Why it matters: Moore’s Law has been alive for a while, but it’s not dead yet. Chipmakers burn the midnight oil to miniaturize transistor designs, and a team of researchers in China has created the smallest one yet.
For decades, scientists and engineers have scaled down transistors so that their smallest features consist of only tens of atoms. Since the first integrated circuits in the 1950s, progress in miniaturizing transistors followed Moore’s Law, which predicted that the density of active components in integrated chips would double every two years.
As many of our readers know, progress in this direction has slowed considerably in recent years. The main reason is that we are rapidly approaching the physical limits of what is possible with existing materials and the most advanced manufacturing processes we have.
More specifically, we can’t make transistor gates – which control current from the source to the drain – much smaller than 5nm because of something called quantum tunneling, which prevents them from working as intended. Materials such as graphene and carbon nanotubes may be vital for making transistors even smaller thanks to their physical properties, but it takes time to build functional devices from there.
In an article published this week, Chinese researchers explain that they have created a transistor with the smallest gate length ever reported. This milestone was made possible by creatively using graphene and molybdenum disulfide and stacking them into a two-step stair structure.
On the higher step you have the source and on top of the lower one you have the drain. Both are made of a titanium-palladium alloy separated by the surface of the trap, which is made of a single layer of a semiconductor material called molybdenum disulfide, which itself rests on a layer of hafnium dioxide that acts as an electrical insulator.
The interior of the higher step is a literal sandwich of aluminum covered with aluminum oxide, which rests on a graphene sheet – a single layer of carbon atoms. The aluminum oxide acts as an electrical insulator, except for a small gap in the vertical wall of the higher step, where the graphene sheet is allowed to contact the molybdenum disulfide. The entire staircase structure rests on a thick layer of silicon dioxide.
The trick to this design is that it uses the edge of the graphene sheet, meaning that when the gate is set to the “on” position, it’s only 0.34nm wide — essentially the width of the graphene layer itself. Another notable feature of this “sidewall transistor” is the negligible current leakage due to a higher off-state resistance. Manufacturers could use this grade for low power applications. Best of all, it would be relatively easy to make, although many of the prototypes required quite a bit of excitement to drive.
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Tsinghua University researcher Tian-Ling Ren co-authored the study and said it could be “the final node for Moore’s Law.” He also believes that going smaller than 0.34nm for the gate size is nearly impossible.
Of course, the researchers behind the new transistor only proved that a functional transistor can be made with thin, one-atom materials without inventing a new process for accurately positioning the required layers. Building billions of these sidewall transistors reliably is still a distant dream, but is a critical step in that direction, fueling hopes for faster, more energy-efficient devices in the future.
In the meantime, Samsung, Intel and TSMC are working hard to realize gate-all-around (GAA-FET) transistors and standardize interconnections for chiplet designs.
This post Chinese researchers create world’s smallest transistor gate
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