Enlarge /. One goal, two behaviors.
Our processors are currently based on silicon. Fundamental limitations of what can be done with this material are causing researchers to look for ways to use materials that have inherently small features, such as nanotubes or atomically thin materials. In theory at least, we can use it to do what we are doing now, only more efficiently and / or with physically smaller functions.
But can these materials allow us to do things that silicon can't? The answer seems to be yes, based on research released earlier this week. In it, the researchers describe transistors that can be reconfigured during operation so that they can perform completely different operations. They suggest that this can be useful for security as it would deter bad actors from figuring out how to implement security features.
Doping for security
Researchers based in Purdue and Notre Dame put forward an argument as to why this type of reconfigurable circuit could have security implications. It depends on the material science of silicon transistors. You need areas of silicon that hold either a negative or a positive charge (creatively called p- or n-type semiconductors). These are created by doping or adding small amounts of certain elements to the silicon. This happens during manufacture and the doping is fixed at this point. This means that the operation of individual transistors is locked when the chip is manufactured.
This becomes a problem for safety-related hardware. If any of the functions were implemented in a silicon chip (rather than being purely software-based), they would have to be physically tied to the chip hardware itself. And since this hardware is static, knowing the chip layout would mean understanding something about how the security hardware works and possibly uncovering its weak points. This is not an abstract fear; We have developed advanced microscopy techniques that can examine hardware at the required level, and there is evidence that they have already been used to do this.
According to the authors, the solution to this is to create transistors that are not assigned to a specific function. And that is not possible with silicon. However, it turns out that atomically thin materials, which have been investigated for other reasons, are not inherently p- or n-type semiconductors. Their behavior is determined by their environment, as they carry a positive or negative charge, depending on what is injected into the material from the metal conductors that wire the transistor. So the researchers decided to test whether they could actually build a reconfigurable transistor.
Although there are a variety of atomically thin materials – graphene, MoS2, and more – the researchers decided to work with something called black phosphorus. The material consists of multilayered layers, each layer being made entirely of phosphorus atoms that are chemically bonded together. In contrast to planar graphene, the chemical bonds of phosphorus cause these sheets to have regular ridges and hollows, like corrugated iron. (The last time we visited this material, it was used to make quick charge batteries.)
Black phosphorus was chosen because it has a small band gap, which means that it does not require a large voltage differential to operate. Unfortunately, this also meant that the difference between the on and off states was small. This problem was compounded by the fact that the hardware was designed to allow power to flow in both directions. When switched off, it became possible for low-level electricity to flow forwards or backwards, which made it more difficult to register "off" as a lack of electricity.
To solve this problem, the researchers fundamentally revised the transistor. In silicon, a transistor has source and drain electrodes to allow current to flow through the transistor and a gate electrode that turns that current on or off. For the reversible version with black phosphorus, the researchers used two gates that amplified the on / off signal. They also added something called a "polarity gate" that would block the flow of electricity when the gate was supposed to be off.
With these, the researchers were able to achieve excellent performance: operation at low voltages and a clear difference between the on and off states, the difference increasing as the voltage increased.
NAND, NOR and XOR
With this, the researchers built a real piece of logic. This had a single bit key that determined the state of the gate. If the bit is in a state, the hardware would perform a NAND (not-and) function. Flip the bit and perform a not-or (NOR) operation instead. And based on the graphics in the paper, it worked exactly as it should. The researchers also showed that it was possible to create a similar device that could switch between exclusive or (XOR) and NOR by just tweaking some details of the configuration.
It is crucial that the status of the bit can be set dynamically at runtime. Without knowing the status of the bit, there is no way to tell what operation these gates are doing by looking at the hardware. Even if you have the full hardware layout, there is no telling what these gates might be doing.
Does that matter? Maybe it doesn't – we are far from implementing any of these new materials anywhere near production hardware. But it's exciting to see people think about it because we haven't seen many such reports. As the authors argue, "research in this area typically focuses on demonstrating operations that can also be achieved with traditional transistors, and attempts to use the unique properties of 2D materials such as ambipolarity to provide new functions, are rare."
Nature Electronics, 2020. DOI: 10.1038 / s41928-020-00511-7 (About DOIs).