This paper presents the comprehensive considerations of CMOS transmit/receive (T/R) switch design towards ultra-wideband and over 15-GHz frequencies. Techniques for minimizing parasitics and increasing linearity are discussed. A customized transistor layout is proposed for T/R switch design and its effects on insertion loss and isolation are studied. The analysis shows that a series-only architecture using the customized transistor layout achieves better insertion loss and reasonable isolation. A double-well body-floating technique is proposed and its effects are discussed. A differential switch architecture without shunt arms is designed and verified by experimental results. Fabricated in 0.13- m triple-well CMOS, the T/R switch exhibits less than 2 dB insertion loss and higher than 21 dB isolation up to 20 GHz. With resistive body floating and differential architecture, the high linearity is of ultra-wideband characteristic, more than 30-dBm power 1-dB compression point (P1dB) is obtained up to 20 GHz in only 0.03 mm2 active die area.