An international team of researchers has demonstrated for the first time that a single molecule can operate as a field-effect transistor when surrounded by charged atoms that operate as the gate. The technology is in its infancy, but points to a future of even smaller transistors than possible now, with computers operating on a near-quantum level.
The researchers used a technique first demonstrated by researchers at IBM in 1990 when they created the letters I, B, and M by moving single atoms around on a metal surface with a scanning tunneling microscope (STM). In order for the molecule to function as a transistor, the researchers had to deposit it — as well as the charged indium atoms that surround it, forming the gate — on a indium arsenide semiconductor surface instead of a metal.
The molecule they used, a dye called copper phthalocyanine, is attached to the semiconductor surface by van der Waals forces, These forces are much weaker than the covalent bonds in conventional semiconductor surfaces. The weaker forces allow the molecule to be moved around easily, and also allows a current to flow from the tip of the STM through the molecule.
In a normal transistor, the current through the channel is controlled by modulating the gate voltage. Instead, the modulation of the electric field can be done by varying the distance between the channel and the gate.
The gating of the channel is different from conventional transistors — the mechanism of how the intensity of electrostatic field controls the current through the channels is not known, and also not known are the specifics behind the change to the quantum state of the molecule. What is known is that the gate controls the charge state in the molecule, which in turn controls the ability of electrons that tunnel between the gate and the STM tip to hop via close molecular orbitals in the molecule.
The results of the experiments are still far from finding applications in real-world devices, not only because of the complexity of the experiments, but also because much of the physics involved is still not fully understood. “These are very basic experiments where we have very ‘ideal’ systems, and it is important to gain a detailed understanding of what is going on,” says Stefan Fölsch, head of the experimentation. At present, the transistor requires a temperature of 4K and ultra high-vacuum.
Despite there not being a direct path to commercialization at this time, similar to breakthroughs in the ’60s for the Apollo space program, the breakthrough is the smallest transistor ever made by a very large margin. Smaller transistors can lead to better power efficiency, and more transistors in the same area, which can lead to better performance.
