  Diamond and cBN are two of the most promising ultra-wide-band-gap (UWBG)
semiconductors for applications in high-power high-frequency electronic
devices. Yet despite extensive studies on carrier transport in these materials,
there are large discrepancies in their reported carrier mobilities. In this
work, we investigate the phonon- and dopant-limited electron and hole mobility
of cBN and diamond with atomistic first-principles calculations in order to
understand their fundamental upper bounds to carrier transport. Our results
show that although the phonon-limited electron mobilities are comparable
between cBN and diamond, the hole mobility is significantly lower in cBN due to
its heavier hole effective mass. Moreover, although lattice scattering
dominates the mobility at low doping, neutral impurity scattering becomes the
dominant scattering mechanism at higher dopant concentrations due to the high
dopant ionization energies. Our analysis provides critical insights and reveals
the intrinsic upper limits to the carrier mobilities of diamond and cBN as a
function of doping and temperature for applications in high-power electronic
devices.
