A team of researchers with the Tsinghua University in Beijing, China, has produced the world's smallest transistor. The research paper, published in Nature, describes how the new transistor design leverages an atom-thin graphene sheet as the transistor's gate, enabling a record-setting gate length of 0.34 nm. The team expects its research - and the resulting sidewall transistors, as they've named the result - to provide a way out from the greatly exaggerated news surrounding the death of Moore's Law.
Transistors are the fundamental units of semiconductor design, requiring three building blocks to be operable: a source (where the electrical current enters the transistor); a drain (the point where that same electrical current leaves the transistor); and a gate, which controls whether or not electrical current makes that particular voyage. However, a transistor by itself is so simple that it isn't actually capable of any useful work. To that end, transistors are crammed together in integrated circuits; a certain number and arrangement of transistors (in scales of billions) then results in a CPU core, while another results in a SRAM cache bank, or a GPU.
It follows that one way to increase performance and available workload complexity lies in cramming ever more transistors into the same space. Another would be to increase the functionality of each transistor. All semiconductor manufacturing improvements are ultimately reaching towards the same: a higher number of transistors, however complex they are, per squared inch. And this is where millions of dollars worth of research are funneled into each year: looking for ways to decrease the size of transistors, so that more of them can be packed in the same area. That in itself is pretty straightforward; however, there are additional benefits to smaller transistors - since electrical current has to endure a smaller journey from the transistor's source towards its drain, it remains in the transistor's features for smaller amounts of time - simultaneously improving power efficiency, temperatures and operational frequency while reducing leakages.
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But how did the researchers achieve such a small gate length? By taking advantage of the fact that graphene sheets, made out of graphene atoms laid out in a 2D plane, are only as thick as the graphene atom. That's how the 0.34 nm gate length was achieved: it corresponds to the height of the graphene sheet.
The classic transistor design was also changed to accommodate this radically new gate (of which the researchers built working prototypes). Imagine two adjacent buildings with different heights. In the transistor design, these buildings are made out of layers of silicon and silicon dioxide (insulators, meaning that they don't carry electrical currents). The graphene layer, one atom thick, is applied on top of the tallest building. But since graphene is electrically conductive (which is why we're using it in the first place), it has to be insulated on both sides; hence, the researchers applied another layer of insulating material: aluminum oxide.
This leaves the very edges of the graphene layer (our building) as the only non-insulated elements, which confers the gate's one-atom thickness. And because the silicon "buildings" are at different heights, we can now apply a layer of semiconductor molybdenum disulfide over both and alongside the sidewall (hence, the name) of the tallest building. It's through this layer that electrical current will flow through the transistor. But remember that the atom-thick layer of graphene is making contact with the molybdenum disulfide electrical highway, which allows it to act as a gate, permitting (or interrupting) the current flow.
Finally, a source is placed on the tallest (silicon) building for electrical current to enter the molybdenum disulfide highway; and a drain, through which electrical current abandons the transistor, is placed on top of the other. That is if the gate (the graphene layer) allows for it to reach its destination. We now have a working transistor.
Crucially, the researchers say these new transistors are easy to make with currently implemented technology and that the change in structure still resulted in compact units. In fact, they should ultimately prove less complex to produce than at least some other currently explored transistor designs since the placement of the layers of graphene and molybdenum disulfide is a relatively low-precision task (compared to etching and other semiconductor manufacturing techniques).
We should always keep in mind that not all promising research turns into a working product, and those that do (including this new transistor design) will take years to reach us. Even so, it's important to have a clear path ahead - or at least, fraught with options - than no option at all. This is especially true given the rate at which the worldwide semiconductor market is developing. That, paired with the increase in computational requirements from general computing, AI, Big Data, Web3, blockchain, and all other technological products, will demand extreme efficiency gains. Having a transistor feature as small as an atom certainly sounds apt for the aim.
