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We present a fully monolithic 52-GHz oscillator integrated circuit (IC) that uses a back-illuminated InP-InGaAs heterojunction phototransistor (HPT). The HPT with an emitter size of 5 μm/spl phi/, where the whole emitter area can be illuminated from the back side of the substrate, exhibits a maximum oscillation frequency of 135 GHz, thus enabling us to achieve a 52-GHz oscillator with a wide margin. Optical injection locking was evaluated by using two laser diodes with a frequency difference of around 104 GHz, which corresponds to the oscillator's second-harmonic frequency. Injection locking is successfully achieved for the second harmonic as well as for the fundamental spectrum. The oscillator IC promises to achieve a half- or full-rate optoelectronic clock extraction from over-100-Gb/s optical data streams.