Ultrasensitive Coplanar Dual-Gate ISFETs for Point-of-Care Biomedical Applications

Abstract

The sensitivity of conventional ion-sensitive field-effect transistors (ISFETs) is limited by the Nernst equation, which is not sufficient for detecting weak biological signals. In this study, we propose a silicon-on-insulator-based coplanar dual-gate (Cop-DG) ISFET pH sensor, which exhibits better performance than the conventional ISFET pH sensor. The Cop-DG ISFETs employ a Cop-DG consisting of a control gate (CG) and a sensing gate (SG) with a common gate oxide and an electrically isolated floating gate (FG). As CG and SG are capacitively coupled to FG, both these gates can efficiently modulate the conductance of the FET channel. The advantage of the proposed sensor is its ability to amplify the sensitivity effectively according to the capacitive coupling ratio between FG and coplanar gates (SG and CG), which is determined by the area of SG and CG. We obtained the pH sensitivity of 304.12 mV/pH, which is significantly larger than that of the conventional ISFET sensor (59.15 mV/pH, at 25 °C). In addition, we measured the hysteresis and drift effects to ensure the stability and reliability of the sensor. Owing to its simple structure, cost-effectiveness, and excellent sensitivity and reliability, we believe that the Cop-DG ISFET sensor provides a promising point-of-care biomedical applications.

High-Sensitivity pH Sensor Based on Coplanar Gate AlGaN/GaN Metal-Oxide-Semiconductor High Electron Mobility Transistor

“…Double gate structure ISFETs based on silicon-on-insulator are drawing attention because they can overcome the Nernst limit via asymmetric capacitive coupling of the upper and lower gate oxides [5][6][7][8]. In addition, in our previous work, we studied a silicon-based device that amplifies the sensitivity through capacitive coupling between the coplanar gate and the floating gate, and here we investigated the capacitive coupling effect according to the coplanar gate area and the corresponding sensitivity amplification [9]. However, owing to the limitations of the physical characteristics of silicon, these transistors cannot easily be integrated with wide transmission bandwidths and at high temperatures.…”

Narrower Nanoribbon Biosensors Fabricated by Chemical Lift-off Lithography Show Higher Sensitivity

“…Ion-sensitive FETs (ISFETs), where FETs respond to changes in environmental ion concentrations, are used for a majority of FET chemical and biological sensing applications. − To investigate the performance of In 2 O 3 nanoribbon ISFETs, we conducted pH sensing by systematically increasing the hydrogen ion concentrations of the solutions contacting FETs. We previously compared the pH sensitivities of 25-μmwide In 2 O 3 ribbon FET sensors having different ribbon heights .…”

Ultra‐thin ISFET‐based sensing systems

“…For example, dual-gate (DG), triple-gate (TG), and extendedgate (EG) ISFET architectures have been developed that increase the capacitive coupling to the gate leading to selfamplification within the device and can exhibit intrinsic sensitivities above 1 V/pH. [42][43][44][45][46] From a material perspective, traditional single-layer metal oxides and nitrides such as SiO 2 , Al 2 O 3 , Si 3 N 4 are being replaced by individual or stacked high-k dielectrics such as SiO 2 /Ta 2 O 5 , HfO 2 , and RuO 2 /SnO 2 that aid in increasing the sensitivity of the device and a complete review on metal-oxide based pH sensors can be found in the work of Manjakkal et al [47] Furthermore, the low-frequency noise developed in ISFETs due to charge trapping and de-trapping in the oxide layer of the transducer FET can be mitigated by adopting the MESFET (Metal-Semiconductor FET) architecture wherein the oxide layer is removed and a Schottky Junction (SJ) is formed and the detector unit comprising of the sensing oxide is fabricated as an extended gate structure. [48] The design of glass-substrate-based ISFETs inspired by thin-film technology has also been explored and an example is based on a 6 nm thick Ta 2 O 5 dielectric and a ZnO channel, exhibiting a sensitivity of ∼55 mV/pH.…”

“…Adapted with permission from Ref. [45]. Copyright 2020, American Chemical Society, (d) Dielectric encapsulated MoS 2 based ISFET with biosensing capabilities.…”

“…Similarly, EG‐ISFETs fabricated on glass with a sensitivity of 304.12 mV/pH, consisting of a floating gate and a silicon channel have been developed and shown in Figure 2c. [ 45 ] Low power devices that utilize the inherent double‐layer capacitive coupling, constructed using electrolyte‐gated structures, have eliminated the need for fabrication of individual high‐k dielectrics as sensing layers. These devices are often developed using thin‐film technology processes with zinc oxide‐derived materials as channels in contact with or electrically coupled to the solution under test.…”