Effect of liquid gate bias rising time in pH sensors based on Si nanowire ion sensitive field effect transistors

Highlights

• Experimental investigation in pH sensors based on SiNW ISFETs.

• The effect of the rising time (T_R) of the liquid gate bias on transient response.

• Rising time (T_R) and pH-dependent ISFET current.

• Importance of retardation of mobile ions in terms of biosensor design.

Abstract

In this study, we investigate the effect of rising time (T_R) of liquid gate bias (V_LG) on transient responses in pH sensors based on Si nanowire ion-sensitive field-effect transistors (ISFETs). As T_R becomes shorter and pH values decrease, the ISFET current takes a longer time to saturate to the pH-dependent steady-state value. By correlating V_LG with the internal gate-to-source voltage of the ISFET, we found that this effect occurs when the drift/diffusion of mobile ions in analytes in response to V_LG is delayed. This gives us useful insight on the design of ISFET-based point-of-care circuits and systems, particularly with respect to determining an appropriate rising time for the liquid gate bias.

Introduction

An important indicator in industries such as food, pharmaceutical, agriculture, biomedical, and environmental monitoring is pH; for example, variations in pH of human blood are signs of serious, potentially fatal diseases [1]. The solubility, stability, and permeability of a drug through biological membrane also depend on pH [2], and therefore, a precise pH measurement technique using a miniaturized sensor is essential. Furthermore, the solubility of heavy metals such as lead, zinc, and copper in soil is dependent on the pH [3], and therefore, pH measurement is important for managing the environment.

Ion-sensitive field-effect transistors (ISFETs) [4] have attracted considerable attention because of their compact structure, low-cost, and ease of fabrication. Recently, ISFET-based pH sensors have been successfully used in human genome sequencing [5], [6], [7]. The resolution of an ISFET-based pH sensor depends on its individual technology and geometry. Therefore, optimum operation of the sensor in complex analog circuits where biasing configuration may change over time needs to be anticipated. The transient response of an ISFET-based pH sensor is governed by two processes: the convection/diffusion of H⁺ ions to the sensor surface (diffusion time) and the response of the sensor to the changes in proton concentration on its surface (response time). Diffusion time depends on various factors that define the ion transport time to the sensor, such as flow rate and the size/shape of the fluidic channel. However, the transient response to time-varying liquid gate bias (V_LG) has been rarely investigated despite its importance. To optimize the details of operation, particularly the choice of the saturation time of signals in the ISFET-based point-of-care (PoC) circuits, the influence of the rising time of V_LG needs to be experimentally characterized. We studied ways to simplify the properties that may affect the way the sensor works. For example, we measured droplets before they entered the microfluidic channel. In this way, we could exclude complicated factors such as convection and diffusion in the analyte.

Conversely, the silicon nanowire (SiNW) ISFET-based biosensors have great potential as essential building blocks for real-time, label-free detection of biomolecules because of their advantages such as direct electrical readouts and high sensitivity and the potential to integrate them with complementary metal–oxide semiconductor (CMOS) circuits [8], [9], [10], [11]. Based on the top-down processing in the SiNW/CMOS hybrid circuitry, researchers have already demonstrated electrical optimization sensitivity [12], sensitivity boosting [13], voltage readout [14], noise cancellation [15], and simulation using commercial technology computer aided design (TCAD) [16]. It should therefore be noted that operating the SiNW ISFET in the saturation region is desirable for a robust analog front-end design, although the ISFET sensitivity itself can be better in the sub-threshold rather than the saturation region.

In this work, the influence of the V_LG rising time (T_R) on transient current response in the top-down processed SiNW ISFET-based pH sensors operating in the saturation region is investigated. By correlating V_LG with the internal gate-to-source voltage of ISFET (V_GS,int) at the electrolyte/insulator interface, we have gained new physical and chemical insights into the drift/diffusion of mobile ions in an electrolyte analyte.