Counteracting Threshold-Voltage Drift in Ion-Selective Field Effect Transistors (ISFETs) Using Threshold-Setting Ion Implantation (original) (raw)
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Low frequency noise and drift in Ion Sensitive Field Effect Transistors
Sensors and Actuators B: Chemical, 2000
Ž. Ion Sensitive Field Effect Transistors ISFETs are currently produced commercially and promise to become the platform sensors for important biomedical applications. The drift in ISFETs is still an important inherent problem that prevents its application to accurate in Ž. vivo measurements. The present paper presents measurements of the drift and the drain current power spectral density PSD of pH ISFETs in the very low frequency range from 5 mHz to 10 kHz. The measurements have been performed in buffer solutions with pH 4, 7 and 10, at room temperature. Above a corner frequency, the measured spectra correspond to 1rf noise introduced by fluctuations at the channel current. Below this corner frequency that depends on the magnitude of the drift, the measured spectra correspond to 1rf 2. The observed corner frequency is ; 1 Hz for a drift of 2 mVrh and shifts to frequencies below 0.01 Hz for a drift of 0.1 mVrh. The measured drift is correlated to leakage currents as well as temperature fluctuations and the inherent behaviour of the ISFET. A method for quality evaluation based on frequency behaviour is introduced.
Improvement of structural instability of the ion-sensitive field-effect transistor (ISFET)
Sensors and Actuators B-chemical, 1993
Three causes involved in tkinstability of the ISFET are proposed in this study. First, it is ascertained that hydmxyl group resident at the surface of the S&N4 film or in the electrolyte solution is most active and subject to gain or loss of electrons. This is one of the main causes for ISFET structural instability. Secondly, the stability of the pH-sensitive FET varies with deposition conditions in the fabrication process of the ISFET. This proves to be another cause of ISFET instability. Thirdly, the pH of the measured solution varies with the measuring process and time, contributing to the instability, but is not a cause of the instability of the pH-ISFET itself. We utilized the technique of readjusting and controlling the ratio of hydmxyl groups to amine groups to enhance the stability of the ISFET. Our techniques to improve stability characteristics proved to be effective in practice.
Ion-sensitive field-effect transistors in standard CMOS fabricated by post processing
IEEE Sensors Journal, 2002
Highly integrated ion-sensitive field-effect transistor (ISFET) microsystems require the monolithic implementation of ISFETs, CMOS electronics, and additional sensors on the same chip. This paper presents new ISFETs in standard CMOS, fabricated by post-processing of a standard CMOS VLSI chip. Unlike CMOS compatible ISFETs fabricated in a dedicated process, the new sensors are directly combined with state-of-the-art CMOS electronics and are subject to continuous technology upgrading. The ISFETs presented include an intermediate gate formed by one or more conducting layers placed between the gate oxide and the sensing layer. The combination of the highly isolating gate oxide of the MOS with a leaky or conducting sensing layer allows the use of low temperature materials that do not damage the CMOS chip. The operation of ISFETs with an intermediate gate and sensing layers fabricated at low temperature is modeled. ISFETs with a linear pH response and drift as low as 0.3 mV/h are reported.
Investigation of Stability of the pH-Sensitive Field-Effect Transistor Characteristics
Sensor Letters, 2011
The drift of threshold voltage of the p-channel ion-selective field-effect transistors with induced channel caused by long-term influence of negative voltage applied to the channel area through the two-layer SiO 2 /Si 3 N 4 gate dielectric is investigated. Based on the experimental data a mechanism of the observed instability is proposed and corresponding design and technology enhancements improving sensor stability for prolonged continuous measurements are outlined.
Silicon
A comprehensive study of the drain current drift mechanism and hysteresis phenomena in fabricated p-channel junctionless ion-sensitive field-effect transistor (JL-ISFET) has been investigated for the first time. The current drift measurements have been performed through transient analysis of drain current, under different pH and liquid-gate bias (V lg). Further, time-dependent gate-capacitance (C G) has also been analyzed to see the effect of hydroxyl ions (OH −) in the sensing film (Al 2 O 3). The hysteresis has also been investigated for different pH loop (7 → 3 → 7 → 11 → 7 and 7 → 11 → 7 → 3 → 7) and times (960s, 1500s, and 1920s). It has been observed that the drift of JL-ISFET occurs because of chemical modification of the sensing film, due to OH −. The proposed device exhibits threshold voltage sensitivity of 58.2 mV/pH that is near to the Nernstian limit. Further, the hysteresis width and maximum drain current drift are measured as ∼ 1.3 mV and 2.4 µA (∼ 75%), respectively.
MODELING ION SENSITIVE FIELD EFFECT TRANSISTORS FOR BIOSENSOR APPLICATIONS
iaeme
During recent decades increasing interest has been shown in the development of biosensors based on ion sensitive field effect transistors (ISFETs). Many ISFET– based pH sensors have been already commercialized and attempts have also been made to commercialize ISFET- based biosensors for applications in the fields of medical, environmental, food safety, military and biotechnology areas. The growing interest for development of these sensors can be explained by the fact that they are manufactured by means of semiconductor techniques which have innovative potential and therefore, may result in the appearance of new biosensor technologies. The basic theoretical principles of ISFET usage in bio analytical practice, the operation principle of ISFET, its modeling and a brief introduction of ISFET technology are considered in this review.
ISFET drawbacks minimization using a novel electronic compensation
Sensors and Actuators B-chemical, 2004
Ion sensitive field effect transistor (ISFET) and membrane field effect transistor (MEMFET) have some drawbacks related to: long-term drift, hysteresis and thermal drift. These factors limit the accuracy of ISFET/MEMFET based measurements systems, specially for continuous or long period measurements. Due to its accuracy, repeatability and easy-to-use features, electronic instrumentation systems are the best tools to design ISFET/MEMFET based measurement systems. A well-designed hardware and a qualified virtual instrumentation software are the key factors to overcome and compensate hysteresis, thermal and long-term drifts ISFET/MEMFET limitations. The paper is dedicated to show an instrumentation system designed to compensate long-term drift on ISFET/MEMFETs. First, this limitation is experimentally shown. As a consequence of it, an electronic hardware is designed to supply correct sensor voltage and current bias levels. On the other hand, an acquisition and control algorithm is implemented. The software acts over the hardware selecting appropriate voltage bias levels compensating sensor drifts. Experimental results demonstrate the effectiveness and feasibility of the instrumentation system proposed.
Biosensors and Bioelectronics, 1991
Recently a new method was introduced to operate an immunological field effect transistor (ImmunoFET). By changing the electrolyte concentration of the sample solution stepwise (the so-called ion-step), a transient diffusion of ions through the membrane-protein layer occurs, resulting in a transient membrane potential, which is measured by the ImmunoFET. It became apparent that the maximum of the membrane potential is a function of pH, owing to a pH-dependent charge density caused by the amphoteric nature of the embedded proteins in the membrane of the ImmunoFET. At a certain pH value. which is called the inversion point (PI'). the membrane potential changes sign. This inversion point is characteristic of the type of protein and the type of membrane and depends on the isoelectric point, the titration curve and the concentration of all amphoteric groups in the membrane. In this paper an attempt is made to establish a theoretical basis for the ionstep method. Because there is no model which describes ion transport in charged membranes and its dynamic behaviour as a result of the ion-step, an existing equilibrium theory has been adapted. The well-known Teorell-Meyer-Sievers (TMS) theory, which describes the membrane potential for charged membranes, is used as a framework. The adapted TM.5 model was verified by experimental data.
ISFET pH Sensitivity: Counter-Ions Play a Key Role
Scientific Reports, 2017
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.