GaAs-based cells, including GaAs single-junction cells, AlGaAs/GaAs two-junction cells, and InGaP/GaAs two-junction cells grown on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) are examined in various levels of concentration and backside cooling temperature. All types of cells have shown boost of efficiency in low and medium ranges of concentration. The cell efficiencies obtained are 31.5 % at 20-suns of AM 1.5 for InGaP/GaAs tandem cell, and 29.2% at 7-suns of AM1,5 for AlGaAs/GaAs tandem cell, respectively. The GaAs single-junction cell is also examined as the reference. A new equivalent circuit model reveals that increase of apparent leakage current is responsible for a rapid efficiency drop in the high-concentration region, It is possible to improve it by reducing contact resistance and using uniform concentrated illumination. (C) 2001 Elsevier Science B.V. All rights reserved.