Research On High-Al-Composition AlGaN/GaN-on-Si Millimeter Wave Devices

With the development of the wireless communication spectrum towards higher carrier frequencies such as mm-wave and terahertz,RF tranceivers raise higher requirements for the core components such as the transistors for power amplifiers.Compared with those RF transistors fabricated by the narrow bandgap semiconductor materials such as Si,Ga As and In P,GaN-based high electron mobility transistors（HEMTs）have demonstrated superior performance in terms of high frequency and high power benefited from the material advantages of GaN such as the high electron mobility,high electron saturation velocity,and high breakdown electric field strength etc.Driven by the civil consumer electronics applications,especially those low voltage and high frequency scenarios,low-cost and large-size wafer GaN-on-Si RF devices are expected to be preferred.However,compared with GaN-on-Si C RF devices,the development and commercialization of GaN-on-Si RF devices for high frequency applications are relatively left behind,and there are still a lot of scientific and technical issues in the materials,structures and processes of GaN-on-Si RF devices that need to be comprehensively addressed.In this thesis,some key issues of GaN-on-Si mm-wave devices are studied from the aspects of device structure designs,process developments and optimization,performance characterizations and analyses etc..Through the research and optimization of device structures and fabrication processes,GaN-on-Si mm-wave RF devices with world-level excellent performance have been achieved and laid an important foundation for the GaN-on-Si mm-wave RF devices to enter the wireless communication as soon as possible.The main research results in this thesis are as follows:（1）A novel technique using an in-situ SiNx insertion layer to enchance the ommic contact was proposed to solve the problem of the realization of low-resistivity Ohmic contacts for high-Al-composition AlGaN/GaN heterostructures.Compared with the conventional methods,the Ohmic contact resistance for the high-Al-composition AlGaN/GaN heterostructures was reduced by half and was as low as 0.17Ω·mm,and the specific contact resistivity was reduced by three-quarters and was 8.53×10^-7Ω·cm².The intrinsic physical mechanism in this Ohmic contacts was studied and it was found that thermal field emission dominates,which is different from the conventional similar structure where the field emission dominates.Further analyses through the EDX characterization reveal that low-work function metal silicides and higher density TiN were formed in this novel Ohmic contacts,which are more favorable to the electron thermal emission.This is consistent with the results of temperature-dependent I-V measurements.This novel Ohmic contact fabrication technology solves the problems of difficult fabrication,complex fabrication processes,and high-cost for the low-resistivity Ohmic contacts in high-Al-composition AlGaN/GaN devices.It lays the foundation for the subsequent realization of high-performance high-Al-composition AlGaN/GaN mm-wave devices.（2）A set of fabrication processes for 45 nm gate length GaN mm-wave devices have been developed,and a GaN-on-Si mm-wave MISHEMT with a cutoff frequency of 320 GHz has been realized.An alignment technique was optimized to realize an alignment error of less than 10 nm during the device fabrication process.An Ar ion implantation planar isolation technique was developed to reduce the device isolation leakage current by two orders of magnitude.The photolithography process to fabricate Ohmic contacts was optimized.A 27 nm length metal gate was fabricated using the developed chromium metal conductive layer process to eliminate the charge accumulation effect and writing field stitching error effect.Based on the novel ohmic contact process and other developed fabrication processes,an Al_0.65Ga_0.35N/GaN-on-Si mm-wave MISHEMT with 45 nm gate length and 650 nm source-to-drain distance has been developed,exhibiting a high transconductance of 704 m S/mm,a low drain-induced barrier lowering（DIBL）of 42 mV/V,a low on-resistance of 0.68Ω·mm,a high output current on/off ratio of 10⁹,a high output current density 2.63 A/mm,and a record high cutoff frequency of 320 GHz.The main performance parameters of this fabricated device have reached the highest values compared with the reports by today.