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This paper describes the design of millimeter-wave wide-band monolithic GaAs passive high electron-mobility transistor (HEMT) switches using the traveling-wave concept. This type of switch combined the off-state shunt transistors and series microstrip lines to form an artificial transmission line with 50-Ω characteristic impedance. A 15-80-GHz single-pole double-throw (SPDT) switch in conjunction with quarter-wavelength impedance transformers demonstrates an insertion loss of less than 3.6 dB and an isolation of better than 25 dB. Another type of wide-band switch was designed by using a series HEMT switch to replace the quarter-wavelength transformer, and the operating band can be extended to dc. With this scheme, dc-80-GHz single-pole single-throw (SPST) and dc-60-GHz SPDT switches are also developed with compact chip size. From dc to 80 GHz, the insertion loss and isolation of the SPST switch are better than 3 and 24 dB, respectively. The SPDT switch has an insertion loss of better than 3 dB and an isolation of better than 25 dB from dc to 60 GHz. The analysis of circuit characteristics and design procedures are also included. It is concluded that the device periphery can be selected for the desired bandwidth, while the number of transistors is decided to achieve the isolation.