The principle, equivalent circuit and performance parameters of GaAs based solar cells are introduced. Based on technology computer aided design（TCAD）tools, two different types of GaAs based solar cells with back surface fields of InAlGaP and InAlP are presented, and their structures and performances are simulated. Meanwhile, the two solar cells are manufactured by molecular beam extension （MBE） device and their IV characteristics are tested. After considering the practical effect of the shunt resistance and series resistance on the IV characteristics of solar cells, the simulation results and the experimental results are basically consistent. When InAlGaP with doping concentration of 2×1018 cm-3 is used as back surface field in GaAs solar cells, the IV curve is the typical IV characteristic of a solar cell. When InAlP with doping atomic concentration of 2×1018 cm-3 is used as back surface field, the IV curve turns into S-shaped. The analysis results show that the drift field, which is formed by back surface field and base layer, can accelerate the transport of photo generated minority carriers in the cell and increase the photo-generated current. Meanwhile, the minority carriers are reflected back to active region because of back surface field, thus reducing the composite of carriers. Due to the existence of hetero junction, the transport processes of the majority carriers with InAlGaP back surface field are affected. The majority carriers can be transported via tunnel with minor voltage. When forward voltage increases, carriers can mainly surmount the barrier through thermal excitation and the S-shaped IV curve appears.