The authors report ultra-high-speed digital IC modules that use 0.1-mu m InAlAs/InGaAs/InP HEMTs for broad-band optical fiber communication systems. The multiplexer IC module operated at up to 70 Gbit/s, and error-free operation of the decision IC module was confirmed at 50 Gbit/s. The speed of each module is the fastest yet reported for its kind.