Electrical and optical properties of gallium arsenide diodes with a P-P0-N structure were studied between 4.2°and 90°K. The diodes were prepared by first diffusing manganese, and then zinc, into n-type gallium arsenide, keeping the zinc junction depth much shallower than that of the manganese junction. The P0 region, dominated by the manganese, becomes one of high resistivity at these temperatures because of freezeout of holes on the deep-lying manganese centers. The low-resistivity N and P regions on either side serve as electron and hole injecting contacts, respectively. The diodes are electroluminescent and exhibit a negative resistance over a portion of their current-voltage characteristics when a forward bias is applied. At 77°K, a typical voltage for the onset of the negative resistance is 5 V at a current of 5 mA; at about 2.2 V the dynamic resistance becomes positive again. The spectrum of the emitted light indicates recombination through both zinc and manganese centers. It appears that the mechanism responsible for the negative resistance is not the one suggested by Lampert. Possibly, the absorption by the P0 region of light emitted at the P-P0 boundary is responsible for the phenomena observed, as suggested by Dumke. At voltages below the onset of the negative resistance, the current-voltage relation in the P0 region obeys Ohm's law at low injection levels but eventually the current depends on the square of the voltage. This behavior, as well as the spatial origin of the light, is in good agreement with the theory of Ashley and Milnes. © 1964 The American Institute of Physics.