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Recently the demand for high-performance low-cost vector modulators for millimeter-wave direct carrier modulation applications is increasing. An efficient technique to improve the performance of the vector modulators has been proposed in this paper accordingly. This technique highlights the effect of the Lange coupler characteristic impedance and the cold pseudomorphic HEMT (pHEMT) total gatewidth on the performance. It is demonstrated by investigating two types of vector modulators, which employ unbalanced and balanced topologies. Furthermore, to reduce the temperature drift, a concise analysis of the thermal behavior of the vector modulators has been carried out. Based on the proposed technique, two types of monolithic vector modulators have been realized at 40 GHz on a 100-mum -thick GaAs substrate with 0.15-mum cold-pHEMT devices. The static constellations were obtained by using an automatic measurement system, and the temperature drifts were characterized by the measurement data from -55degC to 70degC . The results show that the technique is suitable for millimeter-wave vector modulator designs.