  Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and
energy-efficient power-electronics devices. Their wider band gaps result in
higher breakdown electric fields that enable high-power switching with a lower
energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic
materials limitations with regards to their doping asymmetry that impedes their
adoption in CMOS technology. Improvements in the ambipolar doping of UWBG
materials will enable a wider range of applications in power electronics as
well as deep- UV optoelectronics. These advances can be accomplished through
theoretical insights on the limitations of current UWBG materials coupled with
the computational prediction and experimental demonstration of alternative UWBG
semiconductor materials with improved doping and transport properties. As an
example, we discuss the case of rutile GeO$_2$ (r-GeO$_2$), a water-insoluble
GeO$_2$ polytype which is theoretically predicted to combine an ultra-wide gap
with ambipolar dopability, high carrier mobilities, and a higher thermal
conductivity than \b{eta}-Ga$_2$O$_3$. The subsequent realization of
single-crystalline r-GeO$_2$ thin films by molecular beam epitaxy provides the
opportunity to realize r-GeO$_2$ for electronic applications. Future efforts
towards the predictive discovery and design of new UWBG semiconductors include
advances in first-principles theory and high-performance computing software, as
well as the demonstration of controlled doping in high-quality thin films with
lower dislocation densities and optimized film properties.
