This paper presents the practical design methodology of an InP high electron-mobility transistor broadband low-noise amplifier (LNA) using multilayer transmission lines. The LNA consists of high-pass reactive matching circuits and resistive-feedback circuits in order to achieve both low-noise and broadband characteristics. The fabricated five-stage LNA successfully delivered a 43-dB gain with a noise figure of 1.9 dB at 23 GHz, and a gain of more than 40 dB from 18 to 43 GHz. The maximum gain was 49.5 dB at 32 GHz and the chip size was only 1.8 times 0.9 mm2, resulting in a gain density of 30.5 dB/mm 2. To the best of our knowledge, this gain density is the highest performance in any Ka-band LNA reported to date. In addition, a more compact LNA using spiral inductors was also demonstrated