This study investigates the electrical control of neutral and charged excitons in monolayer molybdenum diselenide (MoSe₂), a group-VI transition metal dichalcogenide. The researchers used high-quality monolayer MoSe₂ in field-effect transistors (FETs) to observe and control excitons (X⁰, X⁺, X⁻) via photoluminescence. They found that the trion binding energy is large (30 meV), with a narrow emission linewidth (5 meV) at temperatures below 55 K, indicating strong confinement and heavy effective masses. The charging energies for X⁺ and X⁻ are nearly identical, suggesting the same effective mass for electrons and holes. The study demonstrates that monolayer MoSe₂ is a true 2D semiconductor, enabling the investigation of excitonic physics and optoelectronic applications. The results show that exciton charging can be reversibly controlled by electrostatic means, with the ability to switch between neutral and charged excitons. The observed spectral features are narrow and well-separated, providing high spectral contrast compared to other quasi-2D systems. The temperature dependence of the photoluminescence spectra confirms the 2D nature of the excitons, with the trion binding energy remaining stable at high temperatures. The study also shows that the exciton and trion peak positions follow a standard semiconductor bandgap dependence, and the intensity ratio of trion to exciton is well explained by a mass action model. The findings highlight the potential of monolayer dichalcogenides as a platform for exploring excitonic physics and photonic applications in the true 2D limit. The results open new avenues for the development of optoelectronic devices and the study of phenomena such as exciton condensation and the Fermi-edge singularity.This study investigates the electrical control of neutral and charged excitons in monolayer molybdenum diselenide (MoSe₂), a group-VI transition metal dichalcogenide. The researchers used high-quality monolayer MoSe₂ in field-effect transistors (FETs) to observe and control excitons (X⁰, X⁺, X⁻) via photoluminescence. They found that the trion binding energy is large (30 meV), with a narrow emission linewidth (5 meV) at temperatures below 55 K, indicating strong confinement and heavy effective masses. The charging energies for X⁺ and X⁻ are nearly identical, suggesting the same effective mass for electrons and holes. The study demonstrates that monolayer MoSe₂ is a true 2D semiconductor, enabling the investigation of excitonic physics and optoelectronic applications. The results show that exciton charging can be reversibly controlled by electrostatic means, with the ability to switch between neutral and charged excitons. The observed spectral features are narrow and well-separated, providing high spectral contrast compared to other quasi-2D systems. The temperature dependence of the photoluminescence spectra confirms the 2D nature of the excitons, with the trion binding energy remaining stable at high temperatures. The study also shows that the exciton and trion peak positions follow a standard semiconductor bandgap dependence, and the intensity ratio of trion to exciton is well explained by a mass action model. The findings highlight the potential of monolayer dichalcogenides as a platform for exploring excitonic physics and photonic applications in the true 2D limit. The results open new avenues for the development of optoelectronic devices and the study of phenomena such as exciton condensation and the Fermi-edge singularity.