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Posted: Jul 07, 2016
Probing quantum phenomena in a tiny transistor
(Nanowerk News) Past research has shown heterogeneous silicon-germanium nanowires to be better transistors than their pure silicon counterparts. A team from Michigan Technological University has figured out the most likely reason why. The study, published recently in Nano Letters ("Unlocking the Origin of Superior Performance of a Si–Ge Core–Shell Nanowire Quantum Dot Field Effect Transistor"), focuses on the quantum mechanics in a core-shell nanowire structure. Having a better understanding of the underlying physics could improve efficiency in electronic devices that maximizes existing silicon-based technology.
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.
The material's effectiveness comes down its structure, which is a one-dimensional nanowire with a core of silicon atoms sheathed by shell of germanium atoms. The germanium shell is where the action is at: The close-packed alignment of pz-orbitals between the germanium atoms enable electrons to jump from one atom to another in an atomic game of hopscotch called quantum tunneling. This creates a much higher electrical current when the materials is 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.
Ranjit Pati, a professor of physics at Michigan Tech, led the work along with his graduate students Kamal Dhungana and Meghnath Jaishi. He explains how quantum tunneling—a kind of atomic game of hopscotch—works in the nanowires.
"Imagine a fish being trapped inside a fish tank; if the 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.”
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.
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.