The performances of WR-3.4 monolithic amplifiers fabricated using dual-finger 6 µm InP HBT devices are investigated. While one amplifier uses the dual-finger devices formed by simply connecting two existing standard single-finger HBTs, the second amplifier uses newly formed devices that share a common collector metal on a single merged device isolation area. The amplifiers using two types of devices based on the identical matching networks are fabricated for on-wafer probing tests. The custom merged-device amplifier shows clear performance advantages over the separate-device amplifier, showing a peak gain of 10.5 dB and the maximum output power of 5.2 dBm at 255 GHz.