Wideband amplifiers are employed in many applications such as military radar, electronic warfare and electronic instrumentations and systems, etc. This thesis project aims to build a wideband medium power monolithic microwave integrated circuits (MMIC) amplifier which operates between 6 and 18 GHz by using 0.25 µm Gallium Nitride (GaN) based high electron mobility transistor (HEMT) technology. Fully monolithic microwave integrated circuits realized with gallium nitride (GaN) high electron mobility transistors are preferred for designing and implementing microwave and millimeter wave power amplifiers due to its superior properties like high breakdown voltage, high current density, high thermal conductivity and high saturation current. Large band gap energy and high saturation velocity of AlGaN/GaN high electron mobility transistors (HEMTs) are more attractive features for high power applications in comparison to the conventional material used in industry for power applications- gallium arsenide (GaAs). Besides the high power capability of GaN enables us to make devices with relatively smaller sizes than of GaAs based devices for the same output power. Device impedances in GaN technology are higher than the GaAs technology which makes broadband matching easier. Firstly, GaN material properties are overviewed by mentioning the design and characterization process of the AlGaN/GaN epitaxial layers grown by Bilkent NANOTAM. After the microfabrication process carried out by Bilkent NANOTAM is explained step by step. It is followed by characterization of the fabricated HEMTs. As a final step before going through the design phase, the small signal and large signal modeling considerations for GaN based HEMTs are presented. In the last part, designs of three different multi-octave MMIC amplifier realized with coplanar waveguide (CPW) elements are discussed. In order to extend the bandwidth and to obtain a flat gain response, two different design approaches are followed, the first one is realized with Chebyshev impedance matching technique without feedback circuit (CMwoFB) and the other one is utilized by Chebyshev Impedance matching technique with negative shunt feedback (CMwFB), respectively. To maximize the output power, two transistors in parallel (PT) are used by introducing Chebyshev matching circuit and negative feedback circuit. The design topology which consists of two parallel transistors (PT) is modified to fulfill all the design requirements and it is implemented by taking process variations and the previously obtained measurement results into account. The measurement and the simulation results match each other very well, the small signal gain is 7.9 ± 0.9 dB and the saturation output power in the bandwidth is higher than 27 dBm in this second iteration.