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A circuit-based device noise characterization technique is introduced which uses phase noise data to estimate the power spectral density (PSD) of high-frequency noise in MOSFETs. To apply this technique to a typical CMOS process, an oscillator structure is introduced which provides a predictable phase noise level for a given device noise PSD. The analytical equations governing the phase noise of this oscillator are presented and subsequently verified using circuit simulations. Three oscillators, using transistors of various channel lengths, are fabricated in a commercial 0.18 μm CMOS process technology to study short-channel excess noise. It is shown that, at equal current levels, the noise PSD in minimum-channel-length transistors is 8.7 dB larger than that in 3× -minimum-channel-length devices. The proposed method is especially suitable for applying to a state-of-the-art CMOS process to provide a quantitative analysis of various noise tradeoffs which are sometimes missing in foundry-provided models.