Novel Structure Design And Mechanism Research Of High-voltage GaN HEMT

GaN has great prospects in high frequency,high efficiency and high power density applications,owing to its high critical breakdown electric field,high electron saturation velocity,and two-dimensional electron gas（2DEG）with high density and high mobility along Al GaN/GaN heterojunction.The breakdown voltage（BV）of GaN high electron mobility transistors（HEMTs）is far below its theoretical limit because the electric field crowding at the edge of gate and high leakage current cause premature breakdown.At the same time,the conventional GaN HEMT is always depletion mode（D-mode）due to the high density 2DEG,and many enhancement mode（E-mode）technologies are realized by depleting the 2DEG under the gate,inevitably encountering the tradeoff relationship between a high threshold voltage(V_th)and a large saturated output current(I_d,sat).In order to solve these issues,two high-voltage E-mode GaN HEMTs are designed and optimized based on the Sentaurus TCAD,and high-voltage Al GaN/GaN HEMT with graded fluorine ion terminal and composite passivation layer is proposed and fabricated.Meanwhile,key processes are optimised including passivation layer deposition,electron beam lithography and fluorine ion implantation.1.E-mode GaN HEMT with a grading Aluminum composition Al_xGa_1-xN layerA novel E-mode GaN HEMT with a grading Al composition Al_xGa_1-xN layer（GAL HEMT）is presented for a better tradeoff between breakdown voltage and the specific on-resistance.It features a positive graded Al_xGa_1-xN barrier layer with three-dimensional electron gas（3DEG）to conduct current and a negative graded Al_xGa_1-xN layer with three-dimensional hole gas（3DHG）as back barrier layer.Firstly,the high-sheet density 3DEG greatly increases the on-state current.Secondly,a high breakdown voltage is obtained because the polarization junction formed by the polarization charges in graded Al_xGa_1-xN layer improves the electric field distribution in the drift region.Meanwhile,the leakage current effectively is reduced by the 3DHG in back barrier layer,and thus improves the BV.The proposed device avoids the premature breakdown caused by the high electrical field crowding at the end of the gate,since the source and drain contact locate at the same side of the trench gate,with a source field plate.Thirdly,2DHG at the heterojunction of GaN cap layer and graded Al_xGa_1-xN layer blocks the electron current conduction path between the source and 3DEG so as to achieve E-mode.The specific on-resistance(R_on,sp),breakdown voltage（BV）,Baliga figure of merit（BFOM）and threshold voltage(V_th)of the proposed HEMT is 0.34 m?·cm²,860 V,2175 MW/cm² and 3.2 V in simulation.The new devices combine the advantages of high BV and high current,offering new ideas for the design of GaN power devices.2.High Performance flip-structure enhancement-mode HEMT with face-to-face double gatesA novel double gates flip-structure enhancement-mode（E-mode）high electron mobility transistor with step field plate（DFF HEMT）is proposed based on the substrate transfer technique.It features face-to-face double gates,including a top trench MIS gate with a step field plate and a bottom planar MIS gate,which is shorted together.The face-to-face double gates together deplete the 2DEG by using the work function difference to realize E-mode.In the on-state,the double gates not only can restore the 2DEG by the higher electric potential,but also can form the electron accumulation layers,and thus increase the saturation output current and reduce the on-resistance.Moreover,the step gate field plate modulates E-field distribution and the pinch-off effect of the double gates reduces the leakage current,both of which increase the BV in the off state.The simulation results show that the DFF HEMT achieves a low R_on,spof 0.55 mΩcm²,BV of 465 V,and V_th of 0.8 V.The V_th and output current of the DFF HEMT both increase with the decreasing distance between double gates,breaking the tradeoff relationship between high V_th and low R_on,spof conventional E-mode HEMT and providing a new approach to realize E-mode GaN HEMTs.3.High Performance Al GaN/GaN HEMT with graded fluorine ion terminal and composite passivation layerA novel Al GaN/GaN HEMT with graded F^-implantation terminal and composite passivation layer is presented by experiment.The F^-injection area decreasing from the gate to drain could play a role of variable lateral doping（VLD）technology to modulate the E-field distribution more effective of the drift region in the off-state.By electron beam lithography forming a progressively narrower injection window,the graded F^-implantation terminal is achieved through once F^-implantation.The composite passivation layer is better to confine the F^-in the passivation layer,avoiding the F^-entering the Al GaN barrier layer with the decreasing on the density and mobility of the2DEG.The GFT HEMT achieves BV and R_on,spof 447 V and 0.56 mΩ·cm².Compared with the conventional HEMT,the GFT HEMT increases BV by 94%and R_on,spby only3.7%.Compared with the Al GaN/GaN HEMT with regular F^-implantation terminal and composite passivation layer,the BV and output current of GFT HEMT have been improved,and the BFOM of GFT HEMT increases from 210 MW/cm² to 357 MW/cm².