（3）The fabrication process of 80 nm gate length AlN/GaN-on-Si MISHEMT mm-wave devices for mobile front-end low-voltages applications have been developed,and world-level GaN mm-wave devices that can operate at low voltages of 3.5 V～6 V have been realized.150 nm floating T-shape gates have been realized through one set of lithography,and 80 nm low-resistivity floating T-shape gates have been realized through two sets of lithography.The microwave resistivity of a single 80 nm floating T-gate is as low as about18Ω/mm.Through a series of improvements,the fabricated AlN/GaN-on-Si MISHEMTs with 80 nm floating T-gates and a 1.5μm source-drain diatance exhibite a high transconductance of 500 m S/mm,a high output current density of 1.95 A/mm,and a high output current on/off ratio of 10⁷.A record cutoff frequency（f _T=165 GHz）and oscillation frequency(f_max=255 GHz)in AlN/GaN-on-Si HEMTs were achieved,and high f×L_g values(f _T×L_g=13.2 GHz·μm,f_max×L_g=20.4 GHz·μm)were obtained as well.The continuous-wave large-signal power performances with power densities of 0.36/0.73/1.04 W/mm and peak PAEs of 40%/42%/43%were achieved at drain voltages of 3.5/5/6 V and f=28 GHz,respectively.This study shows the great potential of AlN/GaN-on-Si mm-wave low-voltage power devices to be used in the RF transceiver front-end modules for mobile ends.（4）A high-linearity planar nano-channel AlN/GaN-on-Si MISHEMT mm-wave device was proposed and developed.It meets the linearity requirements of the power amplifier used in RF transceiver front-end modules of the mobile communication systems.A technique based on the argon ion implantation process to realize planar nano-channel structures with Fin-width of as small as 50 nm was developed and successfully involved into 100 nm floating T-gate AlN/GaN-on-Si mm-wave MISHEMTs.It successfully suppressed the short channel effect.Compared with conventional devices,the gate-voltage-swings in the transconductance plateau,f _T plateau,and f_max plateau in the planar nano-channel AlN/GaN-on-Si mm-wave MISHEMTs were increased from 1.8 V to 6.5 V,3.2 V to 8.5 V,and 3.3 V to 6.5 V,respectively.The 1d B compression point OP_{1d B} was increased from 13.9 d Bm to16.7 d Bm,and the power density was increased from 0.85 W/mm to 1.02 W/mm,the AM-PM distortion was reduced from-8.2°to-1.5°,which for the first time suppressed the AM-PM distortion in GaN-on-Si mm-wave devices.In addition,the device exhibits an improvement of OIP3 from 26 d Bm to 33 d Bm and an improvement in linearity figure of merit OIP3/P_DC from 4.1 d B to 9.4 d B during the two-tone intermodulation distortion measurements at 28 GHz.（5）A deep sub-60 mV/dec subthreshold swing and the gate leakage current negative resistance effect independent of gate bias sweep direction were observed in GaN-on-Si mm-wave MISHEMTs,and the intrinsic physical mechanisms were studied.Through I-V and C-V characterizations of those devices with different key feature dimensions,it was found that hot electron effect,buffer layer traps,and interface traps play key roles for the subthreshold swing less than 60 mV/dec.Two kinds of states,“fast state”and“slow state”,were found in the interface traps by AC-conductance method,and the negative resistance effect of gate leakage current was successfully explained by the traps.It was found that the operation mechanisms of this device are similar to a typical floating-gate device,and this provides a new idea for GaN devices in switching applications.In summary,some excellent performance results of high-Al-composition AlGaN/GaN-on-Si mm-wave devices have been achieved from the aspects of device structure designs,process developments and optimization,performance characterizations and analyses etc..However,there are still some works that need further study in the future,such as the efficiency improvement of GaN-on-Si mm-wave devices,the design and manufacture of monolithic microwave integrated circuits（MMICs）to accelerate the development and application of GaN-on-Si MMICs.