET chemical sensor to improve its performance.

Triple Gate Polycrystalline-Silicon-Based Ion-Sensitive Field-Effect Transistor for High-Performance Aqueous Chemical Application

In this paper, we developed a polycrystalline-silicon (poly-Si) thin-film transistor (TFT)-based high-performance ion-sensitive field-effect transistor (ISFET) pH sensor that far surpasses the sensitivity of dual-gate (DG) pH sensors. A triple gate (TG) structure on the same plane as the channel of the ISFET has been proposed to enhance the pH sensitivity. In a poly-Si TFT-based ISFET, a TG is more advantageous than a bottom-gate for increasing the capacitive coupling with the top-gate. As a result, the pH sensitivity by TG mode detection using the TG is much greater than the single-gate (SG) mode detection using the top-gate, or the DG mode detection using the bottom-gate. The sensitivity of the TG mode increased greatly compared with the sensitivity of the conventional SG mode or DG mode. The measured pH sensitivity was 57.75 ± 0.77 mV/pH in the SG mode and 467.08 ± 9.92 mV/pH in the DG mode. In particular, the TG mode gives a maximum sensitivity of 1283.56 ± 45.54 mV/pH for sensing membranes having a theoretical Nernstian pH response (59.15 mV/pH at 25 °C). Furthermore, we measured the hysteresis and drift characteristics, and found that the TG mode has improved non-ideal behavior compared with the SG and DG modes. Therefore, the poly-Si TFT-based TG ISFET pH sensor has the potential to become a promising biosensor application platform, with excellent sensitivity and stability.

ISFET pH Sensitivity: Counter-Ions Play a Key Role

The Field Effect sensors are broadly used for detecting various target analytes in chemical and biological solutions. We report the conditions under which the pH sensitivity of an Ion Sensitive Field Effect transistor (ISFET) sensor can be significantly enhanced. Our theory and simulations show that by using pH buffer solutions containing counter-ions that are beyond a specific size, the sensor shows significantly higher sensitivity which can exceed the Nernst limit. We validate the theory by measuring the pH response of an extended gate ISFET pH sensor. The consistency and reproducibility of the measurement results have been recorded in hysteresis free and stable operations. Different conditions have been tested to confirm the accuracy and validity of our experiment results such as using different solutions, various oxide dielectrics as the sensing layer and off-the-shelf versus IC fabricated transistors as the basis of the ISFET sensor.

Analytical modelling of monolayer graphene-based ion-sensitive FET to pH changes

Graphene has attracted great interest because of unique properties such as high sensitivity, high mobility, and biocompatibility. It is also known as a superior candidate for pH sensing. Graphene-based ion-sensitive field-effect transistor (ISFET) is currently getting much attention as a novel material with organic nature and ionic liquid gate that is intrinsically sensitive to pH changes. pH is an important factor in enzyme stabilities which can affect the enzymatic reaction and broaden the number of enzyme applications. More accurate and consistent results of enzymes must be optimized to realize their full potential as catalysts accordingly. In this paper, a monolayer graphene-based ISFET pH sensor is studied by simulating its electrical measurement of buffer solutions for different pH values. Electrical detection model of each pH value is suggested by conductance modelling of monolayer graphene. Hydrogen ion (H+) concentration as a function of carrier concentration is proposed, and the control parameter (Ƥ) is defined based on the electro-active ions absorbed by the surface of the graphene with different pH values. Finally, the proposed new analytical model is compared with experimental data and shows good overall agreement.

Improvement of integrated Ag/AgCl thin-film electrodes by KCl-gel coating for ISFET applications

To eliminate the need of a separate reference electrode and facilitate the use of ion-sensitive field-effect transistor (ISFET), an all-solid-state reference electrode integrated with ISFET in one chip has been developed. A novel agarose-stabilized KCl-gel membrane was introduced to serve both as a polymer-supported solid reference electrolyte and an ionic bridge for Ti/Pd/Ag/AgCl electrode. Calibrated against the commercial macro Ag/AgCl reference electrode the new miniaturized reference electrode shows its cell potential variation was ±0.9–1.4 mV (equivalent to about ±0.015–0.023 pH) in 1 h, less than 2 mV variation over pH 4–10, and almost insensitive to changes in Cl − ion concentration (about 0.02–0.25 mV/pKCl). The planar solid-state reference electrode also shows a very small offset voltage of 0.45 mV and reproducible to within ±0.5 mV among the batch fabricated electrodes. ISFET pH-sensor with integrated all-solid-state reference electrode characterized in standard pH solutions show a linear sensitivity of 56 mV/pH, these results agree very well with the theoretical value of 59 mV/pH. Dynamic response and hysteresis characteristics of the pH-sensor are also studied and discussed.

A CMOS-integrated 'ISFET-operational amplifier' chemical sensor employing differential sensing

The ISFET (ion-sensitive field-effect transistor) pH sensor is first matched with a MOSFET at the differential input stage of a CMOS operational amplifier (called the ISFET-operational amplifier) to cancel out the temperature sensitivity. Then, the output of an ISFET-operational amplifier with a Ta/sub 2/O/sub 5//SiO/sub 2/ gate (58-59 mV/pH) ISFET is differentially amplified against the output of another on-chip ISFET-operational amplifier with a SiO/sub x/N/sub y//Si/sub 3/N/sub 4//SiO/sub 2/ gate ISFET (18-20-mV/pH). An on-chip noble metal counterelectrode serves as the electrical contact to define the electric potential of the electrolyte. No external reference electrode is required. The difference measurement technique achieves (1) common-mode rejection of the solution potential, and (2) relaxation of the requirement that the on-chip reference electrode be ideal. The CMOS-compatible ISFET process is modified from a standard self-aligned polysilicon gate CMOS process with minimal process redesign. The standard CMOS sequence is unaltered until the contact windows are opened. The complete sensor has 40-43-mV/pH pH sensitivity and demonstrates common-mode rejection to ambient light and noise from the electrolyte.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Open-loop analysis of PaCO2 oscillation in the dog.

Venous CO2 loading and/or unloading using an artificial lung was performed on dogs to evaluate whether changes in the rate of carbon dioxide output (VCO2) can be involved in the control of respiration. As the signal providing VCO2 information, small fluctuations in arterial PCO2 (delta PaCO2) which synchronized with the respiratory cycle were estimated from both the mean PaCO2 and the fluctuations in arterial pH (delta pH) measured with a catheter tip ISFET (ion sensitive field-effect transistor) pH sensor inserted in the common carotid artery. The open-loop method was used to analyze the effects of various respiratory parameters such as VCO2, tidal volume, and respiratory frequency on the changes in delta pH and delta PaCO2. Findings are: (1) delta PaCO2 was linearly proportional to VCO2 while the correlation between delta pH and VCO2 was less linear, (2) magnitude of delta pH was dependent on the changes in tidal volume while delta PaCO2 was not, (3) both the amplitude and the positive slope of PaCO2 oscillation were functions of respiratory frequency. These results suggest the possibility that if VCO2 itself takes part in the control of ventilation, it will be CO2 oscillation which links ventilation with VCO2.