  Bulk and surface state contributions to the electrical resistance of
single-crystal samples of the topological Kondo insulator compound SmB6 are
investigated as a function of crystal thickness and surface charge density, the
latter tuned by ionic liquid gating with electrodes patterned in a Corbino disk
geometry on a single surface. By separately tuning bulk and surface conduction
channels, we show conclusive evidence for a model with an insulating bulk and
metallic surface states, with a crossover temperature that depends solely on
the relative contributions of each conduction channel. The surface conductance,
on the order of 100 e^2/h and electron-like, exhibits a field-effect mobility
of 133 cm^2/V/s and a large carrier density of ~2x10^{14}/cm^2, in good
agreement with recent photoemission results. With the ability to gate-modulate
surface conduction by more than 25%, this approach provides promise for both
fundamental and applied studies of gate-tuned devices structured on bulk
crystal samples.
