A Study on Si-Based Electrolyte-Gated Transistors for Highly Sensitive Biosensing Applications

Abstract

A Si-based electrolyte-gated transistor (EGT) was fabricated, and its sensing capabilities were analyzed. While conventional FET-type sensors use the channel as a sensing membrane, EGT utilizes the gate electrode which can provide a relatively larger sensing area, thereby enhancing sensitivity by increasing the binding probability and receptor density. Device structures and surface treatments in Si-based EGTs are crucial factors in determining the sensing responses. This study investigated the sensing performances of Si-based EGTs, depending on the channel structure, gate materials, and surface treatment in detecting urea. In chapter II, the fabrication and characterization of Si-based EGT were presented. Si EGTs with various nanowire channels were fabricated using microfabrication technology. The subthreshold swing (SS), which is directly related to current-related sensitivity, decreased as the nanowire became thinner, indicating higher gate controllability. In chapter III, the sensing responses of urea detection were presented. Urease was immobilized on Si EGTs with Ag gate electrodes and verified using Atomic Force Microscopy (AFM). The current-related sensitivity was inversely proportional to channel width or SS, while voltage-related sensitivity showed no correlation with the channel width. In devices with minimal channel width, current-related urea sensitivity in the subthreshold regime was over four times higher than other FET sensors, attributed to the superior structure of EGTs. One of the advantages of Si EGT is the lower power consumption operated at ~ 1V, which is promising as a portable sensor. In chapter IV, the dependence of gate materials and surface treatment materials on the sensing performances were investigated. For the surface treatments, various materials with different properties such as pKa, chain length, and functional groups, were considered. A gold nanoparticle test was performed to evaluate the quality of surface treatments; APTES & GA treatment showed outstanding performances in terms of sensitivity and limit-of-detection. It suggests that the pKa value of surface treatments affecting the urease activity is a crucial factor in selecting appropriate surface treatments. Moreover, there are negligible variations among Au, Ag, Pt gate electrodes.