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 1.1. 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 1.1 lists only the shallowest reported ionization energies of each impurity.
TABLE 1.1 Comparison of Selected Important Semiconductor Electronic Properties of Major SiC Polytypes
Property
Silicon
GaAs
4H-SiC
6H-SiC
3C-SiC
2H-GaN
Bandgap (eV)
1.1
1.42
3.2
3
2.3
3.4
Relative dielectric constant
11.9
13.1
9.7
9.7
9.7
9.5
Breakdown field
0.6
0.6
//c-axis: 3.0
//c-axis: 3.2
1.8
2–3
ND = 1017 cm_3 (MVcm_1)
c-axis: 2.5 ⊥
c-axis: > 1 ⊥
Thermal Conductivity (W/cm-K)
1.5
0.5
3–5
3–5
3–5
1.3
Intrinsic carrier concentration (cm_3)
1010
1.8 × 106
~10_7
~10_5
~10
~10_10
Electron mobility at
1200
6500
//c-axis: 800
//c-axis: 60
750
900
ND = 1016 cm_3 (cm2V_1s_1)
c-axis: 800 ⊥
c-axis: 400 ⊥
Hole mobility at
420
320
115
90
40
200
NA = 1016 cm_3 (cm2V_1s_1)
Saturated electron velocity (107 cms_1)
1
1.2
2
2
2.5
2.5
Donor dopants and
P: 45
Si: 5.8
N: 45
N: 85
N: 50
Si: 20
shallowest ionization energy (meV)
As: 54
P: 80
P: 80
Acceptor dopants and
B: 45
Be, Mg,
Al: 200
Al: 200
Al: 270
Mg: 140
shallowest ionization energy (meV)
C: 28
B: 300
B: 300
2005 Commercial wafer diameter (cm)
30
15
7.6
7.6
15
None
For comparison, Table 1.1 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 1.1 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.
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