<p>A compound semiconductor epitaxial substrate which is used for a pseudomorphic high-electron-mobility field-effect transistor and comprises an InGaAs layer as a strained channel layer (6) and AlGaAs layers containing n-type impurities as backside and frontside electron supply layers (3, 9) is disclosed. The strained channel layer (6) is formed to have an emission peak wavelength not less than 1030 nm at 77 K through optimization of the In proportion in the strained channel layer (6) and film thickness.</p>
