Herein, a device study using technology computer‐aided design simulation to theoretically analyze the electrical performance of ultrawide‐bandgap boron nitride (BN)‐based vertical junction devices is performed, including h‐BN Schottky diode, h‐BN pn diode, and h‐BN/AlN pn diode; this is also the first demonstration of the BN power devices in simulation. The material properties of BN are defined with recently reported data, and the physical mechanisms of the device performance are systematically investigated. The h‐BN junctions in this simulation shows excellent performance, especially for breakdown behaviors. Schottky diode shows a turn‐on voltage of 0.6 V for Pt Schottky contact and breakdown voltages over 450 V for 5 μm, 6 × 10¹⁵cm⁻³ p‐type‐doped drift layer; The h‐BN pn diode shows a turn‐on voltage of 6 V and breakdown voltages over 3 kV with a critical electric field of 13.6 MV cm⁻¹ for 2.5 μm, 2 × 10¹⁶cm⁻³ p‐type‐doped drift layer. The h‐BN/AlN heterojunction pn diode shows a turn‐on voltage of 5.8 V and breakdown voltage over 2 kV for 2.5 μm, 2 × 10¹⁶cm⁻³ n‐type‐doped AlN drift layer. Herein, an understanding of the device principles of vertical BN junctions is provided, which can serve as a reference for the future development of robust BN power electronics.