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Broadband Darlington amplifiers using a GaAs heterojunction bipolar transistor (HBT) high electron-mobility transistor (HEMT) process are reported in this paper. The gain-bandwidth analysis of the Darlington amplifiers using HEMT-HBT, HBT-HEMT, HEMT-HEMT, and HBT-HBT configurations are presented. The bandwidth, gain, input, and output impedances are investigated with transistor size, feedback resistances, and series peaking inductance. The design methodology of the broadband Darlington amplifier in the HBT-HEMT process is successfully developed, and the direct-coupled technique is also addressed for high-speed data communications. Furthermore, two monolithic HEMT-HBT and HEMT-HEMT Darlington amplifiers are achieved from dc to millimeter wave, and successfully evaluated with a 25-Gb/s eye diagram. The HEMT-HBT Darlington amplifier demonstrates the best gain-bandwidth product with good input/output return losses among the four configurations.