Research On The High Sensitivity AlGaN/GaN HEMT Sensor

Gallium nitride is a third-generation semiconductor material with large band gap,high critical electric field,high electron mobility and good heat dissipation.It has been widely used in radio frequency devices,power amplifiers and power electronic devices fields.The Al Ga N/Ga N heterojunction structure is based on the difference in polarization strength between aluminum gallium nitride and gallium nitride.The energy band bends downward at the interface of Al Ga N/Ga N and form a thin layer of electrons with both high electron concentration and high electron mobility which is called two-dimensional electron gas(2DEG).The distance between the 2DEG and the surface of the epitaxial layer is the thickness of the Al Ga N barrier layer.Depending on different aluminum composition,it is generally between 8 to 25 nanometers.The 2DEG is very sensitive to the charge on the Al Ga N surface,so it has great potential for applications in the field of sensors.In addition to the high sensitivity of the 2DEG to the surface charges,the high temperature resistance,biocompatibility,chemical stability and optical transparency of the Ga N material have many unique advantanges in different sensor applications.Though gallium nitride sensor is very promising,there are technical problems to be overcome in real applications.The most important one is the packaging problem.Since Al Ga N/Ga N sensor works in the solution environment,its gate sensitive area is exposed to the solution while the adjacent drain/source matals are covered and protected by the packaging mateiral.It is very difficult to achieve both reliable and patternable packaging.Nowadays the researchs on the Al Ga N/Ga N sensor are limited to the biochemical experiments verifying their feasibilities,there is a lack of systematic and theoretical analysis of the Al Ga N/Ga N sensor.For example,what are the characteristics that influence the sensitivity of the Al Ga N/Ga N sensor? where does the background noise come from? How to evaluate the reliability of this solution gate sensor? Though many papers repoted their works on the improvement of sensor performances through optimizations on the sensor structure and fabrication process,these works cannot guide the design and applications of the sensor,nor can they provid answers to the scientific questions such as the theoretical limit of the sensor sensitivity,the dominant source of the noise,and the physical boundaries of the safe operating area.In this paper we have conducted indeph reseach on the packaging problems,the sensitivity optimization,the safe operating area of the Al Ga N/Ga N sensor,and applied the research results to biochemical detection applications.We achieved the following innovations:1.We propose the key limiting factor of the sensitivity and the optimization method of the sensor structure in the packaging Al Ga N/Ga N sensorIn the practical applications,the ultimate limiting factor for the sensitivity of the Al Ga N/Ga N sensor is the series resistance in the sensor system.When the W/L ratio of the sensor channel equals to the channel sheet resistance ?2DEG devides the series resistance RS,the sensitivity of the Al Ga N/Ga N sensor reaches its maximum.We also proposed that there is a maximum thickness of the gate sensitive membrane,if the deposition of the gate sensitive membrane exceeds its maximum value,the sensitivity of the sensor will decrease because of the transconductance gm is reduced.Finally,based on these theory,we achieved a Al Ga N/Ga N sensor with 157 ?A/p H high sensitivity,which is the highest sensitivity in the packaging Al Ga N/Ga N sensor till now.2.We proposed the concept of the safe operating area(SOA)of the Al Ga N/Ga N sensor,also we find out the boundary of the safe operating area and its physical mechanisms.This paper presents the concept of the safe operating area of the Al Ga N/Ga N sensor.We find out the boundaries of the safe operating area and explain their failure mechanisms based on the study of two typical sensor structures.In the p H Al Ga N/Ga N sensor,the boundaries are the quality of the sensor package and the turn-on voltage of the solution/Al Ga N diode.In the biosensor,the boundaries are the oxidation potential and reduction potential of the gate sensitive membrane in the solution.We also developed a method to find out the global optimal quiescent operating point which has the largest sensitivity taking the safe operating area concept in consideration.When the package quality is the key limiting factor,we developed a kind of CMOS-compatible packaging method which reduces the leakage current,enlarges the SOA,improves the reliability and increases the maximum sensitivity.Furthermore,we analyzed the leakage and failure mechanisms of different traditional packaging materials,and proposed a model predicting the failure of the packages.3.We developed an Al Ga N/Ga N immunosensor which achieves ultra-high limit of detection(LOD)of BNP(Brain natriuretic peptide),a biomarker for cardiovascular and cerebrovascular diseases.This paper proposed a four-probe Al Ga N/Ga N sensor detection method which can effectively suppress the background noise and improve the signal-to-noise ratio(SNR).The article also analyzes the possible sources of the background 1/f noise and the way to distinguish their origins.We also give out the SNR curve under differnet reference electrode voltage bias conditions.The best SNR quiescent operating point coincides with the larges gm point.We apply the four-probe method in the BNP detection and prove this method can improve the BNP LOD by 2 orders.By biasing the Al Ga N/Ga N sensor at its optimal SNR point,we finally achieved the BNP sensor with 0.097 ppt LOD.