A Study On Nonlinear Models For 4H-SiC MESFET

Silicon carbide(SiC) has great potential application of high temperature, high frequency, high power and irradiation domains for its superior properties, such as wide bandgap, high conductivity, high saturated electron velocity and high critical breakdown field. 4H-SiC MESFET(Metal Semiconductor Field Effect Transistor) has received increasing attention for its potential application of next generation phased array radar, communication base station and harsh environment oreiented devices. In this dissertation, the 4H-SiC MESFET nonlinear models are discussed, and main contributions are as follows:(1) A 4H-SiC MESFET large signal drain current model based on physical expressions has been developed to be used in CAD tools. This drain current model can be considered as semi-empirical MESFET model, and all parameters in this model are determined by physical parameters of 4H-SiC MESFET. This presented model can be provided the initial value for the parameter extrating procedure of 4H-SiC MESFET large signal model.(2)A novel empirical large signal Direct Current (DC) I-V model is presented considering the high saturation voltage,high pinch-off voltage and wide operational range of drain voltage for 4H-SiC MESFETs. A comparison of the presented model with Statz, Materka, Curtice-Cubic and recently reported 4H-SiC MESFET large signal I-V models is made through the Levenberg-Marquardt method for fitting in nonlinear regression. The results show the new model has the advantages of high accuracy, easy initialization and robustness over other models.(3) An analytical large signal capacitance model for 4H-SiC MESFET based on charge conservation is presented. A new charge conservation empirical large signal capacitance model and an accurate empirical large signal capacitance for 4H-SiC MESFET are developed based on the operational principle of devices. Both tow large signal empirical capacitance models are also very suitably used in CAD tools with high accuracy.(4) A novel self-heating effects model for 4H-SiC MESFET is proposed with a combination of an analytical and a CAD oriented quasi-analytical drain current model. A new method for modeling the variation of the low-field electron mobility and the incomplete ionization of impureity for 4H-SiC with temperature is presented, which leading to simulate the drain current including the self-heating effects for circuit oriented design.(5) Based on an analytical solution of the two-dimensional Poisson equation in the near pinch-off region, the behavior of DIBL(Drain Induced barrier lowering) effect is investigated for short channel 4H-SiC MESFETs. An analytical model of accurate DIBL effect model for the asymmetry short channel 4H-SiC MESFET is developed and verified. According to the presented model, an analysis of short channel device performance on the L/a(channel length/channel depth), channel doping concentration N_D and channel length is made, which provides a good basis for short channel device and circuit design.(6) Test technique is one of key parts in modeling 4H-SiC MESFET. Based on designed test holders and high power test instrument, the DC I-V characteristics of 4H-SiC MESFET is obtained using a combination of gate and drain pulse voltage and quasicent bias under threshold voltage for device. And the DC large signal empirical drain current model of 4H-SiC MESFET(I._emp) is further verifed based on obtained DC I-V characteristics of 4H-SiC MESFET. The test fixture of 4H-SiC MESFET RF small signal and RF large signal characteristics is fabricated, and the RF small signal characteristics of 4H-SiC MESFET are presented. The obtained results can be used to model 4H-SiC MESFET and guide the design of 4H-SiC MESFET RF circuit.