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The fabrication of high-purity layers of Al[sub x]Ga[sub 1-x]As solid solutions in the range 0 ≤x≤0.38 by molecular beam epitaxy is reported. The low-temperature photoluminescence spectra of these layers reveal predominantly the free exciton recombination line (X). The narrow width of the X line, the high intensity ratio of this line to that of the band-acceptor transition line, and the linear dependence of the X line intensity on the excitation power density in the range between 1 × 10[sup -4] and 100 V · cm[sup -2] indicate a low concentration of background impurities in these layers. Using this material in pseudomorphic AlGaAs/InGaAs/GaAs heterostructures for high-power microwave transistors produced devices with a specific saturated output power of 0.9 W/mm at 18 GHz. [ABSTRACT FROM AUTHOR]