Abstract (Click here to see abstract with grids.)
Matthew A. Balmer, P.E.
In the field of power electronics, there is an ongoing search for an ideal solid-state power switch which has a low on-state resistance, low switching losses, high operation frequency, and good thermal capabilities. Silicon-Carbide (SiC) technology is a proven forerunner in the quest for the ideal solid-state power switch.
SiC technology represents a disruptive technological innovation for the 21st century that will establish new trajectories for electronic innovations obsoleting the silicon technology of the 20th century. SiC technology has found a niche in small device applications and is a nascent technology being commercialized into power electronics. Testing of SiC power electronics has demonstrated improved efficiencies, reduced size and reduced weight when compared to conventional Si based technology.
This paper explores
- SiC Technology Introduction,
- SiC Technology Power Electronics Advantages, and
- SiC Technology Commercialization.
Sic technology introduction
The challenge in the semiconductor industry is to maximize efficiency, reduce size, increase power quality and reduce costs. Researchers have learned that using Wide Bandgap (WBG) materials, such as Silicon Carbide (SiC), allows semiconductor components to be smaller, faster, more reliable and more efficient than the existing Silicon (Si) technology .
Characteristics SiC Technology
Silicon carbide morphology is a binary combination of two group IV binary elements having an equal number of silicon and carbon atoms arranged in a hexagonal lattice structure. This atomic structure makes SiC one of the hardest and most thermally stable materials known . Additional, features such as increased avalanche breakdown voltage, high thermal conductivity, and decreased thermal leakage current have created great interest in SiC technology for use in high power applications. Microscopic studies of the stability and rupture of molecular junctions under a high voltage bias discovered that semiconductors having a SiC backbone have higher probabilities of sustaining higher voltages .
Wide Bandgap (WBG) Technology
Bandgaps determines how a material’s electrons behave. Electrons are negatively charged particles that surround a nucleus at different energy levels. When electrons move together in the same direction they form an electric current. Electrons in an atom exist in various states which includes their energy level, momentum and spin. Furthermore, quantum mechanics states two electrons cannot exist in the same state at the same time; therefore, one variable must differ.
These sets of possible states form regions called bands. Sets of states that are not possible form regions between the bands called bandgaps. Bands closest to the nucleus of an atom are called core level and the band containing electrons furthest from the nucleus are called the valence band. Beyond the valence band is the conduction band where the electrons can move freely, figure 1.
Examining various materials (Figure 2), metals have an overlapping valence and conductions bands and electrical current easily flows through them. On the opposite end are insulators, which have a wide gap between the valence band and conduction band reducing the probability to move an electron from the valence band to the conduction band. The third and most interesting materials are semiconductors which can have properties like a metal, an insulator, or properties in between.
When semiconductors were first discovered, they were considered useless because of their erratic unpredictable behavior. Once bandgap properties were understood, physicists and engineers were able to harness bandgaps to innovate new technologies. It was also discovered that valence electrons became exited by heat, light, or an electric field and will jump the bandgap (figure 1) provided the bandgap’s width allows the electrons to jump to the conduction band. Consequently, different electronic devices require materials with different bandgaps which is relative to the energy supplied by the energy source. Table 1 lists common semiconductor materials with their bandgap energy levels and application.
SiC: Power Electronics Advantages
Power Semiconductor Construction
Power semiconductors are unique when compared to that of low power signal semiconductors. Low power semiconductor devices are required to carry a few amperes under forward biased conditions and block small voltages under reverse biased conditions. However, power semiconductor devices are required to carry hundreds of amperes in forward biased condition and block several hundred to thousand volts in the reverse biased condition. These extreme operating conditions require a structural change affecting the operating characteristics. These operating conditions create an inherent contradictory condition for power semiconductors, in that, increased doping reduces the forward losses and also reduces the reverse breakdown voltage, with the converse being true. This contradiction is eliminated by the addition of a lightly doped epitaxial layer (drift region) whose doping density is 1014cm-3 between the heavily doped p and n regions whose doping density is 1019cm-3.
Figure 3 depicts the construction differences between low power and high power semiconductor devices.
This epitaxial layer creates a uniform electric field between the p and n junction that reduces the forward voltage drop and increases reverse blocking voltage.
Reduced Power Loss
Reduced power losses create increased savings for the consumer. SiC wide bandgap has a critical field for avalanche breakdown that is 10 times greater than Si. This reduces the width of the epitaxial layer by one-tenth that of Si. The result is SiC has a specific forward conduction resistance 400 times lower than Si thus reducing power losses .
High Temperature Operation
SiC intrinsic characteristics allow SiC based semiconductors to operate at higher junction temperatures. The intrinsic characteristics include:
- SiC based semiconductors have a melting point of approximately 2,700oC, whereas Si based semiconductors have a melting point of approximately 1,400oC.
- Thermal leakage current is proportional to a compound’s intrinsic carrier concentration. SiC has an intrinsic carrier concentration that is in order of magnitudes less (10-18) than Si .
- SiC has a bandgap three times that of Si prohibiting excessive thermal leakage current.
