This paper presents a 38% tuning bandwidth low phase noise differential voltage controlled oscillator (VCO) using a 0.5 mu;enhancement/depletion- pseudomorphic high-electron mobility transistor (E/D-PHEMT) process. The proposed VCO is based on a differential topology with two common-gate transistors. To achieve a wide tuning range with low phase noise, varactor in the VCO core employs the E-mode PHEMT device. The frequency of the VCO is from 18.8 to 27.5 GHz with a tuning bandwidth of 38% and an output power of higher than 5 dBm. The VCO demonstrates a phase noise of -109 dBc/Hz at 1 MHz offset frequency. This circuit can be compared with the VCOs fabricated using the advanced InP HBT technologies.