Research On Mechanism And Novel Structure Of Enhancement-Mode Gan Based Power Devices

As a wide-bandgap semiconductor material,GaN has become a research hotspot in the area of semiconductors,due to the superior properties such as high critical breakdown electric field,high electron saturation velocity,high thermal conductivity and strong radiation resistance.In recent years,GaN-based devices have been promising candidates in the area of RF/microwave,mobile communication and semiconductor lighting.As representative devices in the field of power electronics,although AlGaN/GaN HEMT has made important breakthroughs in material growth,fabrication process,device fabrication and characterization,there are still many technical problems.Firstly,due to the strong polarization effect of AlGaN/GaN heterojunction,there is two dimensional-electron-gas（2DEG）with high density at the heterojunction interface.Hence,the conventional AlGaN/GaN HEMT is intrinsically depletion-mode（D-mode）,however enhancement-mode（E-mode）operation is strongly desired for power electronics applications.Secondly,the breakdown voltage（BV）of AlGaN/GaN HEMT is far below the theoretical limit,and thus improving the breakdown voltage has been a key problem which needs to be solved urgently.Finally,the fabrication process of GaN power devices has not been systematic,thus it is necessary to develop the fabrication processes for GaN-based power devices.Based on the above scientific problems,two kinds of AlGaN/GaN HEMT are fabricated by developing and optimizing fabrication processes such as cleaning process,ICP etching process and ohmic contact.Two kinds of hole gas enhancement-mode AlGaN/GaN HEMT are proposed,the physical mechanism is analyzed based on Sentaurus TCAD from Synopsys.This thesis provides the following contributions:1.An E-mode high voltage AlGaN/GaN HEMT with trench temernation structure and composite gate dielectric structure is proposed.The trench temernation structure could modulate the electric field distribution,achieving an enhanced BV.The composite gate dielectric structure could improve the quality of etched interface,and thus reducing the interface trap density.The measured BV and on-resistance(R_on)are 412 V and 7.42Ω·mm at L_gd=5μm.One order of reduction in interface trap density from the range of10¹³ to the range of¹0¹²-10¹33 eV^-11 cm^-2 has been observed.2.An AlGaN/GaN HEMT with fluorine ion implantation temernation structure is proposed.Fluorine ion implantation in the thick passivation layer could keep the peak position of fluorine and vacancies distribution far away from the 2DEG channel,thus effectively suppress the current collapse.Fluorine ions in the passivation layer as a termination technique is also used to optimize the surface electric field distribution,and thus increasing the BV.The measured BV and R_on are 803V and 6.17Ω·mm at L_gd=10μm.The test period and stress period are 2 ms and 3 ms,respectively.The dynamic on-resistance(R_on,d)is only increased by 23%at a high drain quiescent bias of 100 V.3.Two kinds of hole gas enhancement-mode AlGaN/GaN HEMT were proposed and analyzed by simulation,in which a novel E-mode operation is realized,meaning two dimensional-hole-gas（2DHG）under the source prevents the electrons injecting from source to 2DEG.Firstly,hole gas enhancement-mode AlGaN/GaN HEMT with single-conduction channel is proposed.The BV and specific on-resistance(R_on,sp)are 705 V and1.18 mΩ·cm² at L_gd=5.5μm.Secondly,hole gas enhancement-mode AlGaN/GaN HEMT with multi-conduction channel is proposed.At L_gd=5.5μm,the BV,R_on,sp and figure of merit（FOM）are 604 V,0.38 mΩ·cm² and 9.6 MW/mm²,respectively.