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Summary form only given. The authors report a study of the impact of buffer layer design on the characteristics of Al0.48In0.52As-Ga0.47In0.53As HEMTs (high-electron-mobility transistors). The aim of the study is to understand and correct the problem of high output conductance observed in devices with a high transconductance. Devices with 1.0- mu m gate lengths were fabricated using modulation-doped Al0.48In0.52As-Ga0.47In0.53As epitaxial layers which had sheet charge densities between 3*1012 and 3.5*1012 cm-2 and mobilities at 300 K between 9000 and 10000 cm V-1 S-1. The different buffer layer designs used were: (1) a standard undoped Al0.48In0.52As buffer 250-nm-thick; (2) an Al0.48In0.52As buffer with a 20-AA thick highly doped p-type region 50 AA below the channel; (3) a Ga0.47In0.53As buffer with a 20-AA-thick highly doped p-type region below the channel; and (4) a low-temperature AlInAs buffer layer. The device with the low-temperature AlInAs had the best output characteristics, signifying that it was the best mode of confining electrons in the channel.