Breakdown Mechanism And New Structure Of AlGaN/GaN HEMT With Back-barrier Buffer Layer

Gallium nitride （GaN） has been an attractive material for power electronics for itswide energy band gap, large breakdown field and high electron saturation velocity. Also,large polarization effect within AlGaN/GaN hetero structure result in two-dimensionalelectron gas （2DEG） of high density and large mobility. However, undoped GaN showsn-type conductivity, with carrier density of1014¹015cm-3though with growthoptimization, leading to a very severe buffer current leakage, limits the breakdownvoltage to several hundred volts.With theory analysis and simulation, I studied effects of AlGaN buffer layer,introducing of acceptor traps and polarization doping on the leakage current ofAlGaN/GaN HEMT in this paper. Results showed that the leakage current decreases asAl mole fraction increases in the AlGaN buffer layer, simultaneously the decreases ofsaturation output current. With simulation, I optimized the Al mole fraction for AlGaNbuffer layer is0.05for the trade-off. Results of simulation based on Al0.05Ga0.95Nshowed good output and pinch-off characteristics. Breakdown voltage obtained forL_gd=3μm is293V and increases as L_gdincreases, reached535V for L_gd=8μm.Impact of acceptor traps on AlGaN/GaN/HEMTs’ leakage current and breakdownvoltage was studied systematically, including the trap density and trap energy level.Simulation results show that deep level acceptor traps in AlGaN buffer layer caneffectively restrain the leakage current and thus improve the high breakdown voltage,and the breakdown voltage increases linearly as the trap density increases, and alsoincreases with a deeper level energy. For L_gdequals25μm, the breakdown voltage is2300V. After optimization for different L_gd, high breakdown voltage of3800V wasobtained for L_gd=30μm.Polarization doped AlGaN buffer layer can suppress the leakage current and reducethe peak electric field compared to conventional AlGaN/GaN HEMT. Simulation resultsshowed that, polarization doping can restrain the leakage current effectively and theleakage current can be lowered4orders when the maximum Al mole fraction is0.1. Breakdown voltage obtained for L_gd=3μm is640V and increases as L_gdincreases,reached3800V for L_gd=27μm.