Abstract: X-ray lithography and its application in GaAs device fabricating have been studied in this paper. The GaAs PHEMT device was fabricated with mix-and-match lithography process, in which the gate lift-off structure was patterned by X-ray lithography and the others by optical lithography. An Au/Cr/SiNX mask system was used in synchrotron radiation lithography. A novel ohmic contact matellization system, In/AuGeNi/Ag/Au, was used to get good topography on GaAs substrate to satisfy the requirement of X-ray lithography on the aligning mark. A self-aligning exposure process was carried out to get T-shaped patterns which were widely used in GaAs PHEMT fabricating. The AlGaAs/InGaAs/GaAs PHEMT transistor with 0.15 μ m gate length was fabricated by X-ray lithography with mix-and-match process.