Why in most power transistors in plastic casing, the middle terminal is connected to the tab and corresponds to the collector?
This is done this way purposely, because in the collector is that the transistor generates the greatest amount of heat. Thus, this terminal is normally made so as to correspond to the middle pin and also to the heat dissipating tab. In these transistors we must therefore take care when installing the heatsink, always remembering that the collector is connected to the heatsink. The same is true for the anode of SCRs in many cases.