Silicon Carbide bipolar power devices - potentials and limits
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Silicon Carbide bipolar power devices - potentials and limits Ranbir Singh Cree Inc., 4600 Silicon Dr., Durham NC 27703. Ph: 919-313-5540; Fax: 919-313-5696, E-mail: [email protected]. ABSTRACT Bipolar devices made with SiC offer 20-50X lower switching losses as compared to conventional semiconductors, and a comparable on-state voltage drop at sufficiently high current densities. To exploit the tremendous advantages offered by SiC for bipolar power devices, it is important to understand the relevant voltage/current range, fundamental limits and technological challenges in order to develop this technology commercially. The opportunity of operating a device at a high current density (>300 A/cm2) to increase total current with reasonable yield, the poor reliability of MOS at high temperatures, and the relatively low channel mobilities obtained in 4H-SiC MOSFETs may make certain bipolar devices more attractive even as low as 1700 V. The total power loss in various bipolar devices is analyzed and compared to fundamental operational limits in order to find the applicability of various devices to advanced applications. INTRODUCTION Power devices made with silicon carbide are expected to show great performance advantages as compared to those made with other semiconductors. This is because SiC has an order of magnitude higher breakdown electric field than conventional materials. A high breakdown electric field allows the design of SiC power devices with thinner and higher doped blocking layers. The large bandgap of SiC also results in a much higher operating temperature and higher radiation hardness. The requirement that a power device must be able to dissipate a significant amount of heat indicates that the thermal characteristics of the semiconductor are also of fundamental importance. The thermal conductivity of SiC is 4.9 W/oC-cm at room temperature, which is greater than that of any metal. The high value of thermal conductivity for SiC allows dissipated heat to be readily extracted from the device. This, in turn, allows a corresponding increase in power to be applied to the device for a given junction temperature. Most of the heating in a semiconductor switch occurs either in the turn-on state or during switching transients. To increase the efficiency and decrease the cost, weight and volume of power conversion systems, it is important to reduce this heat generation. While the on-state voltage drop determines the turn-on heat generation, the switching power is primarily determined by the efficiency with which the current carrying charge can be withdrawn. Often, there exists a design trade-off between the switching speed and the on-state voltage drop in a semiconductor switch. There are other considerations that may be of paramount importance for the circuit designer while making the choice of the right semiconductor switch for a particular application. These include: high temperature capability; radiation hardness; ease of current control; ease of protection under abnormal modes of operation and whether the device i
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