The transporter unit ionizer was a component necessary to the transporter's function. On Constitution-class ships, the ionizer was located with the main transporter circuits on the lower level of the engineering deck.
In 2266, the ionizer was severely damaged due to a phaser blast, stranding an away team on the surface of planet Alfa 177, where the temperature was dropping to a dangerously low level. Fortunately, Spock and Scotty were able to attach some bypass and leader circuits to compensate for the difference, tied directly into the impulse engines. (TOS: "The Enemy Within")