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Electrical and light emission characteristics of pulsed InAs p-n junction lasers, prepared by diffusing Zn into Sn-doped n-type crystals with a substrate concentration of 1018cm-3, were studied at 4.2, 20.4, and 77°K. Data of variation of capacitance with bias showed these to be abrupt junctions. At injection currents below threshold, the temperature independence of the slopes of the I-V characteristics and the observed variation of the integrated light intensity with current suggest the injection mechanism be photon-assisted tunneling. The peak of the spontaneous radiation shifts with the applied voltage in the expected manner. The time-resolved laser spectra are seen to shift to longer wavelengths during pulsed operation of the laser. The line shift results from joule heating of the junction, and follows the variation of bandgap with temperature. From a plot of threshold current density as a function of reciprocal length gain and loss factors have been calculated. An independent value of the gain factor was obtained from the observed variation with current of the superradiantly narrowed linewidth from a nonlasing structure. Mode-confinement studies indicate that the mode confinement is due to the optical gain in the junction region.