||With the rapid progress made in material growth and device processing, high electron mobility transistor (HEMT) in AlGaN/GaN system is emerging as a promising candidate for next generation RF/microwave power amplifiers. There are also recent interests in extending the applications to other RF/microwave front-end building blocks including LNAs, VCOs, and mixers. In order to develope complete GaN-based RFIC and MMIC, HEMTs as well as passive components are needed to integrate together with fully compatible fabrication process. In addition, high performance RFIC and MMIC can only be realized with high performance HEMTs and passive components with high Quality-factor (Q-factor). Passive components including varactors (voltage-controlled capacitor) and air-bridged spiral inductors were fabricated on AlGaN/GaN system with HEMT compatible fabrication process. Varactor, with its tunable capacitance, is essential for tunable circuits such as VCOs and multipliers. It is important to maintain high Q-factor within a wide voltage-controlled tuning range for varactors to be useful in RF/microwave circuits. For example, a high efficiency VCO with low phase noise requires a high quality LC-tank, in which, both the varactor and inductor need to possess high Q-factor. Several methods were utilized to boost up the Q-factor of varactor include Optimization made in Device Structure (Single and Double Channel HEMT structures), Physical Design and Fabrication Process. Different types of varactors were fabricated on AlGaN/GaN system for the comparison of Q-factors. Spiral inductor is a common type inductance component used in RFIC and MMIC. Air-bridged spiral inductors were fabricated on AlGaN/GaN system and bare sapphire with high Q-factors achieved. There were totally four types of pillar structures made to support the air suspended spiral inductors. With different types of pillar structures, trade-offs are made in Q-factors, reliabilities and mechanical supports. The fabrication of varactor and air-bridged spiral inductor were fully compatible with AlGaN/GaN HEMT process, allowing easy integration with HEMT circuits without additional fabrication steps. The fabricated varactors and air-bridged spiral inductors can be used for RFIC and MMIC applications.