Single-layer molybdenum disulfide (MoS₂) field-effect transistors (FETs) exhibit electroluminescence (EL) when operated under electrical bias. The study confirms that EL, photoluminescence (PL), and absorption all involve the same excited state at 1.8 eV. EL is localized at the contacts and shows a threshold behavior. Single-layer MoS₂, a direct band gap semiconductor, is promising for optoelectronic applications such as 2D light detectors and emitters. MoS₂ is a layered chalcogenide material with strong PL, controllable valley and spin polarization, and a large on-off ratio in FETs. Single-layer MoS₂ has a direct band gap of 1.8 eV, while bulk MoS₂ has an indirect band gap of 1.3 eV. The absence of interlayer coupling in single-layer MoS₂ leads to strong absorption and PL bands at ~1.8 eV. Single-layer MoS₂ FETs have unipolar and ambipolar transport characteristics with high mobility and on-off current ratios. The study reports EL in single-layer MoS₂ FETs, showing that it occurs via hot carriers and is localized at the contacts. The EL and PL arise from the same excited state at 1.8 eV. Single-layer MoS₂ is produced by micromechanical cleavage and transferred onto glass substrates. The PL spectrum of single-layer MoS₂ shows two bands at 2 eV and 1.8 eV, associated with excitonic transitions. The Raman spectrum confirms the monolayer structure. The study also investigates the gate dependence of the electrostatic potential in the device. Single-layer MoS₂ behaves as an n-doped semiconductor with a Fermi level at 4.7 eV. A Schottky barrier is formed at the MoS₂-Cr/Au interface, leading to a strong photocurrent response at the contacts. The EL spectrum of single-layer MoS₂ shows a peak at ~685 nm, matching the PL peak at ~680 nm, indicating that EL and PL involve the same excited state. The study finds that the EL threshold depends on the exciton binding energy and thermal properties of the channel material. The conversion efficiency of photons to carriers is estimated to be ~10⁻⁵, which is lower than that of semiconducting nanotubes. The EL is spatially localized near the contacts, and the study maps the EL spatial distribution. The efficiency of light detection and emission needs to be improved for practical optoelectronic devices. Novel device designs are needed to enhance the efficiency and control charge carrier injection and extraction. The study concludes that single-layer MoS₂ transistors can detect and emit visible light, with both PL and EL arising from the same excited state atSingle-layer molybdenum disulfide (MoS₂) field-effect transistors (FETs) exhibit electroluminescence (EL) when operated under electrical bias. The study confirms that EL, photoluminescence (PL), and absorption all involve the same excited state at 1.8 eV. EL is localized at the contacts and shows a threshold behavior. Single-layer MoS₂, a direct band gap semiconductor, is promising for optoelectronic applications such as 2D light detectors and emitters. MoS₂ is a layered chalcogenide material with strong PL, controllable valley and spin polarization, and a large on-off ratio in FETs. Single-layer MoS₂ has a direct band gap of 1.8 eV, while bulk MoS₂ has an indirect band gap of 1.3 eV. The absence of interlayer coupling in single-layer MoS₂ leads to strong absorption and PL bands at ~1.8 eV. Single-layer MoS₂ FETs have unipolar and ambipolar transport characteristics with high mobility and on-off current ratios. The study reports EL in single-layer MoS₂ FETs, showing that it occurs via hot carriers and is localized at the contacts. The EL and PL arise from the same excited state at 1.8 eV. Single-layer MoS₂ is produced by micromechanical cleavage and transferred onto glass substrates. The PL spectrum of single-layer MoS₂ shows two bands at 2 eV and 1.8 eV, associated with excitonic transitions. The Raman spectrum confirms the monolayer structure. The study also investigates the gate dependence of the electrostatic potential in the device. Single-layer MoS₂ behaves as an n-doped semiconductor with a Fermi level at 4.7 eV. A Schottky barrier is formed at the MoS₂-Cr/Au interface, leading to a strong photocurrent response at the contacts. The EL spectrum of single-layer MoS₂ shows a peak at ~685 nm, matching the PL peak at ~680 nm, indicating that EL and PL involve the same excited state. The study finds that the EL threshold depends on the exciton binding energy and thermal properties of the channel material. The conversion efficiency of photons to carriers is estimated to be ~10⁻⁵, which is lower than that of semiconducting nanotubes. The EL is spatially localized near the contacts, and the study maps the EL spatial distribution. The efficiency of light detection and emission needs to be improved for practical optoelectronic devices. Novel device designs are needed to enhance the efficiency and control charge carrier injection and extraction. The study concludes that single-layer MoS₂ transistors can detect and emit visible light, with both PL and EL arising from the same excited state at