MOCVD Growth And Device Fabrication Of AlGaN/GaN Current Aperture Vertical Electron Transistors(CAVET) On Bulk GaN Substrate

Owing to the high critical electric field,high electron saturation velocity,high thermal conductivity,and high density density of the 2-D electron gas(2DEG)of AlGaN/GaN high electron mobility transistors(HEMTs)have become a subject of intense research and demonstrated great potential for high-power high frequency applications in recent years.However,serious current collapse induced by surface states,large chip areas required by the higher breakdown voltages and lower specific on-resistance,and difficult package generated by the lateral geometry are the hurdles in the way of lateral GaN power electronics application.Vertical topology of GaN is far superior to a lateral structure because of high breakdown voltage with small size chip,high thermal conductivity of homoepitaxy,ease for package with Si MOSFET and current collapse induced by alleviated electric field in vertical direction.AlGaN/GaN Current Aperture Vertical Electron Transistor(CAVET)combines high eletron mobility of 2DEG of laterial AlGaN/GaN HEMT in the lateral direction with the high eletric field distribution of vertical GaN device in vertical direction.A CAVET is consisting of current to follow from source in laterial direction through the 2DEG,until it reaches gate,to the drain on GaN substrate through a conductivity aperture.It is critical that high resistivity GaN implanted by Al ions or p-GaN doped by Mg or implanted by Mg ions as current blocking layer(CBL)blocks gate modulated current traveling vertically down through other ways but the aperture.The high resistivity GaN implanted GaN or condutive p-GaN by the use of a revised p/n junction has adopted as current blocking layer.However,the conductance of the aperture region(Lap · q · Nap ·Wg/tCBL),the CBL blocking ability,the interface pollution in the regrowth process,the lattice damage by ion implantation,the Mg memeory effect,the ohmic contact of N-polar GaN,the leakagecurrent and the DC-RF dispersion of AlGaN/GaN CAVET will be disscued.Silvaco Altas and TRIM simulation,optimizatation parameters of MOCVD regrowth,device fabrication and electrical characterization have combined for further discussion.The detailed research and major results have shown as follow:1.Unintentionally C doped GaN with sheet resistivity RSH=106 ?/? is deposited to introduce edge dislocation with deep acceptor impurities at a growth of nuclear layer switching carrier gas H2 toN2 ambient and a function of epitaxial growth temperature and?/? ratio.Morever,Al content and thickness of AlGaN barrier,AlN space layer and GaN caplayer AlGaN/GaN HEMT have been optimizatied for the growth of AlGaN/GaN HEMT.The sheet resistance,the electronic mobility and the 2DEG density of as-grown epitaxial layer obtained by Hall measurement were 332.9 ?/?,1920 cm2 V-1 s-1 and 9.7 × 1012 cm-2,respectively.AlGaN/GaN HEMT has achieved excellent electrical characteristics:a saturated drain current density IDS of up to?478,9 mA/mm,a peak transconductance Gm of 60 mS/mm,an on/off ratio IOnIOff of 0.96×108 and a high breakdown voltage VBR of 600 V@0.01 mA/mm.2.The interface pollution of MOCVD regrowth annealed at 1180? for 900s has been studied,the background carrier concentration by C-Vmeasurement is lower,by one order,than that without anneal treatment.The crystal quality?DC electrical characterization and current collapse of AlGaN/GaN HEMT on HVPE GaN substrate and sapphire/GaN tenplate have been investigated3.We have investigated the effects of surface passivation on electrical characteristics and current collapse of AlGaN MISHEMT by the different thickness of LPCVD SiNx for a comparative study.It is theoretically analyzed by the threshold voltage with a nearly linear VTH =-0.25x-2.93 decrease to extract the interface charge density QSiNx/AlGaN =2.87×1013 cm-2 at SiNx/AlGaN interface.The energy level of traps below the GaN conduction band,denoted as EC-ET the time constant Tit,and the trap state density Dit have extracted through the frequency-dependent measurement of capacitance and conductance on the MIS diodes.Silicon ions at a dose of 1×1015 cm-2+1.6×1014 cm-2 with energy of 80 keV+30 keV were implanted to source-drain regions to achieve a ohmic contact resistance Rc as low as 0.5?·mm,it annealed at 1280? in a nitrogen and ammonia ambient for 5min.4.It can calculate that the A1 implanted GaN with a conduction band offset ?EC=0.07 eV compared with the as grown GaN is equivalent to an Al0.058Ga0.942N barrier.SIMS has shown no obvious Al atoms diffusion into AlGaN/GaN channel and it proves that Al ion implantation has thermally stable property even after high temperature annealing process.The AlGaN/GaN CAVET has been successfully demonstrated by using Al-implanted GaN as a current blocking layer.A maximum source to drain current IDS of 107 A/cm2@VGS =0 V,VDS=10 V,an extrinsic transconductance Gm of 2 mS/mm and a specific ON-state resistance ROn-A of 49.1 m?·cm2 were achieved.The breakdown voltage characterization of high resistivity R>0.5 ?·-mm of HVPE GaN substrate has been studied by thinning?grinding and polishing GaN substrates process.5.We found that the insertion of a low-temperature GaN interlayer was more effective to suppress the Mg redistribution than AIN interlayer in the GaN regrown layer by C-V measurement.AlGaN/GaN CAVET was successfully demonstrated by using Mg doped GaN as a current blocking layer.Device were achieved a maximum source-drain current of IDS =769 A/cm2@VDS =10 V,VGS =2 V,an extrinsic transconductanceGm of 35.1 mS/mm,an on/off ratioIOn/IOff of 2.5X107.The high Knee voltage of AlGaN/GaN CAVET in I-V characteristics has shown that the aperture region was not conductive enough diffused by Mg in laterial direction.6.AlGaN/GaN CAVET has been discussed by using Mg implanted p-GaN as current blocking layer.This device had a maximum source-drain current density IDS of 2894 A/cm2@VDS =20 V,VGS =2 V,in extrinsic transconductanceGm of 62.4 mS/mm and a specific ON-state resistance ROn-A of 7.2 m?·cm2.For AlGaN/GaN CAVET with Al-implanted GaN,Mg doped p-GaN and Al-implanted p-GaN as current blocking layer,the DC and pulsed IDS-VDS measurements,with pulsewidths of 5ms a applied at the gate,were discussed in this dissertation.