The Theoretical And Experimental Study For 6H-SiC Schottky Barrier Source/Drain MOSFET

Silicon carbide is an attractive wide band semiconductor material in high-temperature, high-frequency, high-power and radiation resistant applications due to its excellent physical properties such as high breakdown voltage, high thermal conductivity and high saturation electron drift velocity. A novel SiC Schottky Barrier Source/Drain Metal-Oxide-Semiconductor Field-Effect Transistor (SiC SBSD-MOSFET) is proposed in this dissertation. This kind device can effectively suppress short channel effects for the scaling down of conventional MOSFETs, avoiding the serious effect of the steps of ion implantation and annealing at high temperature on the conventional SiC MOSFET. There are few papers about this device so far. The main studies and contributions of this dissertation are as follows.(1) The study on the model of SiC schottky contacts. The electron tunneling probabilities through triangular barrier to be accurately solved by the one-dimensional time-independent Schro|¨dinger equation is calculated. The presented method is more accurate than the usually used method with WKB approximation. The model of SiC schottky contacts is optimized in which Femi distribution is adopted instead of Boltzmann distribution based on the tunneling probabilities calculated with the presented method. The proposed model has the advantages of more universality, more suitability for SiC in high-field application and seamless calculation of thermionic emission and tunneling current.(2) The study on the theoretical model of SiC SBSD-MOSFET. A numerical-analytical model for SiC SBSD-NMOSFET is presented based on the analysis of the operational mechanism of this device. The device performance is correctly described by presented model including the effect of tunneling current and the barrier lowering in which an equivalent channel thickness is proposed for describing equivalent cross-section of current. The simulated results show that the barrier height at source contact greatly affects on the device performance and its feature will have more improvement as operational temperature rises. Threshold voltage of SiC SBSD-MOSFETs is analyzed. The threshold voltage of the device is defined as the gate voltage at which carriers from the source contact enter the channel by field emission mode while the channel is strongly inverted.(3) The study on the structure of SiC SBSD-MOSFET. The width of sidewall strongly affects on the device performance from the first experiment. The effect of sidewall on the performance of this kind device is simulated with 2-D simulator. The simulated results show that sidewall with the width less than 0.1μm slightly affects on the device performance. However when the width of sidewall exceeds 0.1μm, the conduction does not occur until the drain voltage is high enough and saturation current is pretty low. A novel SiC SBSD-NMOSFET with field-induced source/drain extension is proposed and demonstrated for the first time. In the new device the FISD extension is induced by a metal field-plate lying on the top of the passivation oxide and the width of Schottky barrier is controlled by the metal field-plate. The new structure not only eliminates the effect of the sidewalls but also significantly improves the on-state current. Moreover the performance of the presented device exhibits very weak dependence on the widths of sidewalls, so process control becomes easier and more reliable.(4) The study on the experiment of 6H-SiC MOSFET with polysilicon as source/drain contacts. A new front-to-front measurement structure is used for High-frequency C-V measurement. Experiment results show this structure is feasible with advantages over conventional test structure. The scheme of polysilicon as source/drain contacts is proposed for the first time. 6H-SiC MOSFETs with polysilicon as source/drain contacts are designed and fabricated. Measurement and analysis results show that the serious effect of etching process on the surface of SiC is the cause of inferior quality of gate oxide which strongly depresses the control of gate voltage on the channel. The further modification of the process is put forward based on above analysis.