At its most basic form a transistor in an IC is just a simple switch. As transistors have been miniaturised, these switches have both become faster and used less power to go from one state to another. The regularity of the steps in this shrinking process has been so consistent that it even has its own “law” – Moore’s Law, which states that the number of transistors in a IC doubles around every two years. Moore’s Law was first observed by Intel founder Gordon Moore in 1965 and it has held true to the present day. However, we are now coming closer to the physical limits of the silicon processes that we use, with transistor gates measuring only a few atoms in thickness and it soon won’t be possible to reduce the size of the transistors themselves. To keep moving forward and getting the additional performance increases and lower power operation that we expect every couple of years, new ways of designing ICs need to be investigated. That is what researchers at the Technical University of Vienna have attempted in developing a way to change the function of transistors so that we need fewer transistors overall.
The researchers have added a control terminal to the drain, gate and source terminals normally found on the FETs that make up ICs. The control line turns the device into a new type of adaptive transistor that can be switched in a flash to perform different logical tasks as needed. To achieve this, the researchers did not use the usual silicon technology, but germanium.
In the transistor, both electrons and holes are manipulated simultaneously in a very special way: "We connect two electrodes with an extremely thin wire made of germanium, which is connected to metal on both sides with special, extremely clean interface. Above this germanium segment, we place a gate electrode like the ones found in conventional transistors. What is decisive is that our transistor also has another control electrode, which is placed on the interfaces between germanium and metal. It can dynamically program the function of the transistor," explains Dr. Masiar Sistani, who is a postdoctoral researcher in Prof. Walter Weber's team at the Institute for Solid State Electronics at TU Wien.
Germanium has a special electronic structure. When voltage is applied, the current flow initially increases. After a certain threshold, the current flow decreases again – an effect called called negative differential resistance. The control electrode provides a way to modulatie that threshold. This allows the transistor to be given the properties that are required at that specific time - for example, a NAND gate can be switched to a NOR gate.
"Until now, the intelligence of electronics has come simply from the interconnection of several transistors, each of which had only a fairly primitive functionality. In the future, this intelligence can be transferred to the adaptability of the new transistor itself," says Prof. Walter Weber. "Arithmetic operations, which previously required 160 transistors, are possible with 24 transistors due to this increased adaptability. In this way, the speed and energy efficiency of the circuits can also be significantly increased."
The materials used in the new transistor are already used in the semiconductor industry today, and no completely new manufacturing processes are necessary. In some respects, the technology would even be simpler than before: today, semiconductor materials are doped, but this is not necessary with the germanium-based transistor; pure germanium can be used.
Original publication
M. Sistani et al., Nanometer-Scale Ge-Based Adaptable Transistors Providing Programmable Negative Differential Resistance Enabling Multivalued Logic, ACS Nano 2021, 15, 11 (2021).
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