Millimeter-wave voltage-controlled oscillators (VCOs) are presented which are fully integrated in a SiGe bipolar production technology. The low-cost differential circuits have been designed and optimized for low phase noise and wide tuning range. As an example, by varying the bias voltage of the on-chip varactor, the oscillation frequency can be changed from 36 to 46.9 GHz (i.e., by 26%). In this wide frequency range, phase noise between -107 and -110dBc/Hz at 1-MHz offset frequency and single-ended voltage swing of about 0.95Vpp ±10% (differential: 1.9Vpp) were measured. The circuit consumes 280mW at -5.5-V supply voltage. The high oscillation frequency and low phase noise at wide tuning range are record values for fully integrated oscillators in Si-based technologies. The basic oscillator was then extended by a cascode stage as an output buffer. Now the VCO performance is no longer degraded if nonperfectly terminated transmission lines are driven. Thus, the chip can be mounted in a low-cost socket; however, at the cost of increased phase noise and power consumption.