  We find that electron states at the bottom of the conduction bands of
covalent semiconductors are distributed mainly in the interstitial channels and
that this floating nature leads to the band-gap variation and the anisotropic
effective masses in various polytypes of SiC. We find that the channel length,
rather than the hexagonality prevailed in the past, is the decisive factor for
the band-gap variation in the polytypes. We also find that the floating nature
causes two-dimensional electron and hole systems at the interface of different
SiC polytypes and even one-dimensional channels near the inclined SiC surface.
