Design And Key Technologies Of Novel AlGaN/GaN HEMTs With Electric Field Modulation Effect

Among the power semiconductor technologies,AlGaN/GaN HEMTs power semiconductor devices are a well-known structure.The breakdown voltage is an important parameter when it is applied to high-voltage and high-power applications.Different from traditional Si-and SiC-based devices,the breakdown mechanism of AlGaN/GaN HEMTs is very complicated,which can be divided into the impact-ionization-induced Schottky junction avalanche breakdown,the high gate leakage current components,the source-drain leakage current through the Ga N buffer layer,and vertical breakdown,which can be particularly pronounced when AlGaN/GaN HEMTs are grown on a Si substrate.For different breakdown mechanisms,corresponding measures should be taken to improve the breakdown characteristics of AlGaN/GaN HEMTs.AlGaN/GaN HEMTs with the electric field modulation effect has gradually become a research hot-spot.By modulating the concentration distribution of 2DEG,the channel electric field distribution is modulated.A new electric field peak is introduced,and the high electric field near the gate edge is effectively reduced.The electric field distribution becomes more uniform,and the breakdown voltage is dramatically increased.In this paper,based on the electric field modulation effect,the performance of the novel AlGaN/GaN HEMTs is in-depth discussed combined with simulation design,modeling analysis,experimental research,and the high temperature on-state DC characteristics measurement:(1)Firstly,a novel AlGaN/GaN HEMTs with a partial GaN cap layer was proposed,and enhancement-mode AlGaN/GaN HEMTs using a partial GaN cap layer was also presented with a P-type GaN gate.Through software simulation,the breakdown voltage dependence on the length and thickness of the partial Ga N cap layer was discussed.The partial GaN cap layer is introduced at the top of the AlGaN barrier layer and is partly or entirely located between the gate and drain drift regions,a 2DEG density reduction is obtained,and the 2DEG concentration changes from a uniform distribution into a gradient distribution.As the length of the partial Ga N cap layer increases,the low-density 2DEG region becomes longer,the depletion region extends and is completely depleted,and the electric field modulation effect is gradually strengthened.Therefore,the new electric field peak increases and shifts toward the drain electrode,and the high electric field peak near the gate edge is reduced,the breakdown voltage increases.With the increase in the partial GaNcap layer thickness,the 2DEG density is reduced and tends to saturate,the concentration gradient of 2DEG increases,and the electric field modulation effect is gradually strengthened.Hence,the new electric field peak increases,and the high electric field peak near the gate edge is reduced,the breakdown voltage increases.Compared with the conventional AlGaN/GaN HEMTs,under the optimal parameters,the simulation results have shown that the breakdown voltage of the novel AlGaN/GaN HEMTs was increased by 125%,the threshold voltage was almost constant,and the output saturation current was reduced by 10%.The cut-off frequency kept changed and the maximum oscillation frequency was increased by 12%.While for the enhancement-mode novel AlGaN/GaN HEMTs,the breakdown voltage was increased by 192%,the threshold voltage was almost constant,and the output saturation current was reduced by 11%.The cut-off frequency kept changed and the maximum oscillation frequency was increased by 44%.Secondly,under two assumptions of complete depletion and incomplete depletion,a two-dimensional analytical model for the channel potential and electric field distributions of novel AlGaN/GaN HEMTs with a partial GaN cap layer was developed on the basis of Poisson's equation and proper boundary conditions.In comparison with the simulation results,the effect of parameters on the breakdown characteristics of novel AlGaN/GaN HEMTs was discussed,a fair consistency between the analytical and numerical results indicates the validity of the proposed analytical model.Thirdly,an experimental research of novel AlGaN/GaN HEMTs with a partial GaN cap layer has been performed.Based on the high-quality epitaxial wafer,samples were successfully fabricated through the process of mesa isolation,GaN cap layer etching,ohmic contact of source and drain electrodes,and gate electrode Schottky contact.The conventional AlGaN/GaN HEMTs exhibited a breakdown voltage of 378 V and output saturation current of 538 mA/mm.When the thickness of the partial GaN cap layer is 23 nm and the length is 7 ?m,the breakdown voltage of the novel AlGaN/GaN HEMTs with a partial GaN cap layer was 656 V,and the output saturation current was 558 mA/mm.It can be seen that the breakdown voltage was increased by 74% and the output saturation current was increased by 3.7%.By the Hall measurement,it has been proved that although the density of 2DEG was decreased,the mobility was increased.Therefore,the product of the 2DEG density and mobility was improved,the output saturation current was slightly increased.Finally,the high-temperature characteristics have been conducted at four temperatures of 27?,90?,150? and 200?.The output saturation current of conventional AlGaN/GaN HEMTs was reduced by 53%,while the novel AlGaN/GaN HEMTs with a partial GaN cap layer was 36%.This is due to the optimization of the electric field distribution and the corresponding optimization of the thermal field distribution,which improves the thermal reliability.(2)In the same way,firstly,a two-dimensional analytical model for the channel potential and electric field distributions of AlGaN/GaN HEMTs with a etched AlGaN barrier layer was established.The effect of the etch length and height on the breakdown characteristics was discussed through software simulation.By etching the AlGaN barrier layer near the gate edge,the 2DEG concentration is reduced,and the concentration distribution of 2DEG is partitioned.Employing the electric field modulation effect,with the increase in the etch length and height,the electric field is more uniformly redistributed and the breakdown voltage gradually improves.Then,experimental study of AlGaN/GaN HEMTs with a etched AlGaN barrier layer has been conducted.We obtained a breakdown voltage of 151 V,a output saturation current of 359 mA/mm for the conventional AlGaN/GaN HEMTs,and a breakdown voltage of 621 V,a output saturation current of 466 mA/mm for the AlGaN/GaN HEMTs with a etched AlGaN barrier layer when the etch height is 10 nm and the length is 7 ?m.It can be seen that the breakdown voltage was increased by 311% and the output saturation current was increased by 30%.Finally,the high-temperature characteristics have been tested.The output saturation current of conventional AlGaN/GaN HEMTs was reduced by 54%,while the AlGaN/GaN HEMTs with a etched AlGaN barrier layer was 47%.Owing to the optimization of the thermal field distribution,the degradation of the output characteristics of the AlGaN/GaN HEMTs is alleviated to some extent.