Use of WNX as the diffusion barrier for interconnect copper metallization of InGaP-GaAs heterojunction bipolar transistors (HBTs) was studied. The WNX (40 nm) and Cu (400 nm) films were deposited sequentially on the InGaP-GaAs HBT wafers as the diffusion barrier and interconnect metallization layer, respectively, using the sputtering method. As judged from the data of scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy, and sheet resistance, the Cu-WNX-SiN and Cu-WNX-Au structures were very stable up to 550 °C and 400 °C annealing, respectively. Current accelerated stress test was conducted on the Cu-WNX metallized HBTs with VCE = 2 V, JC = 140 kA/cm2 and stressed for 55 h, the current gain (β) of these HBTs showed no degradation and was still higher than 100 after the stress test. The Cu-WNX metallized HBTs were also thermally annealed at 250 °C for 25 h and showed no degradation in the device characteristics after the annealing. For comparison, HBTs with Au interconnect metallization were also processed, and these two kinds of devices showed similar characteristics after the stress tests. From these results, it is demonstrated that WNX is a good diffusion barrier for the interconnection copper metallization of GaAs HBTs.