We report the first gallium nitride (GaN)-based broad-band power amplifier. The circuit was fabricated on an AlN substrate using AlGaN/GaN power high-electron mobility transistors (HEMT's), grown on sapphire substrates, which were flip-chip bonded for thermal management. The amplifier employed a modified traveling-wave power amplifier (TWPA) topology that eliminated the backward wave of conventional TWPA's. Using four HEMT's each with 0.75-m gate length and 0.75-mm gate periphery, a

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... gain of 7 dB was obtained with a bandwidth of 1-8 GHz. At mid-band, an output power of 3.6 W was obtained when biased at V ds = 18 V and 4:5 W when biased at V ds = 22 V. Index Terms-Flip-chip, GaN, power amplifier.