Here we investigate the topological properties of moiré bands in twisted bilayer ${\mathrm{WSe}}_2$ $\left(\mathrm{t}{\mathrm{WSe}}_2\right)$ by means of quantum oscillation experiments. Magnetoresistance measurements manifest clear Shubnikov–de Haas (SdH) oscillations in $\mathrm{t}{\mathrm{WSe}}_2$ under moderate displacement fields. Landau-level fan diagram unveils a Berry phase of $\pi$ at half filling. Temperature-dependent measurements indicate a hole effective mass $m^{*}\sim 0.17m_e$, which is much smaller than that of monolayer and bilayer ${\mathrm{WSe}}_2$. Increasing the hole density away from half-filling state leads to an increased $m^{*}$ accompanied with a trivial Berry phase. These observations can be well understood by considering a Dirac-like band crossing at low-energy scales induced by a nonzero displacement field in $\mathrm{t}{\mathrm{WSe}}_2$. Our work reveals the modulation of band topology by a perpendicular displacement field, offering avenues to engineer moiré bands for gate-tunable twist transistors.

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• Quantum transport
• Shubnikov-de Haas effect
• Topological phases of matter
• Twistronics
• Two-dimensional electron system
• Quantum oscillation techniques