Environmentally Robust Biosensor Using Field Effect Transistors of Heteroepitaxial Diamond

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In this research, ion sensitive field effect transistors (ISFETs) have been fabricated on the hydrogen terminated (H-terminated) diamond surface resistant to electrochemical environment and operated as transducers of biosensors.1. The electrolyte solution gate FETs have been formed on H-terminated surfaces. The gate structure is just thet H-terminated diamond exposed to directly. This type of FET structure where semiconductor surface is exposed to electrolyte solution is firstly developed in the world. Compared with conventional ISFET, the higher current controllability is achieved.2. The FETs exhibit ideal current-voltage characteristics in not only homoepitaxial and heteroepitaxial diamonds, but also polycrystalline diamonds which is easier to synthesize. In the current-voltage characteristics, perfect pinch-off and the saturation of drain current have been measured. The leakage current at the off state is extremely small and the current ratio between on-off is more than 4 orders of magnitude. The FETs operate stably in the above mentioned characteristics between pH 1-14.3. The threshold voltages of the FETs are independent from pH values. From this result, an appropriate ion sensitive base can be immobilize on the pH insensitive surface leading to high selective molecular recognition.4. On the other hand, the threshold voltages strongly depend on the density of Cl ions in the solution and vary by 30-60 mV according to the density change by one order The Cl ion detection has been observed from 1 Mol/L to 10^<-6> Mol/L exhibiting the possibility of Cl ion sensitive FET.The diamond solution gate FET has been developed for the first time. Their operation in hard environment has been also confirmed and the highly sensitive detection of Cl ions has been achieved.