This letter presents the design and fabrication of a Ku-band differential-mode voltage-controlled oscillator (VCO) with extremely low power consumption and good phase noise characteristics based on a monolithic InP-based resonant tunneling diode/heterojunction bipolar transistor (RTD)/(HBT) technology. In order to reduce the power consumption, an InP-based RTD is used for microwave power generation, which shows the negative resistance characteristics at a low voltage. The fabricated VCO shows an extremely low dc power consumption of 1.42 mW and fairly good phase noise performance of -112 dBc/Hz at 1-MHz frequency offset. The figure-of-merit (FOM), which is used for comparing the performance of the fabricated oscillator with the previously reported results, was obtained to be -195 dBc/Hz. The obtained FOM is one of the best values among Ku-band differential VCOs reported so far.