Design and Fabrication of 4H-Silicon Carbide High Voltage Devices

This research focuses on the design, fabrication, and ruggedness improvement of 15kV 4H-SiC devices. An extensive study of the material properties of 4H-SiC and parameters for numerical simulations was first carried out.;The most important aspect in designing high voltage devices is to realize an efficient edge termination technique. The so-called Multiple Floating Zone Junction Termination Extension (MFZ-JTE) was proposed to simplify the number of process steps and to alleviate the sensitiveness to processing conditions; its concept has been experimentally verified by the fabrication of MFZ-JTE terminated 10kV PiN rectifiers. A systematic approach to design Non-Equally Spaced Floating Field Rings (NES-FFRs) was also proposed and experimentally verified.;The optimization of on-state loss and switching energy loss of 15kV 4H-SiC MOSFETs and IGBTs have been studied by examining their frequency capability. The frequency capabilities of the MOSFETs and IGBTs are compared as a function of active area. This study concludes that when they are used in a Full-Bridge AC-DC rectifier and a Dual Half-Bridge DC-DC converter in Solid State Transformer (SST), the symmetric IGBT becomes the best candidate device up to 10 kHz of operation.;In addition to the frequency capability study mentioned above, the ruggedness of the 4H-SiC 15kV IGBT has been examined. The maximum allowable saturation current density is extremely low (<500A/cm²) for the device to survive a short-circuit condition for 20us. To achieve enough margin of short circuit capability, a new 15kV 4H-SiC IGBT with an embedded JFET region (JE-IGBT) has been proposed. The proposed device provides a greatly improved trade-off between saturation current and on-state voltage drop. In addition, switching simulations of the JE-IGBT show that it can achieve soft turn-on characteristics.;To further improve upon the device electrical characteristics, a novel 4H-SiC MOS controlled thyristor with current saturation capability is proposed. A thyristor structure with a built-in lateral JFET region, the so-called JFET Embedded Base Resistance Controlled Thyristor (JE-BRT) is proposed to reduce the on state forward voltage drop. The JE-BRT has inbuilt current saturation capability due to its embedded JFET region. The thyristor regenerative action and the current saturation capability greatly improve the trade-off between saturation currents and forward voltage drops.;Another application of 15kV 4H-SiC devices is power distribution system protection. Using solid state switching devices with 15kV blocking capability, one can isolate a faulted section much faster than with a traditional mechanical circuit breaker. The concept and the requirements of a solid-state circuit breaker aimed to use in the power distribution system are explained. The 4H-SiC Field Controlled Diode (FCD) has been chosen as an ideal candidate device for this application.;To verify the characteristics of the lateral channel concept as well as the blocking capability of the proposed FCD, a normally-on JFET was fabricated on a 10kV rated 4H-SiC n-type wafer. The fabricated JFETs demonstrated perfect pentode-like I-V characteristics with a blocking gain of 522. A comparative study on the various gate structures shows that the surface gate type device can provide the lowest energy loss for both on-state and transient state. The fabricated JFET can also serve as a switching device for the primary side of the SST.;In addition, to deliver an efficient freewheeling diode for the SST, a new concept of JBS diode was proposed and experimentally verified. The new JBS is capable of handling high surge current by injecting holes from the p+/n junction. It also provides a low forward voltage drop during normal unipolar operation.;In summary, 10-15kV rated 4H-SiC devices such as MOSFETs, IGBTs, FCDs, JFETs, JBS diodes have been extensively investigated with the aim of optimize and improve their on-state, off-state, transient, and short-circuit performance.