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

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.