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F
ragments of silicon carbide (also called carbo- rundum), the second-hardest natural mineral after diamond, were discovered in 1893 by French
By Filippo Di Giovanni
in the “on” state produced by current flow through the on- resistance of the power MOSFET. Thus, the BFOM applies to systems operating at lower frequencies in which the conduc- tion losses are dominant.
In high-frequency systems, switching losses are more sig- nificant. Their higher values are due to the charging and dis- charging of the input capacitance of the MOSFET.2
The SiC material can be synthesized in different variants called polytypes. Polytypism is akin to polymorphism; the latter refers to the possibility of an element or compound crystallizing in a range of structures, while polytypes differ by virtue of the stacking sequence of atomic layers along a given crystalline direction without change in the chemical composition. For SiC material, the basic block is a tetrahe- dron with a C atom at the center surrounded by four Si atoms connected through strong covalent bonds, where an electron pair is shared. In the Ramsdell notation, each polytype is labeled after the number of Si-C bilayers in the unit cell and the lattice structure, which can be cubic (C), hexagonal (H), or rhombohedral (R).
The Physics, Structure,
and Properties of
Silicon Carbide
chemist Henri Moissan in a crater at Canyon Diablo, Arizona, that had been created by the impact of a meteorite 50,000 years ago. SiC was artificially synthesized just two years after its discovery.
American chemist Edward Goodrich Acheson invented a pro- cess for synthesizing SiC from silica, carbon, and some salt additives. The so-called Acheson process was a technique for producing SiC in the form of an abrasive powder for cut- ting, grinding, and polishing — the first industrial applica- tion of this material, still in use today. At the same time, SiC was used to explore its physical and chemical properties.
SiC is an IV-IV wide-bandgap compound (that is, with rigid 50% Si, 50% C stoichiometry) material used in opto- and power electronics. Its physical properties — a high break- down field, high-saturation drift velocity, and high thermal conductivity — make it ideal for applications in which it could need to operate at high temperature, high intensity of radia- tion, high voltage, or high power dis-
sipation. The bar diagrams in Figure 1 show SiC’s main physical parameters compared with both silicon and gallium nitride.
One figure of merit for power devices can be expressed as εμEB3 (Baliga fig- ure of merit, or BFOM), where ε is the dielectric constant, μ is the mobil- ity, and EB is the breakdown or critical field. The figure of merit of 4H-SiC is 560× that of silicon (240× for 6H-SiC), indicating the great potential of SiC for power-device applications. The BFOM assumes that the power losses are solely due to the power dissipation
FIGURE 1: PHYSICAL PROPERTIES OF SiC VERSUS Si AND GaN
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ASPENCORE GUIDE TO SILICON CARBIDE