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The electrical current between source and drain in a nanowire FET cannot be understood using classical physics. That's because electrons do strange things at such a tiny scale.
"Imagine a fish being trapped inside a fish tank; if fish has enough energy, it could jump up over the wall," Pati says. “Now imagine an electron in the tank: if it has enough energy, the electron could jump out—but even if it doesn’t have enough energy, the electron can tunnel through the side walls, so there is a finite probability that we would find an electron outside the tank.”
This is known as quantum tunneling. For Pati, catching the electron in action inside the nanowire transistors is the key to understanding their superior performance. He and his team used what is called a first-principles quantum transport approach to know what causes the electrons to tunnel efficiently in the core-shell nanowires.
The quantum tunneling of electrons—an atomic-scale game of hopscotch—is what enables the electrons to move through the nanowire materials connecting the source and drain. And the movement gets more specific than that: the electrons almost exclusively hop across the germanium shell but not through the silicon core. They do so through the aligned pz-orbitals of the germanium.
Quantum tunneling of electrons across germanium atoms in a core-shell nanowire transistor. The close-packed alignment of dumbbell-shaped pz-orbitals direct the physics of tunneling.
Simply put, these orbitals, which are dumbbell-shaped regions of high probability for finding an electron, are perfect landing pads for tunneling electrons. The specific alignment—color-coded in the diagram above—makes quantum tunneling even easier. It's like the difference between trying to burrow through a well with steel walls versus sand walls. The close-packed alignment of the pz-orbitals in the germanium shell enable electrons to tunnel from one atom to another, creating a much higher electrical current when switched on. In the case of homogeneous silicon nanowires, there is no close-packed alignment of the pz-orbitals, which explains why they are less effective FETs.
Nanowires in Electronics
There are many potential uses for nanowire FETs. Pati and his team write in their Nano Letters paper that they "expect that the electronic orbital level understanding gained in this study would prove useful for designing a new generation of core−shell nanowire FETs."
Specifically, having a heterogeneous structure offers additional mobility control and superior performance over the current generation of transistors, in addition to compatibility with the existing silicon technology. The core-shell nanowire FETs could transform our future by making computers more powerful, phones and wearables smarter, cars more interconnected and electrical grids more efficient. The next step is simply taking a small quantum leap.
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