The advancement in technology in semiconductor materials significantly contributed in improvement of human life by bringing breakthrough in fabrication of optoelectronics and power devices which have wide applications in medicine and communication. The Gallium Arsenide (GaAs) and Gallium Nitride (GaN) are versatile materials for such applications but with relative merits and demerits. GaAs transistors are suitable for both narrowband and wideband applications due to very wide operating frequency range (30 MHz to millimetre-wave frequencies as high as 250 GHz). They are highly sensitive, generate very little internal noise and have power density typically around 1.5 W/mm. But low break down voltage (5x105V/cm), low output power (5-10W) and inability to withstand higher temperatures are the main limitations. On the other hand, GaN possess the improved physical and chemical characteristics, with high output power, high operating temperature (1000°C in vacuum), fast heat dissipation, high breakdown voltage (4x106V/cm), high power density (5-12W/mm), high frequency characteristics and large band gap (3.4eV) which allow significant reduction of devise size. Also high breakdown voltage increases the overall impedance which make it suitable in matching process and enables efficient operation in broad band region. The present paper critically analyses the GaAs and GaN semiconductors in relation to their significant physical and chemical properties, which make them suitable to make efficient power and optoelectronics devices for applications in communication, space and medicine.