When electron-hole pairs are excited in a semiconductor, it is a priori notclear if they form a fermionic plasma of unbound particles or a bosonic excitongas. Usually, the exciton phase is associated with low temperatures. Inatomically thin transition metal dichalcogenide semiconductors, excitons areparticularly important even at room temperature due to strong Coulombinteraction and a large exciton density of states. Using state-of-the-artmany-body theory including dynamical screening, we show that theexciton-to-plasma ratio can be efficiently tuned by dielectric substratescreening as well as charge carrier doping. Moreover, we predict a Motttransition from the exciton-dominated regime to a fully ionized electron-holeplasma at excitation densities between $3\times10^{12}$ cm$^{-2}$ and$1\times10^{13}$ cm$^{-2}$ depending on temperature, carrier doping anddielectric environment. We propose the observation of these effects by studyingexcitonic satellites in photoemission spectroscopy and scanning tunnelingmicroscopy.
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