Investigation of AlGaN/GaN Heterostructure Ion-Sensitive Field-Effect-Transistor for pH Sensor Application

Abstract

This thesis focuses on the investigation of AlGaN/GaN heterostructure Ion-Sensitive Field-Effect-Transistors (ISFETs) and their application for pH sensing This work provides three kinds of methods which are (NH4)2S2O8 treatment hydrogen peroxide (H2O2) treatment and Ultrasonic Spray Paralysis Deposition method to improve the pH sensing characteristics of the conventional AlGaN/GaN ISFETs For the reason to better understand the chemical composition oxide layer thickness surface roughness hydrophilicity and the variation of sheet concentration and mobility of two-dimensional electron gas concentration (2DEG) after applying the methods mentioned above the (1) Electron Spectroscopy for Chemical Analysis (ESCA) (2) Transmission Electron Microscopy (TEM) (3) Atomic Force Microscopy (AFM) (4) Contact Angle Measurement and (5) Hall Measurement were utilized By ESCA we could observe the shift of the binding energy of Al 2p peak after apply those methods At the same time we confirmed the formed oxide layers were Al2O3 by quantitative analysis Next the thickness of the formed oxide layers could be obtained through TEM Moreover by applying AFM the optimal treatment time and temperature of (NH4)2S2O8 could be acquired After that by using Contact Angle Measurement the hydrophilicity of the samples could be observed Finally we could figure out the variation of sheet concentration and mobility of the samples through Hall Measurement In addition to material analysis the Metal-Oxide-Semiconductor (MOS) diodes were also fabricated in this work to figure out k value of the oxide layers which were formed by H2O2 oxidation and USPD method from capacitance-voltage measurement Besides we also observed the hysteresis of the diodes to confirm the improvement of surface defect density and utilized high-low-frequency method to derive interface state density Finally after the AlGaN/GaN ISFETs were fabricated we compared the pH sensing characteristics of the untreated device the device with (NH4)2S2O8 treatment the device with H2O2 treatments and the device with USPD Al2O3 First of all the sensitivity of the devices were measured by contacting pH2 ~ pH12 buffer solution Following the order mentioned above the sensitivity was 41 6 mV/pH 48 6 mV/pH 55 2 mV/pH and 55 6 mV/pH respectively Next the response time of the devices were calculated by contacting buffer solution in the order pH7→pH4→pH7 and pH7→pH10→pH7 which was 10 6 s 9 7 s 7 2 s and 6 7 s respectively Furthermore Hysteresis effect and the stability of the devices were investigated By contacting buffer solution in the order pH7→pH4→pH7→pH10→pH7 and pH7→pH10→pH7→pH4→pH7 the Hysteresis voltages could be obtained which were 11 1 mV 9 6 mV 6 5 mV and 5 8 mV respectively As for the stability by observing the variation of the reference voltage of the devices which were put in pH2 pH7 and pH12 buffer solution over 12 hours the stability could be determined For instance the stability were 10 1 ?mV/h、6 3 ?mV/h、1 91 ?mV/h and 1 25 ?mV/h respectively in pH7 buffer solution over 12 hours Finally the life time of the devices were measured The decay rate of sensitivity were -0 2514 mV/pH?Day -0 2386 mV/pH?Day -0 1685 mV/pH?Day and -0 1643 mV/pH?Day respectively As the experiment results showed (NH4)2S2O8 treatment could increase the sensitivity of the device but couldn’t effectively improve other pH sensing characteristics because it could not form sensing film on the surface On the contrary H2O2 treatment was able to form Al2O3 sensing film in short time and low cost However due to the uniformity problem the stability of the device with H2O2 treatment was not as good as the device with USPD Al2O3 As for USPD method it could deposit good uniformity Al2O3 sensing membrane in non-vacuum ambient and low temperature (350℃) Therefore the improvement of pH sensing characteristics of the device with USPD Al2O3 was the best among all three methods

Date of Award	2014 Jul 31
Original language	English
Supervisor	Wei-Chou Hsu (Supervisor)