Quantum Dot Transistor

An isolated conductor may take on a charge, measured in coulombs for large objects. For a nano-scale isolated conductor known as a quantum dot, the charge is measured in electrons. A quantum dot of 1- to 3-nm may take on an incremental charge of a single electron. This is the basis of the quantum dot transistor, also known as a single electron transistor.

A quantum dot placed atop a thin insulator over an electron rich source is known as a single electron box. (Figure 3433 (a)) The energy required to transfer an electron is related to the size of the dot and the number of electrons already on the dot.

Figure 3433: (a) Single electron box, an isolated quantum dot separated from an electron source by an insulator. (b) Positive charge on the gate polarizes quantum dot, tunneling an electron from the source to the dot. (c) Quantum transistor: channel is replaced by quantum dot surrounded by tunneling barrier.

If the quantum dot is surrounded by a tunnel barrier and embedded between the source and drain of a conventional FET, as in Figure 3433 (c) , the charge on the dot can modulate the flow of electrons from source to drain. As gate voltage increases, the source to drain current increases, up to a point. A further increase in gate voltage decreases drain current. This is similar to the behavior of the RTD and Deltt resonant devices. Only one kind of transistor is required to build a complementary logic gate.