A low-noise microwave oscillator employing a self-aligned AlGaAs/GaAs HBT

A study is made of the application of heterojunction bipolar transistors (HBTs) to low-noise microwave circuits. Design considerations and the low-noise performance of a Ku-band free-running oscillator using a self-aligned AlGaAs/GaAs HBT are described. The device has a novel structure in which, by utilizing SiO/sub 2/ sidewalls, the base surface area, which is the main cause of low-frequency noise, is drastically reduced. For a collector current of 1 mA, the fabricated device has base current noise power densities of 4*10/sup -20/, 6*10/sup -21/, and 2.5*10/sup -21/ A/sup 2//Hz at baseband frequencies of 1, 10, and 100 kHz, respectively. A prototype oscillator operating at 15.5 GHz has a measured output power of 6 dBm and SSB FM noise power densities of -34 dBc/Hz at 1 kHz, -65 dBc/Hz at 10 kHz, and -96 dBc/Hz at 100 kHz off-carrier, without using high-Q elements such as a dielectric resonator. The results of this study demonstrate the suitability of HBTs for low-phase-noise microwave and millimeter-wave oscillator applications.<
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