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This paper describes an ultra high speed 1:2 demultiplexer in optical fiber communication system utilizing SiGe BiCMOS technology. The transient frequency fT of the 0.25 μm SiGe:C npn-HBT's devices is up to 190 GHz and the fmax is 220 GHz. The Latch of the demultiplexer core circuit has been researched. Limited by existing measure condition, a high gain and wide bandwidth clock buffer is designed to drive large load. Transmission line theory of ultra high speed circuits is utilized to design match network and to solve the match problem of input signal, output signal and internal signal. The transient analysis shows this demultiplexer can demultiplex one 100-Gb/s input data into two 50-Gb/s output data. The chip area of this 1:2 demultiplexer is 0.7 mm × 0.47 mm, the input and output data are all standard 400 mVP-P PCML voltage level, and power consumption of the IC is 900 mW at a power supply of -4 V.