Hydrogen Sensor Based On AlGaN/GaN HEMT

In recent years,hydrogen,as a clean energy alternative to fossil fuel,has been widely concerned.Because hydrogen is an explosive gas,the requirements of hydrogen sensor are fast response,high signal amplification and repeatability.The traditional hydrogen sensor is mainly resistance type,its sensitivity is high,but its output signal is weak,so it needs to design additional back-end circuit.And the requirements of the working conditions are more stringent,can not be widely used in a variety of environments.The novel AlGaN/GaN high electron mobility transistor(HEMT)has wide band gap,high mobility,high temperature stability,and chemical and physical inertness in harsh environment.The hydrogen sensor based on the HEMT can realize fast sensing and good stability.In the current HEMT-based hydrogen sensor,the H₂molecule is catalyzed by the gate to convert the metal into H atoms,which are then diffused and adsorbed at the gate/semiconductor interface.Through the study of hydrogen diffusion,it is inferred that with the decrease of the gate metal thickness,the concentration of H atoms adsorbed at the gate/semiconductor interface may increase,thus improving the device performance Based on the above background,the response characteristics and stability of AlGaN/GaN HEMTs modified by different catalytic metals(Pd,Pt,Pd Pt)to different concentrations of hydrogen in a wide temperature range were studied.The fabrication process of AlGaN/GaN HEMT millimeter size devices is studied by using semiconductor technology,mainly focusing on the preparation of ohmic contact.The experimental results show that ohmic contact can be obtained stably by annealing in nitrogen atmosphere for 3 min at 860?.On this basis,we test the response to hydrogen by depositing Pd and Pt in the gate region of the device.The experimental results show that the Pd modified device has a good response to hydrogen when the hydrogen concentration is 0.1%at room temperature,but its recovery performance is poor.The Pt modified device can basically recover to the initial reference value,but has little response to hydrogen.According to the study of hydrogen diffusion in the thin film,it can be inferred that the thickness of Pd and Pt metal layer greatly affects the response of the device to hydrogen.In 0.1%hydrogen atmosphere,with the decrease of Pd(Pt)thickness,the change of response current to hydrogen increases from 0.077 mA to 0.27 mA(from 0.0668 mA to 0.0785 mA),and the response time decreases from 92s to 51s(63s to 50s).The optimization of the sensitive layer thickness(2-5nm)significantly improves the performance of the device.In order to improve the disadvantages of single metal modified devices,the millimeter-scale gateless Pd/Pt modified AlGaN/GaN high electron mobility transistors hydrogen sensors were fabricated and the response of the devices to different hydrogen concentrations was tested.The hydrogen sensing characteristics of the device show that the hydrogen response and recovery rate of the device modified by Pd/Pt alloy are significantly higher than those of the device modified by Pd and Pt alone,and the device with Pd/Pt ratio of 2(mg):1(mg)has the best hydrogen sensing performance.This conclusion is further confirmed by the steady-state analysis of adsorption equilibrium.When the concentration of hydrogen is 0.1%at room temperature and the flow rate of hydrogen is 200 sccm,the current change of Pd/Pt(2mg:1mg)sample is 0.249 mA,the response time and recovery time are41s and 42s respectively.In addition,the current variation of the device annealed at 200?is 20%higher than that of the device without annealing.Increasing the test temperature to55°C further improves the response.The device can work normally in 200,400 and 600?hydrogen atmosphere for a long time,and has good high temperature stability.