Study Of Structure Optimization And Theoretical Model For4H-SiC MESFET

Silicon Carbide (SiC) is a very promising candidate for high frequency, high power,high temperature，and radiation hardness applications due to its excellent physicalproperties such as a wide bandgap，high breakdown voltage，high saturation electrondrift velocity, and high thermal conductivity．With excellent figure of merits comparedwith other types of SiC,4H-SiC metal semiconductor field effect transistor (4H-SiCMESFET) has attracted much interest all over the world since it is well suited for powerand microwave application.However, as far as the improvement of power density is concerned, there is apotential trade-off between the further increment of drain current and breakdownvoltage under unvaried device width: large drain current requires a large product of thechannel doping and thickness, nevertheless, which will reduce the breakdown voltage.A new Buffer-Gate4H-SiC MESFET is proposed, which overcomes this challenge andimproves the current density and breakdown voltage simultaneously.The subthreshold regime of operation of the4H-SiC MESFET is very importantfor analyzing the operation of4H-SiC MESFET since it represents one of the twostationary stages, i.e., off-state. Under this regime, the subthreshold characteristics ofthe4H-SiC MESFET are influenced significantly by the DIBL effect.The main studies and achievements of this dissertation are as follows.(1) The study on the models of4H-SiC MESFET. The analytical models todescribe the current-voltage (I-V) characteristics and alternating current (AC) smallsignal parameters are developed based on the SiC material properties including theincomplete ionization of the dopants, as well as the high field region between the gateand the drain and the Channel-Length-Modulation Effect of the MESFET. Goodagreement is obtained between the calculated results using our models and theexperimental data.(2) The study on the model and performance of the Buffer-Gate4H-SiC MESFET.The analytical models of I-V characteristics and AC small signal parameters areestablished and the breakdown characteristic is simulated. Based on the analyticalmodels and the simulated results, the DC and AC performance of this structure areanalyzed and the results are compared with that of the conventional structure. Thecalculated results are in agreement with the experimental data. For a0=0.2μm, themaximum saturation current density is twice as large as that of the conventional structure. The breakdown voltage and the maximum output power density of theBuffer-Gate MESFET are increased by28%and144%, respectively. The cutofffrequency and maximum oscillation frequency are improved from8GHz to20GHz andfrom45GHz to100GHz, respectively. So, the buffer-gate structure improves thepower and microwave performance.(3) The study on the model of the subthreshold characteristics for the dual-materialgate4H-SiC MESFET (DMGFET). A new DMGFET structure is proposed and itsanalytical models of the subthreshold characteristics are derived. Based on these models,the channel bottom potential distribution, the shift of the threshold voltage, thesubthreshold slope factor, as well as their dependence of the DIBL effect are analyzed.The theoretical predictions are verified by the simulated results using ISE simulator, andcompared with the corresponding results of the conventional single-gate4H-SiCMESFET (SGFET). Good agreement between the calculated and simulated results isobtained. The DMGFET suppresses the DIBL effect significantly, and thus improvesthe subthreshold performance.(4) The study on the model of the subthreshold characteristics and structureparameters optimization for the Buffer-Gate4H-SiC MESFET. The analytical modelsof the subthreshold characteristics are presented. According to these analytical models,the DIBL-dependent subthreshold properties are investigated and the structureparameters are optimized. The results reveal that the optimization has a pronouncedeffect in reducing the DIBL and improving the subthreshold properties.The proposed new structures of the4H-SiC MESFET and the theoretical models inthis dissertation provide novel ideas and methods to improve the power microwavecharacteristics and subthreshold performance.