1. A stack chip comprising: a first semiconductor chip; and a second semiconductor chip stacked on the first semiconductor chip, wherein the first semiconductor chip includes: a first semiconductor substrate having a first active surface and a first back surface opposite to the first active surface; at least one first internal circuit disposed in the first active surface of the first semiconductor substrate; at least one first chip pad connected to the first internal circuit via an input/output buffer, the at least one first chip pad including a first input/output pad; and at least one first connection pad disposed on the first active surface of the first semiconductor substrate and connected to the at least one first chip pad, the at least one first connection pad including at least one first input/output connection pad connected to the first internal circuit, wherein the at least one first connection pad of the first semiconductor chip is electrically connected to at least one second connection pad of the second semiconductor chip, and at least one of the first and second semiconductor chips has a first through electrode, the first through electrode having a connection end exposed from a back surface of the at least one of the first and second semiconductor chips.