NOVELTY - A hexagonal boron nitride sheet having two-dimensional planar structure with a sp2 boron-nitride (B-N) covalent bond, and showing Van der Waals bond between boron-nitrogen layers, root mean square surface roughness of less than or equal to 2 nm, and length of greater than or equal to 1 mm.    USE - As a hexagonal boron nitride sheet, used in a stacked structure, an electronic device and transistor (claimed). Also used as insulating layer in e.g. solar cell or display devices; and in a variety of electronic devices e.g. sensor, bipolar junction transistor (BJT), field effect transistor (FET), heterojunction bipolar transistor (HBT), a single-electron transistor, light-emitting diode (LED), or organic light-emitting diode (OLED).    ADVANTAGE - The hexagonal boron nitride sheet has increased area and improved quality; has high purity i.e. greater than or equal to 70%, and reduced amount of impurities e.g. allotrope having different crystalline structure, thus provides improved physical properties when used in device; and has smooth surface with fewer defects, thus used in various electronic devices. The desirable properties of the hexagonal boron nitride (h-BN) improve performance of the graphene-based electronic device without any significant performance deterioration in the graphene, due to the improved transparency and flexibility of the h-BN sheet.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) preparing a hexagonal boron nitride sheet involving: preparing a metal catalyst in sheet form; first thermally treating the metal catalyst to increase a grain size of the metal catalyst; and second thermally treating and contacting the first thermally treated metal catalyst with a nitrogen source and a boron source to form the hexagonal boron nitride sheet;    (2) a stacked structure comprising: the hexagonal boron nitride sheet; and graphene;    (3) an electronic device comprising the stacked structure; and    (4) a transistor comprising the stacked structure.