Wide band-gap materials such as Gallium Nitride (GaN) and Silicon Carbide (SiC) offer improved performance for power electronic devices compared to traditional Silicon (Si) power semiconductor devices. This paper investigates a 600V GaN & Silicon CoolMOS transistor application in 400W single phase continuous conduction mode (CCM) Boost PFC converter circuits with GaN, SiC and ultrafast silicon diodes and compares power semiconductor device efficiency benefits. The 600V GaN & Silicon CoolMOS transistor power efficiency improvement study and loss analysis for CCM Boost PFC converter circuits are introduced in detail. Based on the experimental study, the 600V GaN transistor enables higher energy efficiency compared with 600V Si CoolMOS for CCM Boost PFC converter at 680 kHz switching frequency: around 1% higher at low line and about 0.5% higher at high line due to low switching loss. 600V SiC diode typically outperforms ultrafast Si diode & GaN diode for CCM Boost PFC converter due to low device charge, and forward voltage. 600V GaN diode suffers lowest efficiency at light load due to large device capacitive charge. The ultrafast Silicon diode has the highest efficiency at light load due to low device charge, but suffers from low efficiency at heavy load due to largest forward voltage and reverse recovery loss. According to the investigations, the combination of a 600V GaN transistor for the boost switch and a SiC rectifier for the freewheeling diode achieves the best efficiency for CCM Boost PFC converter with the lowest power loss.@en