||With the capability of achieving high breakdown voltage and high current density at microwave frequencies, the AlGaN/GaN based high electron mobility transistor (HEMT) becomes a promising candidate for RF/microwave power amplifiers. Recently there are increasing interests in extending the utilization of AlGaN/GaN HEMTs to other RF/microwave front-end building blocks such as LNAs, VCOs, mixers, and switches. The conventional AlGaN/GaN HEMTs are depletion-mode (D-mode) devices featuring negative threshold voltage. This characteristic requires a negative DC power supply in the system in order to pinch off the device. Thus RF/microwave circuits using these devices require both the negative and positive voltage supplies, which lead to increased circuit complexity and system cost. The enhancement-mode (E-mode) AlGaN/GaN HEMT with positive threshold voltage can lead to circuits with single supply voltage, but has been challenging to develop because of the inherently strong charge polarization in III-nitride material system and the lack of reliable and low-cost gate-recess techniques. We have recently developed a novel and robust approach, namely the fluoride-based plasma treatment technique which can effectively convert D-mode HEMT to E-mode HEMT without degrading other device characteristics. This achievement makes the elimination of the negative DC power supply doable in AlGaN/GaN based applications. In this thesis, design, fabrication and characterization were carried out in applying the E-mode AlGaN/GaN HEMTs to dual-gate mixers, MSM varactors and LNAs. The dual-gate mixer feature an E-mode gate as the LO port and a D-mode gate as the RF port, with both gates biased at positive voltages. This configuration not only help eliminate the negative power supply but also provide the dual-gate mixer a better performance with low VDD bias when only positive bias is available. When positive and low voltage VDD (1~3V) is applied, the E/D-mode dual-gate mixer shows higher conversion gain than the D/D mode dual-gate mixer at 2 GHz. For the MSM inter-digitated varactors, various electrode configurations including E/D, E/E and D/D were investigated. It is found that the D/E configuration features a smoother capacitance tuning profile and lower tuning voltages compared with the conventional D/D configuration. A monolithic integrated E-mode HEMT LNA featuring 1.5μm gate length is designed, fabricated and tested. The E-mode LNA only requires a positive voltage supply. A maximum power gain of 5.14dB, minimum noise figure of 3.88dB and associate gain of 3.42dB at 6GHz are achieved.