Axially-doped silicon nanowire field effect transistors for real-time sensing in physiologically relevant buffer solutions

Directed assembly of bioprobe-coated nanowire arrays onto silicon integrated circuits may enable multiplexed detection of biological targets in a compact, low-power chip-based platform [1]. Silicon nanowire field effect transistors (FETs) are attractive biosensor candidates because they promise extremely high sensitivity and label-free, real-time detection [2 - 4]. Previous sensing experiments conducted using silicon nanowires grown by the vapor-liquid-solid (VLS) technique have been limited to measurements of individual heavily-doped nanowires in deionized water or low salt concentration buffer solutions. Despite their small diameters, the heavy nanowire doping limits the change in channel conductance induced by varying the solution pH or by biomolecule binding. Here, we present the first measurements of chemically- gated axially-doped n⁺-p^--n⁺ silicon nanowire FETs and show stable and reproducible transfer characteristics in physiologically relevant buffer solutions (up to 100 mM KCl). The sensitivity of nanowire FETs having bare oxidized and chemically-functionalized surfaces to changes in pH was greatest when the devices were biased in the subthreshold regime during sensing.