A systematic study of the photoluminescence (PL) of Se‐dopedn‐type GaAs grown by metalorganic chemical vapor deposition is reported. A new method is presented to determine the electron effective mass ofn+‐direct‐gap semiconductors from the PL spectrum. GaAs samples with electron densities from 1015to 8×1018cm−3were investigated over the temperature range of 13 to 353 K. The PL spectra ofn+‐GaAs are analyzed using a physical model which for the first time explains in a consistent manner both the energy of the peak and the full width at half‐maximum, and accounts for the electron density. An accurate fit of the PL spectra is obtained by invoking band‐to‐band transitions withoutkselection. The electron exchange and correlation interactions account for all the observed band shrinkage, which reaches 48 meV forn=8.0×1018cm−3. No significant density of band‐tail states is observed. The Fermi energy is obtained directly from the PL fitting and is used with the measured Hall electron densitynto determine the energy‐dependent effective massm*. An increase inm* beyond the value expected from the nonparabolicity of the conduction band in pure GaAs is observed. The mass at the minimum of the conduction bandm@B|0increases from 0.0636mefor pure GaAs to 0.073meforn=8.0×1018cm−3, wheremeis the free electron mass. The increase inm*0is given empirically asm@B|0/me=0.0635+2.06×10−22n+1.16×10−40n2. The change inm* is interpreted as a distortion of the conduction band produced by the impurities. The small dilation of the lattice, 0.0035% atn=8.0×1018cm−3, indicates that this distortion is not mechanical, but electrical in nature. Also, the PL linewidth is abnormally small for samples in which inhomogeneities are believed to exist.