This dissertation presents innovative circuit techniques for improving wideband and energy-efficient mmWave radio-frequency integrated circuits (RFICs). At mmWave frequencies, strong parasitic effects, such as capacitance and the skin effect, pose serious challenges to maintaining performance. While earlier efforts have enhanced metrics like power consumption, tuning range, and harmonic suppression, many designs still face trade-offs that limit broadband effectiveness. To overcome these limitations, this work introduces new design approaches for Wilkinson power dividers, voltage-controlled oscillators (VCOs), and frequency multipliers using SiGe BiCMOS technology. The focus is on reducing trade-offs between bandwidth, phase noise, efficiency, and harmonic rejection. Key contributions include a Wilkinson power divider achieving 159% 10-dB fractional bandwidth up to 170 GHz, using folded inductors and custom capacitors to minimize insertion loss and imbalance. A 14 GHz dual-core VCO is proposed with inverse-active biasing for improved tuning and phase noise. A V-band VCO also achieves a 31% tuning range using a high-Q capacitor switch. Additionally, a wideband frequency doubler demonstrates 64% bandwidth and strong harmonic suppression through a novel transformer balun and capacitor array. Finally, a 50 GHz frequency quadrupler uses a cascaded architecture to deliver top-tier harmonic rejection and total efficiency. Together, these results advance the performance of next-generation mmWave systems.