  Planar tunneling spectroscopy of the Kondo insulator SmB$_6$ suggests that an
interaction between the surface Dirac fermions and the bulk spin excitons
results in incompletely protected topological surface states. To gain further
insight into their true nature, it is necessary to study other topological
Kondo insulator candidates. Calculations of electronic energy bands predict
that the Kondo insulator YbB$_{12}$ hosts topological surface states protected
by crystalline mirror symmetry. In this study, we present tunneling conductance
spectra obtained from the (001) surface of YbB$_{12}$ single crystals and
discuss them in comparison to SmB$_6$. The linear conductance at low bias
provides strong evidence for the existence of surface Dirac fermions. The
double-hump structure in the negative bias region is associated with hybridized
band edges, in agreement with a calculated band structure. While these
similarities with SmB6 are suggestive of the existence of topological surface
states in YbB$_{12}$, in agreement with other experiments, some discrepancies
are also observed, which we attribute to a difference in their exact nature
from those in SmB$_6$.
