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A low-phase-noise 76-GHz planar Gunn VCO using flip-chip bonding technology has been developed. This device demonstrates sufficient RF performance for use in automotive radar systems. The power consumption is 1.9 W at a bias voltage of 3.3 V. The measured output power is 13.5 dBm at 76.4 GHz, with a DC-RF conversion efficiency of 1.2%. The Gunn oscillator achieves a significantly low phase noise of -104 dBc/Hz at a 1-MHz offset frequency, because of its fundamental-mode operation. To our knowledge, this is the lowest reported phase noise in this frequency range. A tuning range of 210 MHz is obtained at a center frequency of 76.5 GHz in the tuning voltage range from 0 to 10 V. The output power averages 13.9 dBm and exhibits variation of less than 0.8 dB. The device exhibits excellent modulation linearity of less than 2.4%. Overall, this Gunn VCO exhibits excellent phase-noise performance, high output power, and good modulation linearity. The adoption of flip-chip bonding technology is expected to enable low-cost mass production and good reproducibility.