Silicon carbide (SiC)-based semiconductor electronic devices and circuits are presently being developed for use in high-temperature, high-power, and high-radiation conditions under which conventional semiconductors cannot adequately perform. Silicon carbide’s ability to function under such extreme conditions is expected to enable significant improvements to a far-ranging variety of applications and systems. These range from greatly improved high-voltage switching for energy savings in public electric power distribution and electric motor drives to more powerful microwave electronics for radar and communications to sensors and controls for cleaner-burning more fuel-efficient jet aircraft and automobile engines. In the particular area of power devices, theoretical appraisals have indicated that SiC power MOSFET’s and diode rectifiers would operate over higher voltage and temperature ranges, have superior switching characteristics, and yet have die sizes nearly 20 times smaller than correspondingly rated silicon-based devices. However, these tremendous theoretical advantages have yet to be widely realized in commercially available SiC devices, primarily owing to the fact that SiC’s relatively immature crystal growth and device fabrication technologies are not yet sufficiently developed to the degree required for reliable incorporation into most electronic systems. This chapter briefly surveys the SiC semiconductor electronics technology. In particular, the differences (both good and bad) between SiC electronics technology and the well-known silicon VLSI technology are highlighted. Projected performance benefits of SiC electronics are highlighted for several large-scale applications. Key crystal growth and device-fabrication issues that presently limit the performance and capability of high-temperature and high-power SiC electronics are identified.
SILICON CARBIDE (SiC) materials are currently metamorphosing from research and development into a market driven manufacturing product. SiC substrates are currently used as the base for a large fraction of the world production of green, blue, and ultraviolet light-emitting diodes (LEDs). Emerging markets for SiC homoepitaxy include high-power switching devices and microwave devices for S and X band . Applications for heteroepitaxial GaN-based structures on SiC substrates include LEDs and microwave devices. These exciting device results stem primarily from the exploitation of the unique electrical and thermophysical properties offered by SiC compared to Si and GaAs. Among these are: a large bandgap for high-temperature operation and radiation resistance; high critical breakdown field for high-power output; high saturated electron velocity for high-frequency operation; significantly higher thermal conductivity for thermal management of high-power devices.
Silicon carbide occurs in many different crystal structures, called polytypes. Despite the fact that all SiC polytypes chemically consist of 50% carbon atoms covalently bonded with 50% silicon atoms, each SiC polytype has its own distinct set of electrical semiconductor properties. While there are over 100 known polytypes of SiC, only a few are commonly grown in a reproducible form acceptable for use as an electronic semiconductor. The most common polytypes of SiC presently being developed for electronics are 3C-SiC, 4H-SiC, and 6H-SiC. The atomic crystal structure of the two most common polytypes is shown in the schematic cross section in Figure. As discussed much more thoroughly in References 9 and 10, the different polytypes of SiC are actually composed of different stacking sequences of Si–C bilayers (also called Si–C double layers), where each single Si–C bilayer is denoted by the dotted boxes in Figure. Each atom within a bilayer has three covalent chemical bonds with other atoms in the same (its own) bilayer, and only one bond to an atom in an adjacent bilayer. Figure 5.1a shows the bilayer of the stacking sequence of 4H-SiC polytype, which requires four Si–C bilayers to define the unit cell repeat distance along the c-axis stacking direction (denoted byMiller indices). Similarly,the 6H-SiC polytype repeats its stacking sequence every six bilayers throughout the crystal along the stacking direction.The direction depicted in Figure is often referred to as one of (along with ) the a-axis directions. SiC is a polar semiconductor across the c-axis, in that one surface normal to the c-axis is terminated with silicon atoms while the opposite normal c-axis surface is terminated with carbon atoms. As shown, these surfaces are typically referred to as “silicon face” and “carbon face” surfaces, respectively. Atoms along the left-or right-side edge of Figure would reside on “a-face” crystal surface plane normal to the direction. 3C-SiC, also referred to as β-SiC, is the only form of SiC with a cubic crystal lattice structure. The noncubic polytypes of SiC are sometimes ambiguously referred to as α-SiC. 4H-SiC and 6H-SiC are only two of the many possible SiC polytypes with hexagonal crystal structure. Similarly, 15R-SiC is the most common of the many possible SiC polytypes with a rhombohedral crystal structure.
Owing to the differing arrangement of Si and C atoms within the SiC crystal lattice, each SiC polytype exhibits unique fundamental electrical and optical properties. Some of the more important semiconductor electrical properties of the 3C, 4H, and 6H SiC polytypes are given in Table 5.1. Much more detailed electrical properties can be found in References 11–13 and references therein. Even within a given polytype, some important electrical properties are nonisotropic, in that they are strong functions of crystallographic direction of current flow and applied electric field (for example, electron mobility for 6H-SiC). Dopant impurities in SiC can incorporate into energetically inequivalent sites. While all dopant ionization energies associated with various dopant incorporation sites should normally be considered for utmost accuracy, Table 5.1 lists only the shallowest reported ionization energies of each impurity.
