Novel Vertical GaN Power Electronic Devices:Design And Optimization

GaN is widely studied by academia and industry communities by featuring wide bandgap,high critical electric field and high electron saturation velocity.Currently,the GaN products are favored by the market since owing to their incredibly promising performances.However,most of the GaN HEMT products are designed for the applications,of which the voltage levels are generally below 650 V.Only few products can prossess breakdown voltages of?1200 V since the necessary carbon doping could introduce a serious degradation of dynamic performance.Thus,it is believed promising that vertical devices structures can be a solution to kilo-volt level GaN devices,which are carbon-doping free and rather easier to reach the GaN mertial limits.Hence,great efforts have been contributed to studies for GaN vertical devices recently.However,limited by the thickness of epitaxial layers(?10?m)and quality of P-GaN(Low dopant activation ratio),vertical devices with breakdown voltages of kilo-volts are still scarcely reported.In this case,considering the limits on P-GaN and epitaxial layers,there are two possible novel devices were proposed to realize a kilo-volts GaN vertical devices in this work,which are a novel kilovolts GaN vertical super-junction MOSFET and a novel hybrid channel GaN MOSFET.(1)A novel kilovolts GaN vertical super-junction MOSFET with trench gate is proposed in this work.The following features are demonstrated.1.The P-GaN/UID-GaN/N-GaN stack was introduced to the proposed devices in order to take the place of the conventional P+/P/N/N+stack in Si super-junction devices.It is known that the P+field stop layer can be hardly realized owing to a low activation ratio of Mg dopants,hence the P-GaN is introduced to device for depletion and lateral electric field.In the meantime,the UID layers are inerted between N-GaN and P-GaN to form a PIN junction that a uniform electric field distribution can be realized.2.By introducing a gated depletion channel placed between gate and P-pillar,a positive threshold is obtained with a promising trans-conductance,which can also possess an outstanding controlling capability of the gates and a high on/off ratio.According to the results of TCAD simulation,the high breakdown voltage and the low specific on-resistance of the optimized devices with a given drift-region thickness of 12?m are 2774 V and 0.79m?*cm~2,which also obtains a Baliga figure of merit of 9.46 GW/cm~2.(2)A novel hybrid channel GaN MOSFET is proposed as well.The devices can be considered as a combination of the conventional HEMT devices and the quasi-veritical,of which a lateral HEMTs is placed on a U-shaped channel quasi-vertical GaN devices.In this case,a 2DEG channel and its gate are merged to a bulk material channel,which can share a common inserted P-GaN on N-GaN drift region of the quasi-vertical device.Benefitted by the bulk material channel and 2DEG channel simultaneously,the proposed device can obtain an excellent saturation current of 282 A/mm~2 in on-state.Meanwhile,the shared inserted P-GaN region can be able to modulate threshold voltage in on-state and block leakage current in off-state,which provide an outstanding on/off ratio and a promising breakdown voltage.In this work,the impacts of two channels were studied repectively,particularly the contribution to on-resistance and the breakdown.Key parameters(e.g.doping concentration,gate length,and thinkness of drift region)were discussed as well.By analysing the mechanisms of device breakdown,a concept of optimizing the proposed devices was established,which could provide the guidance for future designing.The sample devices can obtain a high breakdown voltage of?1700 V and a BFOM of 1.12 GW/cm~2.It is noteworthy in this work that a breakdown voltage of?1600 V can be obtained by the device with a thin P-GaN thickness of 2?m,which can be fabricated relatively easier compared to recent reports.