Energy harvesting using triboelectric nanogenerators (TENGs) is an effective strategy to supply power to microelectronics, the Internet of Things, etc. Generally, the output performance of a triboelectric nanogenerator is limited by the breakdown of air. Hence, we systematically investigate a TENG operating in the contact-separation mode and single-electrode mode from the perspective of the electric field that builds up between the metal electrode and dielectric material. Finite-element simulations are conducted to illustrate the difference between such devices in terms of the static electric field and output performance. The TENG operating in the contact-separation mode has a lower built-in electric field but can deliver much higher transferred charges, short-circuit current, and open-circuit voltage. Furthermore, the output performance of the TENG operating in the single-electrode mode can be enhanced by reducing the gap distance. These findings not only illustrate the process of contact electrification but also show the great potential of such devices for realizing noncontact energy conversion.