This paper presents an improved small-signal model and a W-band monolithic low noise amplifier (LNA) using 100 nm InAlAs/InGaAs/InP-based high electron mobility transistors (HEMT) technology. For improving the fitting accuracy of S-parameters in low frequency, the small-signal model takes into account differential resistances of gate-to-source and gate-to-drain diodes, which modeled by resistances Rfs and Rfd. A W-band LNA monolithic millimeter-wave integrated circuit (MMIC) has been designed and fabricated based on this model to verify the feasibility of this model. The amplifier is measured on-wafer with a small-signal peak gain of 14.4 dB at 92.5 GHz and 3-dB bandwidth from 85 to 110 GHz. In addition, the MMIC also exhibits an excellent noise characteristic with the noise figure of 4.1 dB and the associate gain of 13.8 dB at 88 GHz. This MMIC amplifier shows wider 3-dB bandwidth and higher per-stage gain than others results at the similar band.