A unique design of a hexagonal boron nitride (h‐BN)/HfO2 dielectric heterostructure stack is demonstrated, with few‐layer h‐BN to alleviate the surface optical phonon scattering, followed by high‐κ HfO2 deposition to suppress Coulombic impurity scattering so that high‐performance top‐gated two‐dimensional semiconductor transistors are achieved. Furthermore, this dielectric stack can also be extended to GaN‐based transistors to enhance their performance.