Estimation of the Depletion Layer Thickness in Silicon Nanowire-Based Biosensors from Attomolar-Level Biomolecular Detection

Silicon nanowire (SiNW) biosensors have attracted a lot of attention due to their superior sensitivity. Recently, the dependence of biomolecule detection sensitivity on the nanowire (NW) width, number, and doping density has been partially investigated. However, the primary reason for achieving ultrahigh sensitivity has not been elucidated thus far. In this study, we designed and fabricated SiNW biosensors with different widths (10.8-155 nm) by integrating a complementary metal-oxide-semiconductor process and electron beam lithography. We aimed to investigate the detection limit of SiNW biosensors and reveal the critical effect of the 10-nm-scaled SiNW width on the detection sensitivity. The sensing performance was evaluated by detecting antiovalbumin immunoglobulin G (IgG) with various concentrations (from 6 aM to 600 nM). The initial thickness of the depletion region of the SiNW and the changes in the depletion region due to biomolecule binding were calculated. The basis of this calculation are the resistance change ratios as functions of IgG concentrations using SiNWs with different widths. The calculation results reveal that the proportion of the depletion region over the entire SiNW channel is the essential reason for high-sensitivity detection. Therefore, this study is crucial for an indepth understanding on how to maximize the sensitivity of SiNW biosensors.