The Investigation Of Novel Silicon Carbide Power Diodes

With the increase of demand for higher electrical energy conversion efficiency, and the continuous development of power electronic technology, performances of traditional Silicon based devices have been restricted by the physical limits of the Silicon material. Silicon Carbide is an attractive candidate for the next generation of wide bandgap semiconductor devices. Nowadays, remarkable achievements have been made on SiC devices. Si based power electronic devices have been replaced by SiC devices in some important energy field gradually. Among several device structures, SiC diode was the first to be developed and comerciallized. However, technical challenges remain in its processing technologies and some theoretical aspects need to be further analyzed and investigated. As a result, a systematic research on SiC Junction Barrier Schottky (JBS) diodes from aspects of structure design, analytical modeling and experiments is conducted in this dissertation. Furthermore, a new type of SiC diode structure, namely, the Pinched Barrier Rectifier (PBR) is investigated. The main research contents are shown as follows.In order to improve the weak electric field shielding effects and the subsequent soft breakdown characteristics of traditional planar JBS diodes, the trenched JBS structure (TJBS) has been proposed. A systematic research on the comparison between SiC TJBS diodes and JBS diodes is conducted in this dissertation. The two kinds of diodes are analyzed by different methods of numerical simulation, analytical modeling and experimental analysis. It is found that SiC TJBS diodes have a wider design window and improved performance over the traditional JBS diodes.While there have been some good modeling research work on the forward conduction charactersitics of SiC JBS diodes, the same for their reverse electric field distribution and reverse leakage characteristics are needed. Traditional reverse leakage model is based on the fitting of surface electric field to the simulation results. However, such an approach has some limitations in accuracy and can not be applicable in different conditions of various structure parameter designs. Improved analytical models are established in this dissertation, including a mathematical model of reverse electric field distribution with two-dimensional effects, and a subsequent physical model of reverse leakage current. The validity and accuracy of the models are verified by the simulation and experimental results. Based on the forward conduction and reverse leakage models, this work also gives out the theoretical optimization of SiC JBS and TJBS diodes.Since the Schottky barrier in the JBS diode structure is vulnerable to the Fermi level pinning effects and process conditions, its barrier height and the diode onset voltage are restricted by a few optional metal choices. In addition, the Schottky barrier height is affected by the image force lowering effect and can be significantly reduced under high reverse bias voltage. In order to achieve a continuously adjustable diode barrier height and hence gain better flexibility, the Pinched Barrier Rectifier is proposed and investigated. The operation mechanism of the pinched barrier is analyzed through numerical simulation, physical energy band analysis and experiments. This opens up the possibility of a new research direction in addition to the traditional Schottky barrier and PN junction barrier diodes.