Abstract: In this report the diffusion of Zn into GaAs at low temperature has been investigated. The experiments are accomplished in an evacuated and sealed quartz ampoule using ZnAs2 as the source of Zn.The relation among the junction depth (Xj), the time (t) and the temperature (T) of diffusion has been investigated. It is found that the sheet resistance (Rs) of diffu-sion layer increases as the junetion depth (Xj) decreases, the surface concentration (C2) decreases as 1/T increases and the carrier mobility () decreases as Cs increases. The Cs vs. 1/XjRs is plotted, it indicates that the Cs increases as 1/(XjRs) inereases. This is a simple method for determining the Cs of the multiple Ga1-xAlxAs/GaAs epitaxial layer. The mechanism of Zn diffusion in GaAs and InP is discussed. This process has been applied to fabricate GaAlAs/GaAs double heterojunction light emitting idodes and an output power of 2-4 mW and a series resistance of 3-5 are obtained.