Extended-gate MOSFET for High Sensitivity Photodetectors and pH Sensors
Over the past years, semiconductors have been greatly used in sensors. With the development semiconductor technology, the semiconductor sensors showed high sensitivity, large integration and reliable stability. Ion-Sensitive Field Effect Transistor (ISFET) changed the gate electrode of Metal-oxide-semiconductor Field Effect Transistor (MOSFET) from metals to electrolyte. In this dissertation, the perovskite, which is a kind of material with large light absorption coefficient, is used to replace the electrolyte in ISFET based on the structure of ISFET to create high sensitivity photodetector. The perovskite is deposited on a silicon wafer and physically separated with MOSFET. Besides taking both advantages of perovskite with excellent optoelectrical property and silicon as a single crystal with good electrical property to get high responsivity, this extended-gate structure provides convenience for changing the capacitance of perovskite and removing the influence of light on MOSFET. The frequency of electrical signal on perovskite can modulate the capacitance of perovskite, which can be used when the capacitance of perovskite is too high compared with MOSFET. The ionic movement influence, which degrees the performance of this photodetector, can be reduced by adding another MOSFET served as current source at the gate of original MOSFET. Inspired by the ionic movement of perovskite, this dissertation also proves ionic movement in pH electrolyte deteriorates the sensitivity of ISFET by electrical measurement. The extended gate structure is utilized to separate the MOSFET and pH capacitance so the MOSFET is free from changing of temperature. Low temperature can decrease the mobility of ions in pH electrolyte especially after the phase change from liquid to solid. The ions in electrolyte can’t follow the high frequency bias voltage so the ionic movement is less at high frequency. Our results show that the ISFETs have larger sensitivity in low temperature and high frequency since the ionic movement can be suppressed by low temperature and high frequency.