Design And Simulation Of 4H-SiC MESFETs With Multi-recessed Source/drain Drift Regions

In recent years, silicon carbide(Si C) materials have been given more and more interests due to its superior material and electric characteristics. Different from traditional silicon materials, Si C has wide gap, high electron saturated velocity, high critical electric field and thermal conductivity. Thus it can be used in high power and wide radio frequency applications.In microwave and high power applications, 4H-Si C MESFETs(4H-Si C metal semiconductor field effect transistors) show its potential advantages due to its high power density and superior radio frequency performances, and can work well especially in extreme environment such as high temperature above 500℃. Thus 4H-Si C MESFETs show its wide prospect in phased array radar, solid state microwave communication system and aerospace fields. However, the breakdown voltage( VB) of Si C MESFETs is inevitably decreased because of much larger doping concentration of the channel layer(ND) which is the effective technology to improve the drain current(Id) of the devices, thus it need to be carefully traded off between Id and VB.In this paper, a novel structure of 4H-Si C MESFETs(MRD-MESFETs) which combines both the double-recessed gate and multi-recessed source/drain drift region to solve the mentioned questions above deriving from the doping concentration of the channel layer and improve the radio frequency performance is introduced. The novel structure proposed in this paper has superior direct current and radio frequency characteristics as well as sustaining almost the same drain current, and the cut off frequency( f T) and the maximum oscillation frequency(fmax) can also be improved, so both superior direct current and radio frequency performances can be achieved.The novel MRD-MESFETs structure has introduced the double-recessed gate mainly consisting two parts called upper and lower gates respectively, of which the upper one is used to widen the channel layer and increase the amount of the carriers though the channel, thus improving the drain saturated current. Well, the lower one supports that the channel layer under the gate can be successfully controlled by the gate voltage. However, it should be noted that wide channel layer under the upper gate decreases thedevice breakdown voltage, which is not our initial purpose for the power devices. SRD-MESFETs structure which etches part of the gate-source/drain drift region resulting in thinner channel layer, so the breakdown voltage of the devices is increased while sustaining almost the same drain current. Meanwhile, the recessed gate-source/drain drift regions have an effect on preventing the gate depletion layer expending to both source and drain sides, thus the capacitance between the gate and source/drain are decreased and the frequency characteristics are improved.In this paper, the multi-recessed source/drain drift regions based on the structure of double-recessed gate and single-recessed source/drain drift regions structure is introduced, in which the gate-source/drain drift regions are etched twice and finally forming the multi-recessed drift regions structure(MRD-MESFETs). The simu lated results using the tool of ISE-TCAD show that the electric field around the gate corner is modulated due to the existence of the multi-recessed drift regions, thus improving the breakdown characteristics of the devices and increases the VB to 168 V, about 12.8% higher than the SRD-MESFETs structure. In addition, the gate-source/drain capacitance(Cgs and Cgd) of the MRD-MESFETs are 0.423 p F/mm and 0.213 p F/mm, compared to 0.536 p F/mm and 0.243 p F/mm of the SRD-MESFETs, respectively. The f T and fmax of the MRD-MESFETs can be increased to 16 GHz and 32 GHz due to the small value of Cgs comparing with 13 GHz and 28 GHz for the SRD-MESFETs. And the Cgd has negative impact on the alternating current signal gain as the coupled capacitance from output port feeding back to the input signal. So, the feed-back Cgd is drastically reduced because of the existence of the multi- recessed drift regions. At last, the performences of the different structures relating to several main size parameters are simulated and the optimized values are obtained.