Multi-junction solar cells, which consist of several cells with different energy bandgaps, enable conversion efficiency to be increased. By using more than one type of III-V material within a solar cell, different bandgaps can be obtained and each bandgap can convert a specific region of a solar spectrum with low energy loss between incident photon-energy and bandgap energy. The principles of multi-junction solar cells were suggested in 1955 [I] and multi-junction solar cells have been investigated since 1960 [2]. However, there was no remarkable progress in conversion efficiency for the multi-junction cells until 1970’s. Based on progress in technologies for vapor phase epitaxy of semiconductor thin layer, AlGaAs/GaAs tandem cells, including tunnel junctions [3] and metal inter connectors [4], [5], [6 ], were developed in 1980’s. At that time, the predicted conversion efficiency close to 30% was not obtained because of difficulties in making high performance and stable tunnel junctions, and the defects related to the oxygen in the AlGaAs materials [7]. The high performance and stable tunnel junction was proposed with double-hetero (DH) structure by NTT [8 ]. An InGaP material replacing an AlGaAs material for the top cell was proposed by NREL [9], and a GalnP/GaAs tandem cell with an efficiency of 29.5% but small area of 0.25 cm2 was reported [10], More recently, a monolithic InGaP/GaAs tandem solar cell was reached a highest efficiency of 30.3% with a size of 4 cm2 under 1-sun AM 1.5 global conditions by Japan Energy Corporation [II]. Also a 33.3% efficiency was obtained for a triple-junction cell consisted of a monolithic InGaP/GaAs tandem cell and an InGaAs cell by Japan Energy Corporation and Sumitomo Electric Industrial Corporation [12], In addition, a GaAs/GaSb mechanical stacked cell with a 32.6% efficiency was reported for x 100 concentrator system by Boeing [13].