A High Energy-Efficient 4H-SIC MESFET With Lightly Doped Channel At The Gate Edge

As a third-generation semiconductor material,4H-silicon carbide has excellent material characteristics such as wide band gap,high thermal conductivity,strong breakdown field,and high saturation electron mobility.These excellent characteristics make the device made of 4H-SiC material have the advantages of high breakdown voltage,good heat dissipation and excellent high frequency performance.Compared with MOSFET,MESFET has a higher saturation electron mobility.The higher saturation electron mobility means that MESFET has higher current,transconductance and transmission frequency.Therefore,MESFETs are widely used in microwave circuits.It can be predicted that the development and application prospects of 4H-SiC MESFET will be very broad.However,how to improve the RF and DC characteristics of 4H-SiC MESFET at the same time is a difficult problem.In addition,in the application of high power density,it is increasingly important to improve the power efficiency of the device.Because lower power added efficiency will cause a large waste of energy.A novel 4H-SiC MESFET with symmetrical lightly doped（SLDD-MESFET）at the gate edge drain side is first proposed in this paper.Compared with DR-MESFET,SLDD-MESFET introduces a symmetrical lightly doped region on the source side and drain of the gate edge.On the drain side of the gate edge,the lightly doped region makes a new electric field peak appear below the breakdown point,which reduces the peak value of the electric field at the breakdown point,alleviating electric field crowding effect.The breakdown voltage is greatly increased.At the same time,the area of the lightly doped region is small compared to the entire channel,so the carrier concentration in the channel is only slightly reduced.The saturation current density is slightly reduced.However,the breakdown voltage has been greatly improved,so the maximum output power density of the device has also been improved.On the source side of the gate edge,the lightly doped region increases the width of the depletion region between the source and the gate.With a certain area of the gate plate,the gate-source capacitance is greatly reduced,thereby improving the cut-off frequency and maximum oscillation frequency of the device.The doping concentration,length and width of the lightly doped region of the SLDD-MESFET all have important effects on its DC performance and AC performance.In order to obtain the best performance of the SLDD-MESFET,the doping concentration,width and length of the lightly doped region are optimized.The optimization results show that the performance of SLDD-MESFET is best when the lightly doped region has a doping concentration of 3×10¹⁶ cm^-3,a length of 0.2μm,and a width of 0.1μm.Through simulation,it is found that the breakdown voltage of SLDD-MESFET is 151V,which is 38.5%higher than the 109V of DR-MESFET.The maximum output power density is 7.54W/mm,which is 39.1%higher than the DR-MESFET’s 5.42W/mm.The gate-source capacitance is 0.51p F/mm,which is 15%lower than the DR-MESFET’s 0.51p F/mm.This makes the cut-off frequency and maximum oscillation frequency of the device increased from 21.05GHz and56.2GHz to 23.85GHz and 59.8GHz,which have increased by 13.3%and 6.4%respectively.In this paper,the device model of SLDD-MESFET is built by modifying the model parameters of EE＿FET3 in ADS.The influence of the conduction angle on the power added efficiency in the power amplifier circuit is studied.In order to eliminate the influence of the conduction angle on the power added efficiency,the conduction angle is fixed to 180°by building a class B power amplifier circuit.In order to eliminate the interference of different impedances on efficiency,impedance matching was performed on the power amplifier circuit.Then the power added efficiency of SLDD-MESFET is simulated in two aspects.One is to find out the best operating conditions for the best performing SLDD-MESFET.The simulation found that when the input power is 32 d Bm,the operating frequency is 1.2GHz,and the drain-source voltage is 24 V,the poweradded efficiency of the best-performing SLDD-MESFET reaches the maximum,which is 68.43%.The second is to optimize the structural parameters of the SLDD-MESFET again under random fixed conditions to maximize its power added efficiency.When the input power was reduced to 30 d Bm,the frequency was increased to 1.6 GHz,and the source-drain voltage was increased to 28 V,the power added efficiency reaches the maximum,which is 65.73%.Under this condition,the doping concentration become 0 and the length and width are 0.25μm.Under this working condition,the power added efficiency of SLDD-MESFET is 58%higher than 41.56%of DR-MESFET.It fully proves that SLDD has high power added efficiency.