Study On SiC Based Avalanche Diode In D Band

Impact-Ionization-Avalanche-Transit-Time(IMPATT)diodes have been used effectively for RF power generation at frequencies between 10 GHz to 300 GHz.As the most powerful solid-state sources at microwave and millimeter-wave frequencies,they are widely used in transmitters in radar and communication systems.IMATT diodes are two-terminal devices,which are easier to fabricate than transistors.The peak output power of an IMPATT diode at a given frequency is limited by its underlying material properties.Because of the high breakdown field and high electron saturation velocity of silicon carbide(SiC),the peak power capability of a SiC IMPATT diode is expected to be 100×and 350×higher than silicon and gallium arsenide IMPATT diodes,respectively.In this dissertation,the SiC based IMPATT diodes working at D-band are mainly studied,and the author's major contributions are outlined as follows:1.Influence of the anisotropy on the performance of D-band SiC-based IMPATT diodes.According to the theory of IMPATT diode,the ionization rates of charge carriers in the depletion layer play an important role in governing the microwave performance of the IMPATT diode.However,the ionization rates of carriers of<0001>and<1120>directions4H-SiC material exhibit bigger difference.Therefore,it is predicted that 4H-SiC devices for the<0001>and<1120>directions would present more evident anisotropy.Then,a detailed investigation on the dc and the high frequency properties of<0001>direction and<1120>direction 4H-SiC IMPATT diodes which has the same structure though the computer simulation is presented and the results are discussed.Since the 4H-SiC based IMPATT diodes exhibit a predominantly one-dimensional current flow,the performance of4H-SiC based IMPATT diodes can be easily optimized by choosing the appropriate crystal direction for the current flow.The results show that the conversion efficiency of the<0001>direction is higher than that of the<1120>direction.The Higher breakdown voltage and conversion efficiency lead to higher millimeter wave power output for the<0001>direction 4H-SiC IMPATT device as compared to the<1120>direction device.It is also found that the quality factor Q of the<1120>direction 4H-SiC IMPATT device is lower than that of the<1120>direction device,which indicates better stability and higher growth rate of microwave oscillation.Thus the anisotropy of 4H-SiC must be taken into account in designing the 4H-SiC IMPATT devices.2.Dependence of DC and high-frequency characteristics of 4H-SiC based single drift IMPATT diode on operating temperature.Firstly,we have investigated the influence of the operating temperature on breakdown voltage,electric field distribution,avalanche region width(x_A),the ratio of the avalanche region to depletion layer width(x _A/W),optimal operating frequency,DC to RF conversion efficiency(?)and RF power output of the D-band single drift 4H-SiC IMPATT diodes though the MEDICI simulation platform.For the incomplete ionization of impurities of p~+region greatly affect the avalanche region width and the DC to RF conversion efficiency of the p~+nn~-n~+4H-SiC IMPATT diodes,the doping concentration of p~+region is changed in order to investigate the influence of the doping concentration of p~+region to the performance of the diodes at different temperatures.It is found that the avalanche region width decreases and the DC to RF conversion efficiency increases with the increasing p~+region doping concentration at the same operating temperature.What's more,the change of the avalanche region width and the conversion efficiency in the lower p~+region doping concentration diode is more obvious than that with higher p~+region doping concentration in the lower temperature range.Therefore,the incomplete ionization of impurities greatly affects the lower p~+region doping concentration 4H-SiC p~+nn~-n~+IMPATT diode in lower temperature.Furthermore,the optimal operating temperature at which the maximum DC to RF conversion efficiency is obtained would increase when the p~+region doping concentration is rather small.The simulation results show that the 4H-SiC p~+nn~-n~+IMPATT device would be more suitable for applying in the higher operating temperature compared to the Si-based IMPATT diode.3.Investigation of modified high-low structure IMPATT diodes.The avalanche region width is changed,while the more electron-hole pairs are generated in the avalanche region by adjusting the doping distribution of the high-low structure IMPATT diodes.It would lead to a higher DC to RF conversion efficiency and RF output power of the IMPATT diodes.The results show that the conversion efficiency of the modified high-low structure IMPATT diodes is improved by 30%as compared to the traditional high-low structure IMPATT diodes.In addition,the RF output power is also improved greatly.4.Comparative investigation on 6H-SiC based and 4H-SiC based IMPATT diode working at D-band.Firstly,the effect of anisotropy on the performance of the 6H-SiC based IMPATT is also investigated due to the 6H-SiC material also exhibits anisotropy.The results show that<0001>direction 6H-SiC based IMPATT diode provides higher DC to RF conversion efficiency and larger RF output power compared with those for<1100>direction 6H-SiC based IMPATT diode.Finally,6H-SiC based and 4H-SiC based IMPATT diode are comparatively investigated by taking into account the anisotropy of the material.