Studies on GaN HEMT based gas sensors for low concentrations of NO2 and NH3

• Main Author: Ranjan, Akhil
• Other Authors: Radhakrishnan K
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/141573
• Institution: Nanyang Technological University

In recent decades, the demand for gas sensors has increased rapidly because of their large-scale use in medical instruments, automobiles and laboratories. For example, nitrogen dioxide (NO2) and ammonia (NH3) are found to be among the main constituents of exhaust gases and harmful air contaminants. Inhalation and long-term exposure to these gases can cause health issues and may result in life threatening conditions. Hence, there is a need to detect low concentrations of NO2 and NH3. GaN based sensors are gaining momentum due to their unique properties, such as high electron mobility, mechanical stability and wide bandgap that allow high thermal resistance and the ability to withstand harsh chemicals, gaseous and the corrosive environments. Furthermore, AlGaN/GaN high electron mobility transistors (HEMTs) are extremely sensitive due to the proximity of two-dimensional electron gas (2DEG) to the sensing surface and their high sheet carrier concentration (~1013 cm-2), thereby enhancing the detection sensitivity. Surface charges vary due to the interaction of the environmental conditions such as temperature and humidity leading to the modulation of 2DEG. This thesis presents the fabrication and characterization of GaN HEMT based gas sensors with Pt, Pd and Ag functionalization layers that can sense low concentrations of NO2 and NH3 for a large range of concentrations and temperatures. The sensing mechanisms corresponding to the temperature dependent response of these gases have been explained in detail. It was found that the Pt/AlGaN/GaN HEMT based sensors sensed and recovered completely to the steady state value when exposed to various concentrations of NO2 at temperatures above 275 °C. A high sensitivity of 5.1 % with the corresponding ΔI of 1.72 mA was achieved for 10 ppm NO2 at 275 °C, which is better than the reported results of 1 %, but at a higher temperature of 300 °C. A sensitivity of 1.2 % was obtained for a very low NO2 concentration of 50 ppb in this study, which is the lowest NO2 concentration sensed by Pt/AlGaN/GaN HEMT based sensor. The large surface area of the interdigitated electrodes and the catalytic property of Pt functionalization layer enabled high sensitivity for low NO2 concentrations of < 10 ppm. A rapid response time of < 2 min and a recovery time of < 5 min were obtained for all the concentrations which are comparable to or better than the reported results. In this NO2 sensing studies, the activation energy values of 0.33 eV/ion and 0.64 eV/ion were obtained for response and recovery times, respectively, which are lower than the reported values of 0.9 eV and 1.0 eV respectively, for open gate AlGaN/GaN HEMTs. The lower values obtained in this study are mainly due to the presence of Pt functionalization layer, which enhances the reactivity due to its catalytic properties. Low concentration (< 50 ppm) sensing of NH3 over a wide temperature range of 30-275 °C using AlGaN/GaN HEMT based gas sensors with Pt functionalization layer has been systematically studied and other functionalization layers that have not been previously studied such as Pd and Ag have also been investigated. While the current decreased (ΔI is negative) when the sensor was exposed to different concentrations of NH3 at room temperature, it increased (ΔI is positive) above 200 °C. This phenomenon of ΔI being negative at lower temperatures and positive at higher temperatures has not been reported thus far. At low temperature (30 °C), NH3 is adsorbed in a molecular form on Pt surface and donates free electrons to produce negative potential at the Pt/AlGaN interface leading to a decrease in the current. However, NH3 gets dissociated into positively charged hydrogen ions (H+) at high temperature (>200 °C). This results in positive surface potential causing an increase in the current. The observed phenomenon was validated by plotting Arrhenius plots for two different regions and relating the activation energy with the mechanism. The sensor with the Pd functionalization layer exhibited higher sensitivity at 275 °C as compared to Pt and Ag sensors due to higher hydrogen affinity of Pd. The sensor with the Ag functionalization layer was found to respond quickly to the change in NH3 concentration and demonstrated faster response and recovery times (< 3 min) as compared to other sensors using Pt and Pd. This is attributed to the oxidized Ag surface which creates a favourable site for the adsorption of NH3 gas. After the adsorption, charge transfer from NH3 to Ag occurs quickly leading to faster response and recovery times. Having studied the AlGaN/GaN HEMT based sensors, further efforts were made to investigate Pt/AlN/GaN/AlN HEMTs for NO2 and NH3 sensing. This is the first time that thin barrier AlN/GaN/AlN HEMT has been demonstrated as a gas sensor. The sensitivity obtained by this sensor was lower than that obtained for Pt/AlGaN/GaN HEMT based sensor. However, the Pt/AlN/GaN/AlN HEMT based sensor was found to respond quickly to the change in the NO2 concentration. The respective mechanisms have been illustrated.