2016.09.08 SiC Power Device
Silicon carbide (SiC) is a comparatively new semiconductor material. We begin by briefly describing its properties and features.
SiC properties and features
SiC is a compound semiconductor material consisting of silicon (Si) and carbon (C). The chemical bond in SiC is extremely strong, and the material is thermally, chemically and mechanically stable. SiC exists in a variety of polymorphic crystalline structures called polytypes, with different physical properties. The 4H-SiC polytype is deemed optimal for use in power devices. The following table compares 4H-SiC with Si and other semiconductor materials that are recently attracting interest.
The yellow-highlighted part of the table compares Si and SiC. Characteristics and values in blue are parameters that are particularly important for uses in power devices. As the numerical values indicate, where these parameters are concerned SiC is superior. And in contrast with other new materials, but similarly to Si, a feature of SiC is the ability to control the p-type and n-type regions necessary for device fabrication over wide ranges. For these reasons, SiC is anticipated to be a material for use in power devices that go beyond the limits of Si.
Features of SiC Power Devices
With dielectric breakdown electric field strength approximately ten times higher than that of Si, SiC can achieve very high breakdown voltage from 600 V to thousands of volts. Doping concentrations can be made higher than those in Si devices, and drift layers can be made thin. Nearly all of the resistance component of a high voltage power device is the resistance of the drift layer, and the resistance value increases in proportion to the thickness of the drift layer. When using SiC, the drift layer can be made thin, and so a device with a high voltage and extremely low turn-on resistance per unit area can be fabricated. Theoretically, for a given high voltage, the drift layer resistance per unit area can be reduced to 1/300 of that for Si.
In a Si power device, IGBTs (insulated-gate bipolar transistors) and other minority-carrier devices (bipolar transistors) have mainly been used in the past in order to alleviate the increase in turn-on resistance that accompanies higher breakdown voltages. However, the large switching losses give rise to heat generation problems, imposing limits on high-frequency driving. Using SiC, such fast majority-carrier devices as Schottky barrier diodes and MOSFETS can be designed for high voltages, making possible the simultaneous attainment of a high voltage, low turn-on resistance, and fast operation, parameters that entailed trade-offs in silicon devices.
Further, the band gap is about three times that of Si, making possible operation at higher temperatures than for Si devices. At present, due to constraints imposed by the thermal resistance of packages, operation is guaranteed at 150°C to 175°C, but as package technology advances, guaranteed operation at 200°C and above will become possible.
Important points have been explained briefly. The discussion may have been a bit difficult for persons without a background in material properties and processes, but even without understanding all the details, you will be able to make use of SiC power devices.
・The physical properties of SiC are well-suited to power devices.
・Compared with Si semiconductors, losses are low, and dynamic characteristics in high-temperature environments are excellent.