Graphene has been considered as as a high-mobility semiconductor to be the most promising sensitive material in novel sensor development. Its sensor applications for the detection of electrically charged analytes in aqueous-media have made it increasingly attractive in the last decade. Graphene nanosensors designed under the field-effect transistor (GFET) configuration can transduce analyte-induced electric excitation into a device conductivity response that enable simple sensing operations via the measurement of electrical current. In this case, graphene with outstanding carrier mobility confers GFET nanosensors significantly improved sensitivity, compare with other sensitive materials.
While the GFET nanosensors have made great progress, the devices were mostly configured based on a classical liquid-gate transistor structures, which supply gating electrical field through aqueous-solutions. This configuration undesirably requires an external metal wire, as the gate electrode limits the device integration degree and hinders the device practicability in the on-site applications. For a compact structure, the back-gate GFET configuration has also been proposed, however, it usually requests insecure high gate voltage and exhibits low sensitivity. Electrical analyses attribute these shortcomings to the low gating capacitance provided by the SiO2 dielectric layer, compared with solutions. Although the back-gate GFET configuration was rarely used, it lightens an inspiration that a planar gating configuration with high permittivity may enable high device practicability and ideal sensitivity, concurrently.
A newly structured high-κ solid-gate GFET nanosensor is proposed by Prof He and his team. By employing a planar gate electrode buried by high-κ HfO2 dielectric layer, the GFET nanosensor simultaneously achieves a fully integrated structure for practical applications, and significantly enhances the sensitivity performance over the conventional liquid-gate GFET nanosensors under safe gate voltage. Theoretical discussion suggests that the sensitivity improvement is attributed to the enhancement of gating capacitance; the carrier mobility of graphene is well-preserved. These results also suggest that more advanced solid-gating structures which provide either a thinner dielectric layer or higher permittivity may further improve the device’s performance.