Fast and Reproducible pH Detection with Nanowire Field Effect Transistors

Abstract

As the fabrication technology of nano-size material has continued to develop, various biomedical devices, especially detection devices, have been widely researched since the size of biological entities, such as DNA, proteins, and virus, are similar to them. Chemical sensors with fluorescent labeling and parallel optical detection techniques are widely used. However, they have a number of drawbacks such as expensive and time-consuming processing for sample preparation and data analysis. To solve the problem, FET sensors have been developed as promising devices, which have benefits of free-labeling and real-time detection. Owing to its scalability and its low power requirement, complementary metal-oxide-semiconductor (CMOS) processes are dominant in modern integrated circuit manufacturing. The use of the SiNW FET sensor should enable the co-integration of CMOS for high performance sensor system. SiNW FET sensors integrated with CMOS have been demonstrated in our and other researches. However, it is still debatable whether the sensors operate stably in repeated measurements owing to its drift characteristics. Since the drain current increases slowly during DC measurement, it is almost impossible to read precise current value in fast measurement. It takes at least 15 minutes for the drain current to settle down at a constant value. If we would like to make a fast measurement requiring less than 100 s, the drift effect should be removed or the drain current should be saturated under 50 s. To solve the problem, the drift effect is analyzed as building the schematic model using resistors and capacitors. That means that the drift effect is explained by exponential function with a long and short time constant from the model. As a result, the new operation scheme employing 2-step bias voltage is proposed to reduce necessary time for compensation. The method removes drift effect very fast under 50 s by compensating the exponential function with a long time constant.