Field-effect transistors (FETs) based on 2-D materials have great potential for future ultimate-scaled electronics. However, nonideal semiconductor–dielectric interfaces due to interfacial traps and oxide traps have constrained the potential of 2-D semiconductors. Here, we report a new dielectric interface engineering approach for monolayer (1L) MoS2 transistors employing a relatively high- $\kappa $ TaOx interfacial layer ( $\kappa \sim $ 7) whose defect bands are located outside of the operation window of the MoS2 Fermi level. Such band alignment can minimize active interface trap states in top-gate (TG) dielectric stacks. The TaOx interfacial layer can also act as an efficient doping layer, with the highest ON-current ${I}_{\text {on}}$ reaching 861 $\mu \text{A}/\mu \text{m}$ at ${V}_{\text {DS}} $ = 1.5 V and overdrive voltage ${V}_{\text {OV}} $ = 3 V. The lowest contact resistance is down to $230 \Omega \cdot \mu \text{m}$ . Dual-gate (DG) FETs can achieve subthreshold slope (SS) values down to $\sim $ 70 mV/dec in short-channel devices ( ${L}_{\text {CH}} =55$ –75 nm). Our reported SS, ${I}_{\text {on}}$ , and ${R}_{C}$ are among the best-reported values for MoS2 devices. For low-power applications, our devices exhibit a record-high ${I}_{\text {on}}$ of 598 $\mu \text{A}/\mu \text{m}$ at ${V}_{\text {DS}} =0.65$ V. The new dielectric engineering approach proposed in this study can pave the way for realizing high-performance logic devices based on 2-D materials.