  The topological state that emerges at the surface of a topological insulator
(TI) and at the TI-substrate interface are studied in metal-hBN-Bi2Se3
capacitors. By measuring the RF admittance of the capacitors versus gate
voltage, we extract the compressibility of the Dirac state located at a gated
TI surface. We show that even in the presence of an ungated surface that hosts
a trivial electron accumulation layer, the other gated surface always exhibits
an ambipolar effect in the quantum capacitance. We succeed in determining the
velocity of surface Dirac fermions in two devices, one with a passivated
surface and the other with a free surface that hosts trivial states. Our
results demonstrate the potential of RF quantum capacitance techniques to probe
surface states of systems in the presence of a parasitic density-of-states.
