Research On AlGaN/GaN Semiconductor Heterojunction Sensor For Heavy Metal Ions Detection

Real-time monitoring of the living environment,especially the heavy metal content in water quality,can help improve the food safety testing system,thereby maintaining national physical health and ensuring quality of life.The existing mature heavy metal ion detection technology requires a significant investment of manpower,material resources and finances,which needs to be tested with large equipment in a laboratory environment.Additionally,operators need professional training,making it unsuitable for real-time outdoor monitoring.However,high electron mobility transistor（HEMT）sensors based on AlGaN/GaN heterojunction preparation have excellent properties,such as high electron saturation drift rate,wide direct bandgap,high temperature resistance,corrosion resistance,radiation resistance,and ease of integration,and have great development potential in the field of trace heavy metal ion detection.Based on the characteristics of the gate surface modification material binding with the measured heavy metal ions and transmitting information,different types of heavy metal ions can be detected by using different modification materials.In addition,different epitaxial materials and gate structures also affect the device’s electrical properties,directly influencing heavy metal ion detection performance.This article focuses on studying the heavy metal ion detection performance of AlGaN/GaN HEMT sensors with different modification materials and gate structures,and analyzing the detection mechanism.The article uses self-assembled modification materials combined with AlGaN/GaN HEMT to construct sensors,mainly studying the impact of different HEMT device structures and modification materials on specific heavy metal ion detection.The main research results are as follows:1.Firstly,a copper ion(Cu²⁺)sensor based on Al_0.18Ga_0.82N/GaN HEMT was prepared by functionalizing with L-cysteine.By detecting the changes in HEMT drain current after Cu²⁺forms complexes with L-cysteine within the concentration range of 0 mg/L-20 mg/L,the sensor’s current response was analyzed,and a general rule was obtained that the current response decreases as the added Cu²⁺concentration increases.The sensing mechanism was explained through the relationship between surface potential,2DEG concentration and current response.The high sensitivity of the prepared Al_0.18Ga_0.82N/GaN HEMT sensor to Cu²⁺detection was calculated using the current response under a specific drain bias.Therefore,the L-cysteine-functionalized Al_0.18Ga_0.82N/GaN HEMT sensor provides a promising method for efficient,rapid,and convenient detection of Cu²⁺.2.Secondly,to study the performance of HEMT devices in detecting multiple heavy metal ions,a modified AlGaN/GaN HEMT sensor（gate size of 100×70μm²）was prepared and used for comparative detection of Cu²⁺,Fe³⁺,Pb²⁺and Cd²⁺.After introducing heavy metal ions with a concentration range of 0 mg/L to 20 mg/L,the sensor exhibited a rapid and stable response,forming stable complexes with the modifying material,and its current response showed a decreasing trend with increasing ion concentration.By using the same method to modify and use a uniform structure of AlGaN/GaN HEMT to detect Cu²⁺,Fe³⁺,Pb²⁺and Cd²⁺,the sensitivity of the sensor for detecting the four heavy metal ions was calculated based on the measured current response,and the result showed that the sensitivity for detecting Fe³⁺was the highest,while that for detecting Cd²⁺was the lowest.By analyzing the equilibrium constants of metal-ethylenediaminetetraacetic acid complexes,the binding abilities of modifying materials for these four heavy metal ions were found to be different,and therefore,the sensitivity of the HEMT sensor for these four heavy metal ions differed significantly.3.Thirdly,to achieve more precise and specific recognition of heavy metal ions,an enhanced AlGaN/GaN HEMT device with a folded gate was fabricated and modified with2-mercaptobutyric acid for Fe³⁺specific detection.The crystal quality of the epitaxial growth material and the surface morphology of AlGaN were characterized by X-ray diffraction（XRD）and atomic force microscopy（AFM）techniques,confirming that the fabricated HEMT device is suitable for ion detection experiments.Fe³⁺was surface-dropped onto the HEMT sensor with traditional gate,3-fold gate,and 5-fold gate.The change in current response was analyzed with I-V characteristics of the device and constant 7 V drain bias voltage,and the law of the increase in the change in current response with the increase in the introduced Fe³⁺concentration was obtained.The sensitivity of the three devices were calculated based on the current response,and the 5-fold-gate HEMT sensor showed the highest sensitivity.The detection mechanism of the 5-fold-gate sensor was explained based on the 2DEG channel position electric field distribution and the HEMT device’s capacitance model.The higher electric field strength and more uniform electric field distribution during the sensing process give this structure an advantage over other structures.Based on the current response of HEMT after adding Fe³⁺,Cu²⁺,Pb²⁺and Cd²⁺ions,it was proved that this sensor has specific recognition for Fe³⁺.Therefore,the 2-mercaptobutyric acid-modified HEMT sensor with5-fold gate has great potential in real-time detection of trace Fe³⁺.