Copper-metallized AlGaAs/InGaAs pseudomophic high-electron mobility transistor single-pole-double-throw (SPDT) switches utilizing platinum (Pt, 70 nm) as the diffusion barrier have been studied and demonstrated. As compared with the Au-metallized switches, the Cu-metallized SPDT switches exhibited comparable performance with insertion loss less than 0.5 dB, return loss larger than 20 dB, isolation larger than 35 dB, and the input power for 1-dB compression (input P 1 dB ) of 28.3 dBm at 2.5 GHz. In order to evaluate the temperaturedependent impact on dc and RF characteristics of the coppermetallized switches for high-temperature applications, the switches have been tested at different temperatures. The device exhibits low thermal threshold coefficients (δV th /δT ) of −0.25 mV/K from 300 K to 500 K, good microwave performance at 380 K with insertion loss less than 0.5 dB, isolation higher than 40 dB, and the input power for 1-dB compression (input P 1 dB ) of 28.45 dBm at 2.5 GHz. To test the thermal stability of the Pt diffusion barrier, these switches were annealed at 250 • C for 20 h. After the annealing, the switches showed no degradation of the dc characteristics. In addition, after a high temperature storage life environment test at 150 • C, these copper-metallized switches remained capable of excellent power handling. To test the operation reliability of the copper-metallized switches, the copper-metallized switches were subjected to ON/OFF (control voltage = +3/0 V exchange) stress test for 24 h at room temperature. The devices maintained excellent RF characteristics after the stress test. Consequently, it is successfully demonstrated through these tests that copper metallization using Pt as the diffusion barrier could be applied to the GaAs monolithic microwave integrated circuit switch fabrication with good RF performance, high-temperature characteristics, and reliability.Index Terms-Copper metallization, platinum, pseudomophic high-electron mobility transistor (PHEMT), single-pole-doublethrow (SPDT), switch, temperature.