This paper for the first time systematically analyzed the operation mechanism of SiC NPN transistors. Theoretical device figure-of-merits for switching power devices based on the conduction loss and switching loss were developed. The on-state loss and the switching loss of 4.5-kV SIC switching power devices (MOSFET, NPN transistor and GTO thyristor) were then compared by using theoretical and numerical calculations. Special emphasis is placed on comparing the total power loss of the devices at a given current density. Theoretical analyses and simulation results show that GTO thyristors have a large switching loss due to the long current tail at turn-off, hence restricting its maximum operation frequency. High Voltage SiC MOSFETs have a large on-stale power dissipation at high current levels due to the resistive nature of the drift region, restricting their applications at high current densities. SIC NPN transistors have a comparable switching loss as that of SiC MOSFETs, but at the same time, SiC NPN transistors have the lowest on-slate loss. This study indicates that SIC NPN transistor is the most attractive switching power device at 4.5 kV.