Reliable and sensitive functional interface between integrated electronic devices and ions/molecules in solution is the key missing link for sensing and actuation in biomedical device applications. It can be observed that electrostatic forces are not only the basic forces in chemical covalent, ionic and hydrogen bonds, but also the main driving forces for carriers in semiconductor electronic and MEMS devices. Electrostatic forces obtained from applied potential difference are always attractive, except in fringing cases . This will severely limit the device design for interaction with polar molecules. On the other hand, for electronic device structures that can generate, retain and reset nonvolatile charges in the floating gate, there are many more degrees of freedom in design. An illustrative device design modified from a conventional EEPROM cell is shown in the attached figure, and the measured IV characteristics are also shown. First, the interaction force can be either attractive or repulsive [2,3], controlled by the polarity of the nonvolatile charges in the floating gate. Also, since no direct current is necessary, isolation and contamination control can be achieved in a straightforward manner. Furthermore, the extension of the floating gate separates the structure in contact with the solution and the actual sensing transistor, which enables the use of aggressive FET design for ultimate sensitivity. In comparison with the nano-wire FET structure for similar sensing purposes , the new device provides much easier operations, more accuracy and larger dynamic range. As shown in the figure, either the threshold voltage shift can be used for large signal, or the channel current in the different-pair setup can be used for extremely minute traces of polar molecules. Multiple floating-gate extension structures with electrostatic (capacitive) coupling are also the base structure for translinear circuits and neuron MOSFET , which can allow further built-in analog functions with minimal number of devices. The proposed polar molecular sensors also exceed in many aspects in comparison with conventional MEMS-based chemical sensors .
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Z. Liu, M. Kim, N. Shen and E. C. Kan, "Novel Electrostatic Repulsion Forces in MEMS Applications by Nonvolatile Charge Injection, " The 15th IEEE Robotics and Automation Society Conference on Micro Electro Mechanical Systems (MEMS), Las Vegas, NV, Jan. 22-24, 2002.