Design principles of CMOS low-noise amplifiers (LNAs) for simultaneous input impedance and noise matching by tailoring device size for Ropt=50 Ω are introduced. It is found that Ropt close to 50 Ω can be obtained by using small devices (110 μm) and small currents (5 mA). Based on the proposed approach, CMOS LNAs with on-chip input and output matching networks on thin (∼20 μm) and normal (750 μm) substrates are implemented. It is found that the noise figure (NF) (3.0 dB) of the CMOS LNA at 5.2 GHz with 10-mW power consumption on the normal (750 μm) substrate can be reduced to 2.17 dB after the substrate is thinned down to ∼20 μm. The reduction of NF is attributed to the suppression of substrate loss of the on-chip inductors. The input return loss (S11) is smaller than -22 dB across the entire band of interest (5.15-5.35 GHz). An input 1-dB compression point (P1 dB) of -8.3 dBm and an input third-order intercept point of 0.8 dBm were also obtained for the LNA on the thin substrate.