Coulomb blockade occurs when the charging energy (the energy required to add an extra electron) of a small conductive island, such as a quantum dot, becomes significant. This is due to the island's capacitance being extremely small. When the voltage applied to the island is below a certain threshold, adding an extra electron requires more energy than is available, thus blocking additional electrons. When the voltage surpasses this threshold, the blockade is lifted, and electrons can flow freely.
