  The origin of the low current on/off ratio at room temperature in dual-gated
bilayer graphene field-effect transistors is considered to be the variable
range hopping in gap states. However, the quantitative estimation of gap states
has not been conducted. Here, we report the systematic estimation of the energy
gap by both quantum capacitance and transport measurements and the density of
states for gap states by the conductance method. An energy gap of ~250 meV is
obtained at the maximum displacement field of ~3.1 V/nm, where the current
on/off ratio of ~3*10^3 is demonstrated at 20 K. The density of states for the
gap states are in the range from the latter half of 10^12 to 10^13 eV^-1cm^-2.
Although the large amount of gap states at the interface of high-k
oxide/bilayer graphene limits the current on/off ratio at present, our results
suggest that the reduction of gap states below ~10^11 eV^-1cm^-2 by continual
improvement of the gate stack makes bilayer graphene a promising candidate for
future nanoelectronic device applications.
