Highly efficient broadband power amplifiers have been crucial and required in any communication system in recent years, since the power amplifier influences overall system efficiency in terms of output power, linearity, bandwidth, and power consumption. To obtain highly efficient power amplifiers in terms of output power and bandwidth, design approach and selecting a good transistor technology should be considered. However, achieving such efficiency would be difficult due to a trade-off in the design approach between linearity, and better efficiency. A harmonic tuning approach can offer the right harmonic impedance between the input and output circuit networks in order to achieve the optimal trade-off balance. This is accomplished by carefully regulating the harmonics of the voltage and current waveforms at the transistor's input and output terminals. However, reaching such astounding output power efficiency will be profitable. Due to the adjustment, the bandwidth is lowered. Finding an efficient transistor technology is the other most significant technique to build a highly efficient power amplifier. GaN-HEMT transistors have proven themselves trustworthy transistors to achieve high output power and efficiency in every communication system. The main advantage of the GaN-HEMT transistor is its high breaking down voltage and high power density compared to the classic devices like silicon (Si) and gallium arsenide (GaAs). This enables the development of a wide bandgap or broadband extremely efficient power amplifier. Furthermore, GaN HEMT has low resistance, allowing for the creation of extremely efficient power amplifiers at high frequencies. The goal of this thesis work is to construct a highly efficient wideband harmonically tuned GaN HEMT power amplifier with a maximum output power of 50 dBm and tuned in the 1.6-2.4 GHz range. High efficiency has been attained by applying the harmonic tuning design approach. A 100W output power and an average PAE of 60 % were attained by using a big signal continuous wave input signal. wave input signal, a 100W output power and average PAE of 60% has been achieved. Key Words: Highly Efficient Power Amplifier, GaN HEMT, Harmonic Tuning, Output power, PAE, Power Gain, IM3