Study Of New Structures And Models Of Rf/microwave Power 4H-SiC MESFETs

Silicon carbide (SiC) is a representative of the 3rd generation semiconductor and it is a promising material for semiconductor device under high-power, high-temperature, high-frequency, and high-irradiation applications due to its superior properties, such as the wide bandgap, high critical electric field, high electron saturation velocity and high thermal conductivity. It is emerging as a highly promising technology for military and civil applications such as aerospace, nuclear energy, communications, radar, etc. Compared with GaAs and Si devices,4H-SiC MESFETs has better breakdown and power performance. In recent years, with fabrication process and quality improvement of SiC substrate and epitaxial material, impressive results were published after in-depth theoretical and experimental research on 4H-SiC MESFETs. However, the power and frequency performance of available SiC MESFETs still can not meet radio frequency (RF)/Microwave (MW) power system development requirement due to device structure limitation. And with large-periphery SiC MESFETs development, the impact of self-heating become worse and already limit the further improvement of power density of SiC MESFETs.To address the RF/MW power issue, device structure and related model of 4H-SiC MESFETs were studied in this dissertation. Novel 2D and 3D device structures were proposed and experiment for the 2D recessed gate strctures were conducted. The thermal models were proposed. The main innovation work is as following:(1) Two novel 2D 4H-SiC MESFETs device structures were proposed and experiment for the recessed gate strctures were conducted. To overcome the shortage of the double-recessed structure, an improved double-recessed 4H-SiC MESFETs structure with recessed source/drain drift region was proposed. For the proposed structure, the recessed source/drain drift region is to restrain gate depletion layer extension to source/drain as well as reduce channel thickness between gate and drain. The simulated results showed that the cut-off frequency (fT) and the maximum oscillation frequency (fmax) of the proposed structure are 21.8 GHz and 81.5 GHz compared to 19.0 GHz and 76.4 GHz of that of the double-recessed gate structure, respectively. The breakdown voltage of the proposed structure is 145 V compared to 109 V of that of the double-recessed gate structure. The output power density of the proposed structure is about 33% larger than that of the double-recessed gate structure. A new 4H-SiC MESFETs with floating metal strips (FMS) was proposed to address disadvantage of the conventional and filed-plate structures. The simulated results showed that the maximum electrical field of the MESFET gate is clamped after surface depletion layer punch through to FMS. The breakdown voltage of the 4H-SiC MESFETs with one strip and two strips are 95% and 180% larger than that of the conventional one without FMS, respectively. The maximum theoretical output power density of the 4H-SiC MESFETs with one strip and two strips are 10.0W/mm and 14.5W/mm compared to 4.8W/mm of the conventional structure. Although the frequency response of the FMS structure is a little bit worse than the conventional one, it still has comparable frequency characteristics. Based on available recessed gate structure, a multiple recessed gate device was proposed with further optimization of gate strcuture. The buried gate and the multiple recessed gate devices were fabricated and the result showed that the multiple recessed gate structure has better frequency and breakdown performance than that of the buried gate one.(2) Two novel 3D 4H-SiC MESFETs device structures were proposed. A symmetric 3D tri-gate 4H-SiC MESFETs structure was proposed in order to overcome the disadvantage of 2D device. With a vertical channel and 3D tri-gate formation, equivalent channel width for the proposed device is increased and simultaneously the structure maintains other device parameters at almost same level as conventional one. The simulation result showed that the maximum theoretical output power density of the proposed structure with t=2μm, a=0.4μm and w=0.6μm is 15.5 W/mm compared to 4.2W/mm for the conventional one and it maintains comparable frequency performance. With application of the source/drain recess structure, a new symmetric 3D tri-gate device with recessed source/drain drift region was proposed. Similar as the improved double-recessed 2D structure, the recessed source/drain drift region is to restrain gate depletion layer extension to source/drain as well as reduce channel thickness between gate and drain. The simulation result showed that the maximum theoretical output power density, the fT and fmax of the proposed structure are 18.5W/mm,19.3GHz and 74.1 GHz compared to 15.5 W/mm,16.1 GHz and 55.9GHz of that of the symmetric 3D tri-gate structure with no recess, respectively. Considering the effect of gate length on the MESFETs performance, an improved asymmetric 3D tri-gate 4H-SiC MESFETs with a recessed drain drift region was proposed after top and side gate optimization. The simulation result showed that the fT and fmax of the proposed asymmetric structure are 20.6 GHz and 82.4 GHz compared to 16.1 GHz and 55.9 GHz of those of the symmetric 3D tri-gate structure, respectively. The maximum theoretical output power density of the proposed structure is about 36% larger than that of the symmetric 3D tri-gate structure.(3) 4H-SiC MESFETs thermal models were proposed. A 3D transient and steady thermal model for multiple-finger SiC MESFETs was proposed based on the analysis of thermal characteristics of SiC devices. The relationship between structure parameters and temperature were established in the model and the transient and steady temperature of various fingers can be obtained quickly and precisely. The model can be used for designer to perform thermal design to alleviate the impact of self-heating effect as much as possible to improve the stability. It can also be used as a sub-model for electro-thermal analytical model. Based on analysis of available MESFETs DCⅣempirical model, a 4H-SiC MESFETs DCⅣelectro-thermal empirical model including self-heating effect was proposed with temperature related parameters. The analysis result showed that a good agreement between the model and experimental result with wide voltage range can be obtained for non-linear DCⅣcharacteristics.