Study On High Performance AlGaN/GaN Millimeter-wave HEMT

Due to GaN-based semiconductor materials possess superior properties,such as high mobility,high electron saturation velocity and high critical breakdown field strength.It has great potential in millimeter-wave power devices.For millimeter-wave power devices,the maximum oscillation frequency,output power density and power added efficiency are the most important parameters.In order to improve the above parameters,the systematic study of AlGaN/GaN millimeter-wave HEMT is presented.In chapter 2,the polarization effect of GaN-based materials is introduced.Then,the mechanism of two-dimensional electron gas in heterojunction and principle of device is presented.Later,the epitaxial growth of heterojunction and process of device fabrication are studied,and the key process is emphasized,including wafer cleaning,ohmic contact,active region isolation,surface passivation,Grss etch,gate recess,gate metal and interconnect metal.Finally,five parameters of millimeter-wave power device are discussed,namely the maximum cut-off frequency,the maximum oscillation frequency,output power,power gain and power added efficiency,and the way to improve these parameters are also proposed.In chapter 3,the relationship between the gate structure,passivation structure,short channel effect,source-drain distance and frequency characteristics of device is studied by device simulation.In the study of gate structure and passivation layer,the operating frequency of the device increases with the gate length decreases.However,as the gate length decreases,the gate resistance becomes the main factor restricting the frequency.In order to solve the contradiction between gate resistance and gate length,T shaped gate structure is proposed,in which the gate height,gate cap and passivation layer thickness are optimized.As the gate height and gate cap is 120nm and 500nm,the parasitic capacitance introduced by the gate cap can be minimized,meanwhile,the influence of gate resistance on the power gain can also be solved.In the study of short channel effect,the reason for the short channel effect is analyzed,and the scheme of improving short channel effect is proposed.In the study of source-drain distance design,considering the influence of parasitic capacitance and resistance on frequency characteristics,gate-source spacing is determined to be 0.9?m.The design of gate-drain spacing is mainly based on the influence of breakdown voltage,therefore the gate-drain spacing is 1.5?2?m.In chapter 4,according to the gate structure proposed in device simulation,the electron beam lithography process is studied.Electron beam lithography process includes resist and developer study,charge accumulation and main field stitching study,exposure dose and pattern study.Finally,40nm minimum line and 90nm floating T-shaped gate structure is achieved.In chapter 5,aiming at the research of short channel effect in device design,conventional AlGaN barrier structure combined gate recess scheme is proposed.Firstly,the recess gate process is studied.As the ICP coil power and reflected RF power is 100W and 10W,respectively,the gas flow is 15sccm and pressure is 5mTorr,a steep sidewall is achieved.Then,the influence of etching depth on threshold voltage,maximum transconductance,drain current,off-state characteristics is investigated.It is shown that different gate length needs to be matched with different etching depth to suppress short channel effect.For device operating at millimeter-wave,the gate length is generally less than 100nm,thus the corresponding barrier thickness should be less 10nm.By using this configuration,the maximum transconductance of 400mS/mm,off-state leakage of less than 10-3mA/mm,DIBL of less than 30mV/V,breakdown voltage of 85V,fT of 53GHz and fmax of 147GHz is achieved.The performance is greatly improved compared with the non-etched device.In chapter 6,the N2O plasma treatment on access region is proposed.An oxide layer about 3nm is formed by plasma treatment in the gate region,which can prevent recess gate induced gate leakage,RF transconductance collapse,meanwhile,it can partially repair etching induced mobility degradation.The gate leakage is reduced by three orders of magnitude.As the gate voltage is 3V,the gate leakage is still less than ImA/mm,enabling the device working at large gate voltage.In addition,the N2O plasma treatment on gate-source region and gate-drain region can not only suppress passivation free induced effective gate length extension,but also improve the passivation effect.In the double pulsed measurement,the current collapse ratio is less than 5%.In chapter 7,based on the results of previous chapters,three kinds of high performance millimeter-wave are fabricated,including ultra-high millimeter-wave HEMT,high efficiency millimeter-wave power HEMT,and enhancement-mode millimeter-wave HEMT.For the ultra-high millimeter-wave HEMT,90nm T-shaped floating gate and N2O plasma treatment on access region is adopted.A suppressed current collapse of 4%,RF transconductance of 10%,and improved breakdown of 80V is obtained.The small signal measurement shows fT and fmax of 98GHz and 322GHz,the fmax is highest reported for AlGaN barrier HEMT.A record fmax·Vbr of 25THz·V is achieved,which is highest for GaN-based HEMT.For the high efficiency millimeter-wave power HEMT,the recess gate configuration is used to suppress short channel effect,favoring the enhancement of gate control ability and lower buffer leakage.In addition a thin oxide layer formed by plasma treatment on gate region reduces the gate leakage at least three orders of magnitude compared with conventional recess gate HEMT,resulting ultralow off-state current of 9.4×10-7mA/mm,and well suppressed current collapse of around 4%.It is indicated that N2O plasma treatment on access region can reduce gate leakage,off-state leakage,and current collapse.At the operating frequency of 30GHz,the peak power added efficiency of 46.8%accompanied by the power density of 6W/mm,as well as those of 59.4%accompanied by 4.3W/mm at Vds=25V and Vds=15V,respectively.The output density and power added efficiency are the highest reported for GaN-based HEMT at 30GHz.For the enhancement-mode millimeter-wave HEMT,the recess gate is used to achieve enhancement-mode operation.The plasma treatment on gate region is used to suppress gate leakage and increase threshold voltage.The fabricated HEMT shows an outstanding performance,with a Vth of 0.4V,maximum drain current of 965mA/mm,and fmax of 272GHz,indicating this method could be a good technique for fabricating high enhancement-mode millimeter-wave HEMTs.