Energy levels of source, island and drain (from left to right) in a single electron transistor for both the blocking state (upper part) and the transmitting state (lower part). In the blocking state no accessible energy levels are within tunneling range of the electron (red) on the source contact. All energy levels on the island electrode with lower energies are occupied. When a positive voltage is applied to the gate electrode the energy levels of the island electrode are lowered. The electron (green 1.) can tunnel onto the island (2.), occupying a previously vaccant energy level. From there it can tunnel onto the drain electrode (3.) where it inelastically scatters and reaches the drain electrode Fermi level (4.). The energy levels of the island electrode are evenly spaced with a separation of ${\displaystyle \Delta E}$. ${\displaystyle \Delta E}$ is the energy needed to add a subsequent electrode to the island, which acts as a self-capacitance ${\displaystyle C}$. The lower ${\displaystyle C}$ the bigger ${\displaystyle \Delta E}$ gets. It is crucial for delta-E to be larger than the energy of thermal fluctuations ${\displaystyle k_{B}T}$, otherwise an electron from the source electrode can always be thermally excited onto an unoccupied level of the island electrode, and no blocking could be observed.