  We investigate band-gap tuning of bilayer graphene between hexagonal boron
nitride sheets, by external electric fields. Using density functional theory,
we show that the gap is continuously tunable from 0 to 0.2 eV, and is robust to
stacking disorder. Moreover, boron nitride sheets do not alter the fundamental
response from that of free-standing bilayer graphene, apart from additional
screening. The calculations suggest that the graphene-boron nitride
heterostructures could provide a viable route to graphene-based electronic
devices.
