TECHNOLOGY SiC POWER ELECTRONICS
Figure 2. Ultra-high- voltage SiC power devices can enable a substantial trimming of the size and weight of power converters
Why SiC?
Interest in SiC has been driven by its wide bandgap – it is 3.26 eV in the 4H polytype commonly used for making power devices. This wide bandgap is responsible for a breakdown electric field strength ten times that of silicon and a thermal conductivity that is three times that of silicon. Furthermore, SiC is an exceptional wide bandgap semiconductor, which offers the opportunity to control the doping concentration over a very wide range (n-type: 1014 p-type: 1014
– 1019 – 1020 cm−3 cm−3 , ). In addition, SiC devices
can operate at high temperatures, such as in excess of 250°C. Drawing on all of these attractive attributes enables the simplification of the bulky cooling units often required in silicon-based power converters.
The higher field strengths permitted with SiC aid device design. Compared to silicon, blocking- layer thickness can be tens times thinner, while the doping concentration can be increased by two orders of magnitude (see Figure 3). Thanks to this, it is possible to realise huge reductions in the voltage-blocking region resistance, and ultimately achieve low levels of power dissipation.
The tried and tested route for realising a high blocking voltage in any semiconductor device is to increase material thickness and trim doping
concentration in the voltage-blocking region. Calculating the impact of these changes is easy, and helps to guide device designers that must also consider the doping-dependent breakdown electric field of the material (see Table 1).
These back-of-the-envelope calculations also reveal why it is impossible to build a 20 kV device from silicon: the required doping concentration would have to be close to the intrinsic carrier concentration at room temperature, while the required thickness would be impractical. Fortunately, with SiC, it’s an entirely different story – a 20 kV device falls easily within the limits of what is possible.
Manufacturing such a device in high volumes is not out of the question, given the rapid progress in SiC bulk growth processes that has led to the availability of single crystalline SiC wafers of reasonable quality with 100 mm and 150 mm diameters. There have also been remarkable advances in SiC epitaxy and device processing technologies, such as ion implantation and metallization, and, on top of this, it is possible to draw on the development of devices operating at lower voltages. Back in 1991 NASA reported the first 1 kV SiC pin diode, while our group announced the first SiC Schottky barrier diode operating at that voltage two years later, and since then many more groups from all over the world have started to develop high-voltage SiC power devices (see Figure 4 for an overview of the increases in SiC blocking voltage).
Why bipolar?
Table 1. Typical thicknesses and doping concentrations required for specific blocking voltages in silicon and SiC devices. Calculations took into account the doping-dependent breakdown electric field of the materials
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www.compoundsemiconductor.net March 2014
One decision facing designers of power electronic systems and modules is whether to select a unipolar device, such as an SBD or a FET, or deploy a bipolar device, such as a pin diode, thyristor, or insulated-gate bipolar transistor (IGBT). The decision partly depends on the blocking voltage required. An SBD is an attractive
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