Why Ec (Conduction Band) in an Energy Band diagram is Potential energy of an electron ?

[Robotduck]

In an energy band diagram of a uniformly doped n type semiconductor with an externally applied electric field - Ec is considered as potential Energy of an electron and as electron move up in a conduction band that distance from Ec is kinetic energy ? How ? Also how does the electron move in a conduction band with an impact ionization?

Thank you !