Research On Au-free Gate Process Of GaN Based High Electron Mobility Transistors

GaN-based high electron mobility transistors(HEMTs)show great potential in high temperature,high frequency and high power applications of electronic industry.With the worldwide researches,the GaN-based HEMTs have been employed in military radar and other high technique areas,and then,its commercial applications have gradually become more and more attractive.However,the development of large-scale production of GaN-based HEMTs lags behind,which seriously hinders the industrialization process.In order to reduce the cost,the HEMT devices must be mass-produced in the Si based complementary metal oxide(CMOS)process.Conventional GaN devices with Au-based metal are expensive.In addition,Au is not CMOS-compatible,which limits device preparation to dedicated lines.The gate process is critical to device fabrication.The research on the gold-free gate process not only provides a new gate fabrication process physically from the device,but also can promote the mass production of HEMT devices.In this dissertation,the Au-free Gate Process of GaN-based HEMTs was studied,from the several aspects of material selection and optimization,device preparation,electrical test analysis and device performance optimization.Firstly,the deposition process of Au-free gate electrode was analyzed and optimized.Titanium nitride(TiN)with good thermal stability and adjustable workfunction is compatibale to various CMOS processes.Based on experiment results,the good adhension of TiN has been revealed and smooth edge was available after lift-off,which indicates process feasibility.The reactive sputtering process was used to optimize the TiN film.The coating uniformity of the instrument was verified.The dependence of the resistivity on the reactive sputtering power,N2 content,and the chamber pressure was investigated.Electrical performance tests and analysis were performed on the initially TiN gate device.The titanium metallic target was used with 90W DC(Direct Current)power under 5%N2 atmosphere for TiN-gate deposition.However,compared with conventional gate devices,the peak of transconductance of TiN gate devices fabricated under this process condition is reduced by 4%,the threshold voltage is negatively drifted by 1.4V.The Schottky reverse leakage of a TiN device is nearly one order of magnitude higher than that of a conventional device due to the lower Schottky barrier height(SBH)of the TiN gate electrode.Then,the process of the TiN gate electrode was optimized.With using increasing N2 gas contents,5%,8%and 10%,the threshold voltage of the TiN gate device positively drifted,the transconductance peak also increased,and the reverse leakage decreased.The SBH calculated from the thermionic emission theory for the samples created with different N2 sputtering gas content shows that the SHB raised with the increase of the N2 content.The optimized GaN based HEMTs with sputtered TiN gate exhibited good pinch-off behavior and comparable DC output,transfer and breakdown characteristics to the HEMTs with Ni/Au gate.