Three types of GaAs p-n junctions containing diffused copper impurities were fabricated and studied. Careful investigation of their electrical characteristics and optical emission spectra were carried out over a wide range of current between 4 and 300°K. Broad emission peaks near 1.34, 1.29, and 1.06 eV (at low temperatures) are identified with transitions involving the interstitial copper donor and the first and second substitutional copper acceptor levels having binding energies of Ec-Edonor=0.07 eV and Eacceptor-Ev=0.18 and 0.41 eV, respectively. A similar peak near 1.00 eV is attributed to an unidentified donor level lying 0.5 eV below the conduction band. External quantum efficiencies were generally low, indicating the presence of an appreciable amount of nonradiative recombination. The existence of impurity conduction in the copper donor band of graded diodes containing large concentrations of copper (>6×1018 cm-3) is deduced from the disappearance at 77°K of all emission peaks except that at 1.34 eV. These diodes exhibited negative resistance (dVdI0) above a breakdown voltage of 10 to 20 V. Alloy diodes in similar material showed negative resistance of the tunneling type (dIdV0) above 0.3 V at room temperature. Some of the centers frequently seen in laser-type diodes by their emission near 1.0 eV and by their effects on capacitance are shown not to be due to copper. © 1965 The American Physical Society.