Design Of 4H-SiC MESFETs For High Efficiency High Power RF Amplifier

The design of power semiconductor devices based on silicon carbide?SiC?materials has been studied for many years because of its high band gap brought high breakdown voltage and high anti irradiation.The 4H-SiC MESFET device has become a widely studied microwave power device structure because of its high reliability?from no gate oxide layer?and high frequency structural characteristics?from Schottky contact?.The optimization of4H-SiC MESFET device is mainly to overcome the contradiction between the device breakdown voltage and the saturated drain current.In the process of overcoming this contradiction,the gate technology,the field board technology,the drift area surface etching technology and the channel depression technology have been successfully applied in the device structure design.The main work of this thesis is to improve the performance of4H-SiC MESFET power device by using channel doping technology and buffer layer doping technology,and further break the contradiction between breakdown voltage and saturation drain current.At the same time,the effect of channel doping technology and buffer layer doping technique on the energy efficiency of the device is further evaluated by studying the power added efficiency of the device.In this treatise,the channel doping of the device is studied in detail.The maximum saturation drain current of the device is positively correlated with the doping concentration of each region,and the doping sensitivity of the lower channel is the highest.The surface area near the drain side of the gate is the breakdown region of the device,a slight increase in concentration in the two changing concentration channel regions near this region will greatly advance the breakdown time of the device.At the same time,the decrease in doping concentration does not show the same sensitivity as the device increases,but exhibits a fast saturation state.The influence of the doping concentration on the gate-source capacitance of the lower channel region and the gate-drain region is opposite.The influence of the gate-down channel on the gate-source capacitance is almost inversely related,while the latter exhibits almost Positive correlation.Furthermore,when this trend is associated with a change in the breakdown voltage of the device,it can be found that by changing the channel doping concentration below the gate and toward the source direction,the device gate source capacitance increases with the decrease of the breakdown voltage,however,in the gate toward the drain direction area,with the breakdown voltage decreases,the gate-source capacitance is decreasing.There is a positive relationship between the DC transconductivity of the device and the change of the doping concentration in the four regions.It is worth noting that in the lower gate position,due to the influence of the channel concentration on the gate voltage and the saturation drain current,the change of the doping concentration has a negative effect on the transconductance of the device.In this paper,a new structure of a 4H-SiC MESFET device with partial heavy doping channel is proposed for the first time.Due to the larger channel carrier concentration provided by the heavily doped region,the device's saturation drain current increases by18.4%compared to the double depressed buffer layer DRB-MESFET structure,while the maximum output power density of the device increases by 16.5%.The DC transconductivity of the new structure increases by 32%compared with the DRB-MESFET structure due to the heavily doped concentration region modulates the depletion region of the gate and the increase of the channel current.At the same time,due to the increase in the device gate-source capacitance,the cut-off frequency of the device is substantially the same as that of the DRB-MESFET.In this paper,a new structure design of a 4H-SiC MESFET device with partial buffer layer heavy doping region is proposed for the first time.A partially P-type heavily doped buffer layer region of 7 x 10¹⁶ cm^-3 was used in the new structure.Compared to the traditional SiC MESFET structure.The breakdown voltage of the new structure is increased from69.4V to 113.4V due to the modulation of the region's electric field to the device,and the maximum output power density of the device is 60.17%higher than that of the traditional structure.In this paper,the device efficiency is regarded as an important index.Under the S-band,the power-added efficiency of the new structure with partial trench re-doped region is increased from 69.4%to 72.15%compared with the double-depression buffer layer.Compared with the traditional SiC MESFET,the power addition efficiency of the partially doped buffer layer is increased from 63.1%to 64.8%.