One or more quantum dots are used to control current flow in a transistor. Instead of being disposed in a channel between source and drain, the quantum dot (or dots) are vertically separated from the source and drain by an insulating layer. Current can tunnel between the source/drain electrodes and the quantum dot (or dots) by tunneling through the insulating layer. Quantum dot energy levels can be controlled with one or more gate electrodes capacitively coupled to some or all of the quantum dot(s). Current can flow between source and drain if a quantum dot energy level is aligned with the energy of incident tunneling electrons. Current flow between source and drain is inhibited if no quantum dot energy level is aligned with the energy of incident tunneling electrons. Here energy level alignment is understood to have a margin of about the thermal energy (e.g., 26 meV at room temperature).