Study On The Modeling And Fabrication Of 4H-SiC RF Power

Silicon Carbide (SiC) is a promising material in power microwave, high temperature, optoelectronics, and radiation resistant applications due to its excellent properties such as wide band gap, high electron saturated velocity, high breakdown electric field, high power density, high thermal conductivity, etc. With excellent figure of merits such as high electron mobility, low anisotropy and low ionization compared with other types of SiC, 4H-SiC metal-semiconductor-field-effect-transistor (MESFET) is a potential candidate for applications in HDTV systems, phased array radars and next generation mobile communication base station.Modeling of 4H-SiC MESFET is of great importance in design of microwave devices and circuits, wherein characterization of large-signal is complicated and crucial. Many large-signal models are successful in describing the characterization of GaAs MESFET, but not accurate for SiC MESFET. Neither 2D-numerical models nor measurement-based models in resent published papers have satisfactory simulations for SiC devices. It is necessary to develop a n analytical model of SiC MESFET which is clear in device mechanism description and simple in calculation, therefore suitable for the design and optimization of device. In this work, one of the emphases is put on the nonlinear modeling of SiC MESFET, wherein the Drain current Ids and the Gate capacitance Cgs, Cgd are the key parts of the large-signal equivalent circuit. With the properties of the incomplete impurity ionization and the higher saturated electron drift velocity, a quasi-analytical model of DC I-V characteristics for 4H-SiC RF power MESFET is proposed by utilizing empirical hyperbolic tangent description and theory of carrier saturated-velocity under high field. The improved model is used to describe the large-signal nonlinear characterization for short channel microwave SiC MESFET. The comparison between simulation and measurement shows an excellent agreement. A large-signal nonlinear capacitance model for RF power 4H-SiC MESFET is also proposed by combining the physical analysis of devices based on theories of charge controlling with the description of empirical models such as Statz and Angelov models. This model is superior to Statz and Angelov models in describing the strong nonlinear Cgs in the low Vds area and it has less calculations than the numerical models based on physics of devices, therefore suitable for design and optimization of circuits. Meanwhile, the self-heating effect which plays an important role in saturated regionof I-V characteristics is considered in this analytical model so as to analyze the temperature performance of SiC power MESFET. The heat transport between the channel and the substrate is modeled with non-constant temperature T0 at the bottom of 4H-SiC substrate in order to fit the realities of devices. A good agreement between simulation and measurement is obtained. An analytical trapping effect model that utilizes concise parameters to describe the impact of deep level traps in the p-buffer layer on the output characteristics is proposed for 4H-SiC RF power MESFET. This model is simple in calculations compared with 2D numerical model. The description in transconductance decrease, drain conductance decrease and pinch-off voltage dispersion caused by the traps is easily derived with this model. The validity shows good agreements between simulations and measurements. Accordingly, the process related surface-state effect is investigated in fabrication of SiC devices, and a nonlinear model for 4H-SiC power MESFET is proposed which takes into account the surface related parameters. The frequency-dependent and temperature-dependent transconductance dispersion is readily demonstrated with the improved model. The simulation results show that the larger dispersion and the higher transition frequency occur for 4H-SiC MESFET than that for GaAs MESFET. The process technology for SiC device has been put on more attentions in recent years because of great complexity. It is also the important part in th...