The combination of these intrinsic features allows SiC devices to operate at junction temperatures reaching 600oC compared to Si which has a junction temperature limit of 150oC.
High-speed Switching Operation
SiC wide bandgap has a high dielectric breakdown voltage which reduces power losses during switching operation. Figure 4 depicts the forward recovery time for an SiC Schottky Barrier Diode. The Si overshoot represents an accumulation of charge carriers in the epitaxial layer that must diffuse prior to re-switching. In addition, the Si overshoot is also indicative of heat generation in the device created by the simultaneous current and voltage overshoot. SiC has an epitaxial region one-tenth the size of Si which results in a reduction of overshoot, a faster forward recovery time and reduced power loss.
SiC intrinsic thermal conductivity, the ability to remove heat, is twice that of Si . SiC has a thermal conductivity of 3.3W/cmoK contrasted to Si which has a thermal conductivity of 1.5W/cmoK. SiC’s increased heat dissipation means SiC devices can operate with a reduced temperature drop across the device making it ideal for power applications.
SiC: A Disruptive Technology
The Institute of Electrical Engineers (IEEE) states silicon carbide may be to the 21st century what silicon was to the 20th century . Furthermore, the US Department of Energy considers WBG semiconductors to be a foundational technology that will transform multiple markets and industries, resulting in billions of dollars of savings for businesses and consumers when use becomes widespread . WBG semiconductors permit devices to operate at much higher temperatures, voltages, and frequencies making the power electronic modules using these materials significantly more power and energy efficient than those made from conventional semiconductor materials .
SiC technology is a disruptive technology that has change the trajectory of future semiconductor innovations. Topologies of technological change are divided into two distinct categories: Sustained or Disruptive .
A sustained technological change maintains the industry’s rate of product performance trajectory; these are process innovation and less product innovation. An example of sustained technology change was in the early use of computer memory disk drive industry. Computer memory disk drives were introduced circa 1976 and with a recording density of 1 million bits/square inch. These disk drives used particulate oxide disk technology and ferrite head technology (oxide/ferrite) . From 1976 to 1985, oxide/ferrite recording density grew linearly to 10 million bits/square inch. The recording density growth was related to incremental advances in manufacturing techniques such as grinding the ferrite heads, more precise dimensions, and using smaller more finely dispersed oxide particles on the disk’s surface .
In 1985, the oxide/ferrite storage technology reached its maturity and the recording density began to level-off. The maturity of the oxide/ferrite technology created a search for a new technology which introduced an incremental advance to thin film and head technology starting a new trajectory.
A disruptive technological change is governed by an innovation discontinuity which redefines the performance trajectory leading to creative destruction overturning the established industry structures . Furthermore, discontinuous innovations are competency destroying, obsoleting existing know-how because mastery of the old technology does not imply mastery of the new . In the 20th century, the disruptive technology was the introduction of silicon based electronics which replaced vacuum tubes and sparked the evolution electronics that revolutionized the world.
In the 21st century, the integration of WBG technology will set a new course for all industries. The Department of Energy (DOE) has indicated WBG semiconductors will pave the way for exciting innovations in power electronics, solid-state lighting and other diverse applications across multiple industrial and clean energy sectors .
Commercially Available SiC Power Devices
Mitsubishi Electric has commercialized SiC technology into fundamental power semiconductor devices that include Schottky Barrier Diodes (SBD) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET). More important, Mitsubishi has commercialized SiC technology into both full and hybrid Insulated Gate Bipolar Transistors (IGBT) and Intelligent Power Modules (IPM). These full and hybrid SiC IGBT products offer the following advantages over the existing Si technology:
- Reduced switching losses;
- Increased system efficiency;
- High temperature operation;
- Increased operating frequency;
- Reduced cooling requirements;
- Low inductance for increased switching speed; and
- Reduced system size = increased power density.
The full SiC IGBT modules have attained a 70% reduction in inverter power losses and the hybrid SiC IGBT modules have attained a 45% reduction in inverter power losses. Both products have found applications in a plurality of industries which include:
- High power inverters for traction drives, Uninterruptible Power Supplies, and renewable energy applications.
- Small inverters for household appliances and HVAC equipment.
- High frequency inverters for medical equipment and welding.
SiC Power Application: Mitsubishi Ginza Subway Line Retrofit
SiC technology development has focused on small device technology. However, Mitsubishi Electric introduced SiC technology into power electronics through the development and testing of SiC inverters for Japan’s Ginza subway line . Mitsubishi’s test results are summarized as follows:
- The SiC based inverter was 40% smaller and lighter than the conventional inverter;
- Energy savings was 38.6% compared to the conventional system; and
- Increased regenerated power to 51% compared to 22.7% for the conventional system .
These favorable results have prompted Mitsubishi Electric to commercialize the technology and have received 127 orders for the SiC-based inverters .
SiC technology is a disruptive technology that will establish a new trajectory for small device and power electronics. Tests have demonstrated that SiC based power electronics is smaller, lighter, and more efficient than the existing Si based technology.
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