Silicon carbide occurs in many different crystal structures, called polytypes. A more comprehensive introduction to SiC crystallography and polytypism can be found in Reference 9. Despite the fact that all SiC polytypes chemically consist of 50% carbon atoms covalently bonded with 50% silicon atoms, each SiC polytype has its own distinct set of electrical semiconductor properties. While there are over 100 known polytypes of SiC, only a few are commonly grown in a reproducible form acceptable for use as an electronic semiconductor. The most common polytypes of SiC presently being developed for electronics are 3C-SiC, 4H-SiC, and 6H-SiC. The atomic crystal structure of the two most common polytypes is shown in the schematic cross section in Figure 5.1. As discussed much more thoroughly in References 9 and 10, the different polytypes of SiC are actually composed of different stacking sequences of Si–C bilayers (also called Si–C double layers), where each single Si–C bilayer is denoted by the dotted boxes in Figure 5.1. Each atom within a bilayer has three covalent chemical bonds with other atoms in the same (its own) bilayer, and only one bond to an atom in an adjacent bilayer. Figure 5.1a shows the bilayer of the stacking sequence of 4H-SiC polytype, which requires four Si–C bilayers to define the unit cell repeat distance along the c-axis stacking direction (denoted byMiller indices). Similarly, the 6H-SiC polytype illustrated in Figure 5.1b repeats its stacking sequence every six bilayers throughout the crystal along the stacking direction. The direction depicted in Figure 5.1 is often referred to as one of (along with )the a-axis directions. SiC is a polar semiconductor across the c-axis, in that one surface normal to the c-axis is terminated with silicon atoms while the opposite normal c-axis surface is terminated with carbon atoms. As shown in Figure 5.1a, these surfaces are typically referred to as “silicon face” and “carbon face” surfaces, respectively. Atoms along the left-or right-side edge of Figure 5.1a would reside on “a-face” crystal surface plane normal to the direction. 3C-SiC, also referred to as β-SiC, is the only form of SiC with a cubic crystal lattice structure. The noncubic polytypes of SiC are sometimes ambiguously referred to as α-SiC. 4H-SiC and 6H-SiC are only two of the many. FIGURE 5.1 Schematic cross-sectional depictions of (a) 4H-SiC and (b) 6H-SiC atomic crystal structure, showing important crystallographic directions and surfaces. possible SiC polytypes with hexagonal crystal structure. Similarly, 15R-SiC is the most common of the many possible SiC polytypes with a rhombohedral crystal structure.
Owing to the differing arrangement of Si and C atoms within the SiC crystal lattice, each SiC polytype exhibits unique fundamental electrical and optical properties. Some of the more important semiconductor electrical properties of the 3C, 4H, and 6H SiC polytypes are given in Table 5.1. Much more detailed electrical properties can be found in References 11–13 and references therein. Even within a given polytype, some important electrical properties are nonisotropic, in that they are strong functions of crystallographic direction of current flow and applied electric field (for example, electron mobility for 6H-SiC). Dopant impurities in SiC can incorporate into energetically inequivalent sites. While all dopant ionization energies associated with various dopant incorporation sites should normally be considered for utmost accuracy, Table 5.1 lists only the shallowest reported ionization energies of each impurity. TABLE 5.1Comparison of Selected Important Semiconductor Electronic Properties of Major SiC Polytypes with Silicon, GaAs, and 2H-GaN at 300 K For comparison, Table 5.1 also includes comparable properties of silicon, GaAs, and GaN. Because silicon is the semiconductor employed in most commercial solid-state electronics, it is the standard against which other semiconductor materials must be evaluated. To varying degrees the major SiC polytypes exhibit advantages and disadvantages in basic material properties compared to silicon. The most beneficial inherent material superiorities of SiC over silicon listed in Table 5.1 are its exceptionally high breakdown electric field, wide bandgap energy, high thermal conductivity, and high carrier saturation velocity. The electrical device performance benefits that each of these properties enables are discussed in the next section, as are system-level benefits enabled by improved SiC devices.
Two of the most beneficial advantages that SiC-based electronics offer are in the areas of high-temperature and high-power device operation. The specific SiC device physics that enables high-temperature and high-power capabilities will be examined first, followed by several examples of revolutionary system-level performance improvements these enhanced capabilities enable.
The wide bandgap energy and low intrinsic carrier concentration of SiC allow SiC to maintain semiconductor behavior at much higher temperatures than silicon, which in turn permits SiC semiconductor device functionality at much higher temperatures than silicon . As discussed in basic semiconductor electronic device physics textbooks, semiconductor electronic devices function in the temperature range where intrinsic carriers are negligible so that conductivity is controlled by intentionally introduced dopant impurities. Furthermore, the intrinsic carrier concentration is a fundamental prefactor to well-known equations governing undesired junction reverse-bias leakage currents. As temperature increases, intrinsic carriers increase exponentially so that undesired leakage currents grow unacceptably large, and eventually at still higher temperatures, the semiconductor device operation is overcome by uncontrolled conductivity as intrinsic carriers exceed intentional device dopings. Depending upon specific device design, the intrinsic carrier concentration of silicon generally confines silicon device operation to junction temperatures <300°C. SiC’s much smaller intrinsic carrier concentration theoretically permits device operation at junction temperatures exceeding 800°C. 600°C SiC device operation has been experimentally demonstrated on a variety of SiC devices. The ability to place uncooled high-temperature semiconductor electronics directly into hot environments would enable important benefits to automotive, aerospace, and deep-well drilling industries. In the case of automotive and aerospace engines, improved electronic telemetry and control from high-temperature engine regions are necessary to more precisely control the combustion process to improve fuel efficiency while reducing polluting emissions. High-temperature capability eliminates performance, reliability, and weight penalties associated with liquid cooling, fans, thermal shielding, and longer wire runs needed to realize similar functionality in engines using conventional silicon semiconductor electronics.
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