This paper takes metal-insulator-metal system as example and investigates the main types of electrical contact through the view of energy band structure, to analyze the whole process of the transition from ohm contact to barrier contact. Ohm contact, which promotes charges injection from electrode (metal) to insulator, can be used as storage of charge carrier, which is body limited; it can also be regarded as a type of contact that forms an accumulation layer extending from the interface to the interior of the insulator. Whereas, barrier contact is a type of contact which forms a depletion region extending from the interface to the interior of insulator. As for this type of contact, electron injection from metal tends to the state of saturation. The characteristic of neutral contact is that there is no space charge in the insulator, nor band bending, which means the boundary of conduction band and valence band up to the interface is flat.