Study On GaN-based Heterostructure IMPATT Diode In Sub-millimeter Wave Band

The research of microwave solid-state devices has become one of the main research contents of high power devices.The IMPATT devices are one of the most promising high power devices among two-terminal solid-state electron devices.Especially,IMPATT diode based on GaN is the most potential power source device operating at terahertz frequency,and has gained more and more attention and in-depth research.At present,there is yet no relevant experimental research results on GaN based IMPATT diodes in the literatures.But lots of theoretical simulation results indicate that the wide bandgap semiconductor GaN is the most promising material for IMPATT diode fabrication in the future.Based on this background,further clarify the transport process in the GaN based IMPATT diode,and design devices with novel structure to improve the rf output power and dc-to-rf conversion efficiency have become the main research contents and purpose of this paper.The research work is carried out under the SWB,and the main research conclusions are as follows:1.The influence of DDM and HDM on the dc and microwave performance of GaN based homojunction IMPATT diodes was studied in-depth.The dc simulation results indicate that,the values of the peak electric field,carriers'impact ionization rate and breakdown voltage under DDM are larger than the HDM results.Through HDM simulation,carriers'impact ionization process in the GaN based IMPATT diode is further clarified.There exists a hysteresis effect between carriers'impact ionization and the electric field because of the energy relaxation effect in the HDM.Meanwhile,we found that carriers'impact ionization rate was overestimated under DDM.The microwave performance simulation results indicate that,at operating frequency under 310GHz,both models give reasonable simulation results,but at operating frequency over 310GHz,the microwave performance under HDM is superior to DDM,which indicates that the GaN based IMPATT diode can operate over a wider range of frequencies.2.In this paper,a novel structural IMPATT diode configured by GaN/Al_xGa_1-xN/GaN double heterostructure is investigated at the frequency of D-Band,in which the emphasis is put on the influence of Al composition of Al_xGa_1-xN on the diodes performance.The mechanism of improving the device performance because of the incorporation of a GaN/Al_xGa_1-xN heterostructure into the classical GaN homostructure IMPATT diode is analyzed.In the proposed structure,Al_xGa_1-xN and GaN were used to form a compound drift region.The Al composition of x is set to 0.2,0.3,0.4,0.5 and 0.6.The dc simulation results indicate that,carriers'impact ionization rate and the breakdown voltage increase with going on increasing the Al composition.The microwave performance simulation results indicate that,the rf output power and dc-to-rf conversion efficiency of the heterostrcuture IMPATT diodes both have an improvement as compared with the GaN homostructure IMPATT diode.The results indicate that the heterostructure IMPATT diode with Al composition of 0.4 suppresses all other IMPATT diodes with regard to rf output power density(1.56 MW/cm²)and dc-to-rf conversion efficiency(21.99%).We think that,the high density of 2DEG in the AlGaN/GaN heterointerface because of the polarization effect,and the role of AlGaN layer in localizing the avalanche region width because of its wider bandgap play an important role in improving the microwave performance as compared with the GaN homostructure IMPATT diode.3.A lattice-matched GaN/In_0.17Al_0.83N/GaN heterostructure based IMPATT diode is investigated in this paper.Three different IMPATT diodes are designed with different width of In_0.17Al_0.83N layer(40nm,45nm,50nm),in which the emphasis is put on the influence of In_0.17Al_0.83N layer width on the diodes performance.The dc performance simulation results indicate that,the breakdown voltage decrease with increasing the width of the In_0.17Al_0.83N layer.The microwave performance simulation results indicate that,the rf output power decreases with increasing the width of the In_0.17Al_0.83N layer,while the dc-to-rf conversion efficiency firstly increases and then decreases.The largest rf output power density of1.67MW/cm²is for the heterostructure IMPATT diode with 40nm of In_0.17Al_0.83N layer width,while the largest dc-to-rf conversion efficiency of 15.4%is for the heterostructure IMPATT diode with 45nm of In_0.17Al_0.83N layer width.The largest conduction band offset of 0.18e V between In_0.17Al_0.83N and GaN for the heterostructure IMPATT diode with 45nm of In_0.17Al_0.83N layer width leads to a higher density of 2DEG,which leads to a higher rf output power and the highest dc-to-rf conversion efficiency.Finally,we found that the microwave performance of the lattice-matched GaN/In_0.17Al_0.83N/GaN heterostructure IMPATT diode has an improvement as compared with the GaN homostructure IMPATT diode.4.In this paper,the 4H-SiC/GaN based heterostructure IMPATT diode is also investigated.In the proposed heterostructure IMPATT diode,n-type dopant 4H-SiC was used to form the drift region,n-type highly dopant 4H-SiC was used to form the ohmic contact of the IMPATT diode.The 4H-SiC in the structure serves as the substrate of epitaxial GaN growth and the active region of the IMPATT diode,which is beneficial to device's heat dissipation.Through simulation,we found that the optimum operating frequency for the IMPATT diode is 280GHz,the highest rf output power density is 1.36MW/cm²,and the dc-to-rf conversion efficiency is 19.2%,which indicates that the 4H-SiC/GaN hetero-structure is also a candidate material system for IMPATT diode fabrication.