FET Gas Sensor Platform Having a Horizontal Floating Gate

Abstract

Today, there are numerous kinds of gases which are becoming widely used in the various industry fields. However, the harmful gases which a disease of the respiratory system are generated by industrial activity. Therefore, the demand for a gas sensor which can detect the noxious gases is expected to grow. Especially, there will be a growing interest in portable, low-cost, high reliability and low-power gas which can be applied to smart device in the future. Many kinds of gas sensors have been developed by many researcher until now: electrochemical, electrical, mass sensitive, magnetic, optical and thermometric type gas sensors. One gas sensor type of interest is that based on FET (Field Effect Transistor) has been considered as advanced gas sensor which can implant in various smart devices. However, there are many disadvantages in conventional FET type gas sensors. In this thesis, we propose a gas sensor platform having a horizontal floating-gate (FG) to solve the problem shown in conventional FET gas sensor. We first introduce structure and fabrication process sequence of the gas sensor that we proposed. The gas sensor is based on MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and have a horizontal FG and the sensing layer covers partly the control-gate (CG) formed horizontally and the passivation layer formed on the FG in the center. The gas sensor reads out work-function (WF) change in the sensing layer formed between FG and CG (control gate), when the device is exposed to a target gas. The sensing layers in our work are semi-metal or semiconductor. Next, we analysis gas sensing characteristics of the fabricated gas sensor by using various sensing layers and target gases, respectively. We measure transfer and transient curves of the device in target gases. From the measured data, we explain operating principle and show gas sensitivity, selectivity, response and recovery characteristic and Langmuir relationship of the gas sensor. We also show the various methods that various gas sensing layers are formed on the gas sensor and verify gas sensing characteristics of the device in which SnOx (Tin Oxide), ZnO (Zinc oxide) and CNT (Carbon Nanotube) are used as a gas sensing materials by using sputtering, ALD (Atomic Layer Deposition) and ink-jet printing method. Then, we show sensor calibration of the gas sensor by CG bias. Finally, we fabricate FET gas sensor based on wide band gap material (Gallium nitride) which can operate at high temperature (T > 300 °C) and conform gas sensing characteristics of the gas sensor. We think that the proposed gas sensor platform can be applied to various FET type device (ex TFT: Thin Film Transistor, TFET: Tunneling Field Effect Transistor) useful and a key component in an electronic nose system.