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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.

The characteristics of a glucose sensor based on an ion-sensitive TiO2/Ti extended gate electrode field-effect transistor (EGFET) are reported. A glucose oxidase-containing silk fibroin membrane was immobilized on a TiO2/Ti surface as the bio-sensing component. This EGFET-type biosensor was estimated to be able to detect a glucose concentration as low as 0.001 mg/mL in an aqueous electrolyte, which enables the sensing of glucose in the saliva and sweat. The endurance of this sensor was also examined, and it was found that the retention time of the original sensitivity for repeated use at room temperature was more than 30 days, with a high heat tolerance temperature close to 60 °C.

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A new surface-cleaning process for Si-MBE, termed Si-beam radiation cleaning (Rad clean), and a model of the cleaning process have been examined. Epitaxial Si layers of high quality have been obtained as a result of the formation of an inactive, very clean natual oxide, removal of this oxide together with contaminants slightly adsorbed on it, and dispersion of remaining contaminants without their developing into defect nuclei. Epitaxial Si layers with etch pit densities of less than 103/cm2 have reproducibly been obtained by carrying out growth at 500°C following the Rad clean process at 800°C for 2 min.