Zero Voltage in a power semiconductor chip
IGBTs are only available in NPN type, so to make something like a bridge you have to use two of them. Typically the lower one(s) will have the emitter common with the negative rail and the upper one(s) will have the collector common with the positive rail. From this Infineon web page: 



The same is true of MOSFETs when only N-channel types are used. For practical reasons that's the case with almost all high voltage H-bridge, 3-phase bridges, and half-bridges. At lower voltages, such as 12VDC or 24VDC, it may become attractive to use P-channel MOSFETs for the upper switches, which allows the sources of those transistors to be tied to the positive rail, which in turn simplifies the gate drive circuitry.