Highly Improved Response and Recovery Characteristics of FET-type Gas Sensor using Pre-Bias

다국어 초록 (Multilingual Abstract)

Recently, a gas sensor technology has become increasingly important due to the increasing demand for automobiles, air quality control, farming, and household appliances. Thus sensor technology is expected to play an important role in the Internet of Things (IoT) era. Various types of gas sensors such as optical, electrochemical, semiconducting and FET-based gas sensors have been widely reported so far. However, most of the studies on the gas sensors have been focused on resistor-type sensors and the investigation of sensing materials rather than the electrical control scheme. The resistor-type sensor has demerits in size, yield, and integration with CMOS circuits. Thus, to fulfill the demands for low cost, scalable, stable, and CMOS compatibility for the gas sensor, FET-type sensors have been widely studied. Although the FET-type sensors have a higher degree of freedom coming from four terminal features in enhancing the sensing performance, there have been no report on electrical control scheme.
In this dissertation, we propose, for the first time, a pulse pre-bias scheme for enhancing the sensing performance of Si FET-type gas sensor and investigate the pre-bias effect on the response and recovery characteristics. The Si FET type gas sensor having a control-gate (CG) and a floating-gate (FG) in horizontal direction is fabricated to investigate the pre-bias effect. The ZnO film as a sensing material is deposited between the CG and the FG by ALD. By applying the pulse bias scheme to the CG of the FET-type gas sensor, the pre-bias effect is verified, and we analyze the reaction between NO2 gas and the ZnO as a sensing layer deposited on the Si FET-type gas sensor. The mechanism responsible for the pre-bias effect is explained using energy band diagram.
The proposed scheme was verified to be very efficient in improving the gas response and the reduction of the recovery time to NO2 target gas. It is expected that the pre-biasing scheme will be very practical in the commercialization of the FET-type gas sensor.

목차 (Table of Contents)

• Chapter1 1
• Introduction 1
• 1.1 Study background 1
• 1.1.1 Motivation 1
• 1.1.2 Improvement of gas sensing characteristics 7
• 1.2 Purpose of research 11
• 1.3 Thesis outline 12
• Chapter2 13
• Device structures and fabrications 13
• 2.1 Device structure 13
• 2.2 Device fabrication 17
• 2.2.1 Key fabrication process 27
• 2.2.2 Simulation study for buried channel implantation 22
• 2.2.3 Formation of sensing layer 25
• Chapter3 28
• Device characteristics 28
• 3.1 I_V characteristics of FET-type gas sensor 28
• 3.2 Electrical modeling of FET-type gas sensor 32
• 3.3 Nonvolatile functionality 37
• 3.4 Gas sensing mechanisms of the FET-type gas sensor 40
• 3.5 Resistor-type gas sensor 44
• Chapter4 48
• Proposed pulse per-bias scheme and gas sensing characteristics 48
• 4.1 Motivation 48
• 4.2 Proposed pulse pre-bias scheme 50
• 4.3 The pre-bias effect 52
• 4.3.1 The pre-bias effect on gas response 52
• 4.3.2 Mechanism responsible for the gas response 55
• 4.3.3 The pre-bias effect on gas recovery 60
• 4.3.4 Mechanism responsible for the gas recovery 62
• 4.4 Gas response to different concentration of NO2 gas 64
• 4.5 Optimal pulse pre-bias scheme for oxidizing gas 70
• 4.6 The advantages of the pre-bias effect 74
• 4.7 The pre-bias effect on reducing gas in recovery period 76
• Chapter5 78
• Conclusions 78
• Appendix A. 80
• Considerations for the pulse pre-bias scheme 80
• A.1 Effect of the pulse with of pre-bias 80
• A.2 Effect of the carrier gas 81
• Appendix B. 82
• Capacitance change FET-type gas sensor 82
• Bibliography 84
• List of Publications 92
• Abstract in Korean 98