Many electronic devices operate using the effect of quantum tunneling. Flash memory chips found in USB drives use quantum tunneling to erase their memory cells. Some negative differential resistance devices also utilize the quantum tunneling effect, such as resonant tunneling diodes. Unlike classical diodes, its current is carried by resonant tunneling through two or more potential barriers (see figure at right). Its negative resistance behavior can only be understood with quantum mechanics: As the confined state moves close to Fermi level, tunnel current increases. As it moves away, the current decreases. Quantum mechanics is necessary to understand and design such electronic devices.