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-µm gate length and 0.75-mm gate periphery, a small-signal 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 Vds= 18 V and 4.5 W when biased at Vds= 22 V.