Zinc oxide ion-sensitive field-effect transistors and biosensors

Abstract

In order to open up a new application field of zinc oxide (ZnO), characteristics of polycrystalline ZnO-based ion-sensitive field-effect transistors (ISFETs) and FET-type biosensors are studied. These solution-gate FETs were composed of a layered structure on a glass substrate; amorphous tantalum pentoxide (Ta2O5) for the gate insulator/polycrystalline ZnO doped with indium (ZnO:In) for the channel layer/glass substrate. The Ta2O5/ZnO:In bilayer was deposited by a newly developed facing-target sputtering (FTS) method to reduce the damage on the depositing film by the bombardment of gamma electrons and negatively charged ions. The advantage of FTS, high-quality film growth with sharp interface, enabled the use of such thin layers as 8-nm-thick Ta2O5 and 35-nm-thick ZnO:In for the ISFETs with acceptable performance. The ZnO-based ISFET showed high pH sensitivity and high stability comparable to commercially available silicon-based ISFETs with much lower photo-induced error under visible-ray illumination. By modifying biofunctional molecules on the gate electrode, the application of the ZnO-based ISFET to two types of biosensors was demonstrated. One is an aptamer-immobilized immuno FET specific for human immunoglobulin detection, and the other is a glucose oxidase-immobilized enzyme FET for β-D glucose detection. Both of the former and latter FET-type biosensors showed high sensitivities in accordance with the Langmuir isotherm plot corresponding to monolayer absorption. These results stay at the preliminary stage at present, however, by considering the easy integration of these solution-gate FETs on a chip, it is concluded that micro total analysis system chips for clinical use are promising application fields for ZnO devices as an ecologically and economically favorable semiconductor in the next generation.