Author: Zhang, Meng
Title: Advanced SiC power devices with enhanced switching performance based on numerical simulations
Advisors: Cheng, Ching-hsiang (ISE)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Power transistors
Electric current converters
Department: Department of Industrial and Systems Engineering
Pages: xviii, 141 pages : color illustrations
Language: English
Abstract: Power devices are essential and fundamental elements in power electronic circuits for controlling the current flows in switching operations. Traditionally, power devices are fabricatedon silicon. Several representative power switches are thyristors, power bipolar junction transistors (BJTs), insulated gate bipolar transistors (IGBTs), power junction field effect transistors (JFETs), power metal oxide semiconductor field effect transistors (MOSFETs), etc. After development over several decades, silicon MOSFETs have become the dominant power switching devices in low-to-medium voltage range, while silicon IGBTs are the most popular power switch in medium-to-high voltage range. Apart from power switches, power rectifiers are also of significance in power electronics. The performance of these silicon based power devices is gradually reaching their theoretical limitation defined by the semiconductor material. The wide bandgap semiconductor silicon carbide (SiC) promises significant improvements in power device performance in terms of breakdown voltage, on-resistance, and thermal resistance, etc. At present, the most popular SiC power devices are SiC MOSFET as power switches and SiC junction barrier Schottky diode (JBS) as power rectifiers. The development of SiC MOSFETs is challenged by the low electron mobility at the interface between the gate dielectric and SiC which results in a large channel resistance and thus boosts up the total ON-resistance (RON). The trench MOSFET structure is widely considered as a promising method, which allows a more compact cell design, and thus lowers the device RON byahigher channel density. When used in wide bandgap SiC devices, the trench MOSFET faces a critical issue for its commercial success: the high OFF-state oxide field at the trench bottom/corner. An effective approach to reducing the maximum oxide field (Eox-m) to a safe level (< 3 MV/cm) is to introduce a grounded p-shield region under the gate trench. However, the p-shield leads to the formation of a JFET region that presents a corresponding JFET resistance. Furthermore, the down-scaling of the unit cell is limited by the width of the current path in the JFET regions, hindering the effort to increase the channel density. In this thesis work, a SiC trench MOSFET solution is proposed for achieving a low RON in the SiC trench MOSFET by driving the MOS-gate and the JFET gate (the p-shield under the trench gate) simultaneously during device operation. The JFET portion is driven by the same gate-drive signal as the MOS-gate through a self-biasing network. This SiC trench MOSFET with a self-biased p-shield (SBS-MOS) boasts a widened current path in the JFET region at the ON-state, which lowers the JFET resistance and/or allows a further size reduction of the cell pitch.
For the SiC trench MOSFET with a p-shield, an appreciable chip area has to be sacrificed in order to contact the p-shield region. In this thesis work, a trench MOSFET structure with protruded p-bodies (PB-MOS), i.e. p-bodies deeper than the gate trench, is proposed. No additional p-shield regions are required. The lateral pinch-off effect of the protruded p-bodies in the PB-MOS protects the gate trench from the high OFF-state drain voltage, leading to a lower OFF-state oxide field. A low RON and a high breakdown voltage are maintained in the PB-MOS. The PB-MOS further boasts a much improved Crss and better switching performance compared to the conventional SiC trench MOSFET. The junction barrier Schottky diode (JBS) is the most popular rectifier among SiC based rectifiers since it features the merits of the fast switching and low turn-on voltage of a typical Schottky barrier diode as well as the merits of a high breakdown voltage due to the integrated pn junction. Recently, some research studies suggest a Schottky contact to the p-grid in power devices, including the SiC JBS. For the SiC JBS, a rectifying Schottky contact to the p-grid suppresses the minority carrier injection from the p-grid, which avoids the possible bipolar degradation well known in SiC technology. In this thesis work, the role of the contact to the p-grid is comprehensively discussed. It is found whether a Schottky contact or an Ohmic contact do not impact on the static characteristics of the JBS. However, in the switching operation, the JBS with Schottky contact to p-grids would suffer a severe degradation in the I-V characteristics, including a larger turn-on voltage and a higher resistance. The mechanism behind this degradation is identified with the storage of negative charges in the p-grid. The rectifying Schottky contact allows charging the p-grids with a high reverse bias, but hinders the release of the negative charges in the p-grids. Therefore, a non-rectifying contact to the p-grid is essential for the switching operation of the SiC JBS.
Rights: All rights reserved
Access: open access

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