Research On Characteristics Of GaN HEMT PH Sensor Based On Recessed Gate Structure

The pH sensor is an indispensable inspection device for measuring the pH of liquid media,precision monitoring and scientific certification.It has important applications in the field of environmental,medical,industrial,agricultural and biological solutions.With the development of science and technology,all-solid-state pH sensors based on ion-sensitive field-effect transistors are favored due to their small size,non-fragility,high sensitivity,stable performance,and portability.However,the development of Si-based pH sensors is gradually approaching the theoretical limit.Due to the performance defects of the material itself,it cannot work in high temperature(below 150?)and some specific solutions(hydrofluoric acid,etc.).Reliability cannot be guaranteed,which greatly limits its usefulness.The gallium nitride material has a wide band gap(3.4e V),which enables it to prepare high-temperature stable devices;and it is chemically stable and can be used in solutions,so it is considered an ideal choice for developing pH sensors.In addition,the AlGaN/GaN pH sensor has a simple preparation process and is compatible with the integrated circuit preparation process.It can couple and integrate the sensor and subsequent amplification circuits,which has gradually become a potential solution to promote the large-scale industrialization of ISFET structure pH sensors.This paper analyzes the sensing mechanism of pH sensors based on GaN high electron mobility transistors,studies the implementation method of enhanced-mode pH sensors,and focuses on the DC characteristics,sensing characteristics,and temperature characteristics of AlGaN/GaN HEMT pH sensors with recessed-gate structure.The details are as follows:Firstly,the work studies in the implementation method and mechanism of enhanced-mode AlGaN/GaN HEMT devices,and analyzes the advantages of the recessed-gate structure to achieve enhanced-mode devices.Then,the basic characteristics of recessed-gate AlGaN/GaN HEMT device and the conventional HEMT device were fabricated and tested.It was found that with the increase of the recess depth,the threshold voltage of the HEMT device gradually increased,the saturation current decreased and the gate leakage current increased.Secondly,facing the need of the enhanced-mode pH sensor,we designed and fabricated the recessed-gate AlGaN/GaN HEMT pH sensor by combining the AlGaN/GaN HEMT pH sensor with the recessed-gate structure,thereby realizing an enhanced-mode pH sensor.By analyzing the sensing characteristics of the recessed-gate AlGaN/GaN HEMT pH sensor and the conventional HEMT pH sensor in different pH solutions,it was found that the current sensitivity of the recessed-gate AlGaN/GaN HEMT pH sensor increased significantly.Through the analysis and testing of the response speed of the pH sensor,it was found that the response speed of the pH sensor was greatly improved by introducing the recessed-gate.It is proved that the introduction of the recessed-gate structure is of great significance to the improvement of the pH sensor's sensing performance.Finally,the sensing characteristics of the recessed-gate AlGaN/GaN HEMT pH sensors were compared at different temperatures.It was found that with the increase of temperature,the threshold voltage of the pH sensors increased gradually.The increase of the temperature caused no damage to the pH sensors,and the higher the temperature,the greater the voltage sensitivity(S_V)of the sensor,which proves the high temperature application of AlGaN/GaN HEMT pH sensors.