| With the increasing demand for miniaturized,low-loss,and high-reliability power devices,GaN-based diodes have become a research hotspot due to their excellent performance.For solve the problem that GaN planar devices hinder their high voltage and high power performance due to their structural limitations,the discussion studies a quasi-vertical GaN PiN diode with aspects of material design,key process development and device fabrication and analysis,Develop a quasi-vertical GaN PiN diode with both low on-resistance and high breakdown voltage.In the material design section,a four-layer material system is adopted.The effects of the drift layer doping concentration（NI）and the conduction layer doping concentration（NN）on the device characteristics was studied by Silavaco simulation software.It was found that NI affects the on-resistance mainly due to the incomplete conductance modulation effect,and NN affects the on-resistance mainly due to the current crowding effect.The unique current transport mode of quasi-vertical structure and the current crowding effect are studied,A effective current length Le characterization method is proposed to quantitatively analyze the strength of the current crowding effect.It is found that Le decreases with the NI,and Leincreases with the NN.The analysis shows that the current crowding effect is mainly related to the equivalent square resistance of the conduction layer relative to the drift region.The current crowding effect is weakened with a lower equivalent square resistance.In the reverse characteristics,it is found that with the increase of NI,the breakdown voltage of the device shows rapid decrease after initial increases with an optimal doping concentration range;the breakdown voltage of the device shows little dependence on the NN.Combined with the actual material growth technology,it is determined that the 2×1016 cm-3 of NIand 1×1019 cm-3 of NN as the better epitaxial material structure.In the key process development section,by optimizing the photolithography formula,adding thermal conductivity and using low-temperature etching technology,we solved the problem of photoresist carbonization and etched sidewall bending during the etching process,and produced 6.34μm thick photoresist mask layer with relatively vertical sidewall used for deep etching process;Reserch on deep etching process found that reducing the pressure of the reaction chamber can effectively increase the verticality of the etched sidewalls.Research shows that increasing the BCl3/Cl2 gas flow ratio will better solve the phenomenon of sidewall breakpoint but produce columnar structures on the GaN surface meanwhile.Finally,by optimizing the etching formula parameters,a deep etching process with an etching rate of 190 nm/min,a better etched sidewall morphology and higher verticality sidewall was developed;Add HCl solution which used for metal evaporation pretreatment and optimize the annealing formula we obtain a P-GaN ohmic contact with a contact resistivity of 10-5Ω·cm2.In the device fabrication section,The GaN quasi-vertical PiN diode formed by using the developed deep etching and P-GaN ohmic contact process.The device shows a on-resistance of 0.51 mΩ·cm2,a turn-on voltage of 3.16 V,a switching ratio of 1010,an ideality factor of 1.87,a reverse leakage of～10-5 A/cm2@-200V,a peak electric field of 2.09 MV/cm,a breakdown voltage of 670 V@0.1A/cm2,a BFOM of880MW/cm2.which has a excellent performance.Research on reverse leakage found that SCLC is the main leakage mechanism of the diode;research the effect of size on device performance.It is found that with the increases of the mesa size,the current value of the device increases,the current density and the on-resistance both decreases,which confirms the existence of the current edge effect of quasi-vertical devices.Two solutions to the current crowding effect were proposed;the influence of the device size parameters on the device performance is studied,and it is found that the anode size and the change of the anode and cathode spacing have no obvious impact on the reverse characteristics of the device,with the anode size increases in a large range or decreases the distance between anode and cathode will reduce the on-resistance of the device.Thess provide a reliable guide for the design and fabrication of quasi-vertical and full-vertical GaN PiN in the future. |
PDF Full Text Request