Some fundamental properties of beryllium in (GaAl)As/GaAs solar cells were investigated. The diffusion of Be into GaAs during LPE growth of Be-doped p-Ga 0.2 Al 0.8. As was first investigated in order to control the junction depth. The temperature dependence of the diffusion coefficient was found to take the form D = D /SUB o/ exp(-E /SUB o/ /kT), where D /SUB o/ = 11.2 cm/sup 2/sec/sup -1/ and E /SUB o/ = 2.43 eV. The diffusion coefficient was also found to be approximately proportional to the beryllium concentration in the growth melt. The acceptor energy level for Be in Ga /SUB o/ Al 0.8 As found from the temperature dependence of free carrier concentration was 47 meV, being smaller than that for Zn, 65-80 meV. (GaAl)As/GaAs p-p-n solar cells using Be as p-type dopant have high conversion efficiency in excess of 20% (AMI), and the highest output power 5 Wcm/sup 2/ was obtained at 400 suns.