We report the development of an ultrahigh-bandwidth vector network analyzer useful for small-signal characterization of high-speed semiconductor devices. It employs 100-fs optical pulses for making terahertz-bandwidth electro-optic measurements of electrical signals, as well as for sub-picosecond, photoconductive, electrical-stimulus-signal generation. High-bandwidth coplanar strip transmission lines are used for signal transmission. A 0.15 x 50gm gate A1GaAs/InGaAs/GaAs HFET has been characterized over a bandwidth of 100 GHz using this network analyzer. A comparison with conventional RF network analyzer measurements performed to 40 GHz demonstrated good agreement throughout this bandwidth. Such measurements of the actual device characteristics across their entire operating frequency range should improve device development and incorporation into active circuits. ss~ 35/~-~