Borophene has attracted increasing interest for nanoscale electronics because its polymorphic bonding network gives rise to unusual structural and electronic properties. In this work, we investigate the structural stability, electronic properties, and electric field-dependent transport response of bilayer \(\alpha _{12}\) -borophene and its zigzag nanoribbon derivatives using first-principles calculations combined with a Wannier-based quantum transport framework. Starting from the optimized monolayer \(\alpha _{12}\) -borophene structure, AA- and AB-stacked bilayer configurations are first examined to identify the stable parent phase for nanoribbon construction. The AB-stacked bilayer is found to be dynamically stable and semiconducting, whereas the AA-stacked configuration is excluded from further electronic and transport analysis due to its dynamical instability. Maximally localized Wannier functions are then constructed from the AB-stacked bilayer electronic structure, and the resulting Wannier Hamiltonian is validated against the direct DFT band structure before being used to generate finite-width zigzag nanoribbons. The width-dependent nanoribbon calculations show that quantum confinement modifies the electronic gap, with the bandgap increasing rapidly for narrow ribbons and approaching a nearly saturated value for wider nanoribbons. For a representative zigzag nanoribbon with a width of 5.27 nm, the perpendicular electrostatic potential drop progressively reduces the bandgap from 0.587 eV at zero field to 0.110 eV at \(E_z=4.14\) V/nm, followed by complete gap closure at \(E_z=4.83\) V/nm. The density of states confirms finite spectral weight near the Fermi level at the gap-closed field, while the transmission and conductance spectra show strong transport gap narrowing under high-field modulation. These results show that bilayer \(\alpha _{12}\) -borophene zigzag nanoribbons possess strong width and field-dependent electronic tunability, suggesting their relevance for high-field dual-gate and reconfigurable nanoscale switching concepts.
