This research aims to design and implement GaAs HBT MMICs for the two crucial components in W-CDMA transmitters, quadrature modulator and upconverter, with thorough evaluation of the package and PCB effects. To construct a strong theoretical foundation, the small-signal modeling of HBTs and the EM-characterization of package and PCB interconnects are intensively studied. In this dissertation, a novel extrinsic-inductance independent approach is developed for direct extraction of the intrinsic elements in a hybrid-pi equivalent circuit of HBTs. The interconnects of leadless RFIC packages and test PCBs are investigated using the 3-D EM simulation tools and transformed into the equivalent circuits for co-analysis with the designed HBT MMICs. The first HBT MMIC design is a W-CDMA direct-conversion quadrature modulator incorporating a new 90 degrees phase shifter. Although the proposed 90 degrees phase shifter has a remarkable advantage over the others in implementation loss, it is rather susceptible to the package and PCB effects, resulting in a moderate degradation of EVM. The second HBT MMIC design is a W-CDMA upconverter incorporating a popular micromixer. Although the micromixer-based upconverter consumes much less current at low output powers to achieve the same high linearity when compared to a Gilbert mixer-based design, it is quite susceptible to the package and PCB effects, causing a significant degradation in ACPR. Comparison between theory and measurement shows good agreement in evaluating the influences of package and PCB interconnects on both HBT MMICs.