Does a single electron transistor really need the tunneling junctions?

[TriKri]

If you know how a single electron transistor works, can you please explain to me what the tunneling junctions are necessary for? I mean, if we would remove the two tunneling junctions and connect the drain, the source and the island into one long wire, wouldn't still it be possible to create a potential barrier by applying a negative gate voltage, thus preventing the current from going through, and in that way be able to switch on and off the current? Or what is the purpose of the tunneling junctions, really?

 

[Anaxandridas]

The tunneling junctions allow for isolation between the quantum well and the semi-classical leads. Numerous experiments have been carried out at low temperatures in which the "height" of the tunneling barriers was lowered so as to increase coupling between the quantum well and its leads. In some cases, part of the leads (in close vicinity to the quantum well) actually create a hybridized quantum state with the quantum well. In other words, the quantum wavefunction leaks out of the well and into the leads. In such cases, exotic tunneling phenomena (including the Kondo effect and RKKY interaction) have been observed.

The problem is an interesting one in the sense that when you lower the "height" of the tunneling barriers, you can gain more information about the quantum system through conductance spectroscopy. However, by doing so, the quantum system is changed (the wavefunction leaks out of the well). So, you would never be able to probe a quantum well with infinitely high barriers for information.