Highly sensitive electrolyte-insulator-semiconductor pH sensors enabled by silicon nanowires with Al2O3/SiO2 sensing membrane

Low sensitivity and poor reliability currently limit the applications of solid-state bio-chemical sensors. We demonstrate an electrolyte-insulator-semiconductor (EIS) sensor with a large capacitance, near-Nernst-limit pH sensitivity, and good reliability by integrating an ensemble of Si nanowires (NWs) for the first time. The NWs were fabricated by using the electroless wet etching technique. An Al2O3/SiO2 bilayer coating was employed as a sensing membrane. The EIS sensors with 3.8 μm long NWs exhibited about 8 times larger capacitance than that of a planar type EIS sensors that were fabricated using the same fabrication scheme without integrating NWs. The measured pH sensitivity at room temperature was 60.2 mV/pH, which is higher than the theoretical Nernstian of 59 mV/pH. Our results and analysis clearly indicate that ultra-sensitive pH sensing can be realized with optimized NW integrated EIS sensors.