Affords AlxGa(1-x)As (0≦x≦1) substrates epitaxial wafers for infrared LEDs, infrared LEDs, methods of manufacturing AlxGa(1-x)As substrates, methods of manufacturing epitaxial wafers for infrared LEDs, and methods of manufacturing infrared LEDs, whereby a high level of transmissivity is maintained, and through which, in the fabrication of semiconductor devices, the devices prove to have superior light output characteristics. An AlxGa(1-x)As substrate (10a) as disclosed is an AlxGa(1-x)As substrate (10a) furnished with an AlxGa(1-x)As layer (11) having a major surface (11a) and, on the reverse side from the major surface (11a), a rear face (11b), and is characterized in that in the AlxGa(1-x)As layer (11), the amount fraction x of Al in the rear face (11b) is greater the amount fraction x of Al in the major surface (11a). The AlxGa(1-x)As substrate (10a) may additionally be provided with a GaAs substrate (13), contacting the rear face (11b) of the AlxGa(1-x)As layer (11).