SiC Power Diodes and Junction Field-Effect Transistors: Testing, Characterization, Modelling and Applications

Abstract

Power electronic devices are the most important element in any power electronic system. In the last decade, silicon carbide-based devices have emerged as an alternative to silicon-based devices promising a new era in power conversion. The focus of many studies has been on improving material technologies to reduce production cost and defect density. This development has produced devices with superior characteristics. However, the unique properties of silicon carbide favour the development of relatively new power devices, such as the enhanced-mode junction field-effect transistor, in order to exploit the full potential of the material. Following the introduction of such devices, power electronics engineers face new challenges.

In this thesis, the characteristics of SiC power devices, including the PiN diode, the Schottky diode and the JFET, are investigated. Simplified behavioural models for these SiC devices are established and implemented in commercial circuit simulators. The work focuses on the SiC enhancement-mode power VJFET as it is a potential candidate to replace Si MOSFETs and IGBTs especially for medium voltage applications.

Breakdown voltage and drift region design of the SiC PiN diode are studied. Due to the conductivity modulation in the drift region, the PiN diode achieves low on-resistance. At the same time, the high critical electric field of SiC allows high breakdown voltage. However, a saturation voltage of approximately 3 V appears during the forward condition mode as a result of the wide bandgap of SiC. The high forward voltage leads to more conduction loss making this type of diodes only attractive for high voltage applications where the voltage drop over a Schottky diode with same breakdown voltage becomes prohibitively high. The characteristic and the modelling of both PiN and Schottky diodes are discussed in this thesis. Suggested models are implemented in commercial circuit simulators and feature a satisfactory accuracy over a wide temperature range.

By studying the SiC VJFET structure, the physics of the device and gate drive requirements are investigated. Several driving technologies are tested. However, for an effective driving, employing a two-stage driver is recommended. With this driver, high switching speeds are achievable with minimal switching energy loss. Using the SiC JFET, a high efficiency energy conversion, that combines low switching losses with low conduction loss, is possible.

Finally, a model for SiC VJFET is established and implemented on LTSpice. Model parameters are determined by measurements and datasheet values. There is a good agreement between simulation results using this model and measured static and dynamic characteristics of the SiC VJFET.