Researchers have discovered that nanowire structures under tension exhibit increased electron mobility. What challenges do modern transistors present, what did the researchers discover, and how can they be used for future transistors?

What challenges do modern transistors present?

As the demand for semiconductors continues, so does the pressure on semiconductor manufacturers to improve transistor technology. Smaller transistors have smaller gate charges meaning that they can operate faster, but this comes at the cost of reduced gate/channel performance.

Smaller transistors also operate at smaller voltages, which is particularly advantageous for reducing energy requirements for semiconductors. This can be seen with the advances in mobile technology whereby a computer of the past that would use hundreds of kilowatts of power can be shrunk down into a small semiconductor powered by a tiny battery, all seated on a small watch.

But reducing the size of modern transistors presents several challenges that are increasingly becoming difficult to solve. One of these is the effects of quantum tunnelling, whereby electrons can effectively teleport from one side of a boundary to the other if the boundary is narrow enough. This effect is negligible for transistors whose features are in the µm, but transistors in the nm scale feel the full effect of electron tunnelling.

Another challenge faced is the inherent properties of commonly used semiconductor materials. One of these is electron mobility which describes how easy it is for electrons to move through a material. The higher the electron mobility, the faster that material can respond to changes in electrical current. Thus, future transistor devices will need to explore materials with higher electron mobility to help increase their operating frequency.

Researchers improve electron mobility of nanowires using tension

Recently, researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the TU Dresden and NaMLab have demonstrated how the electron mobility of nanowires can be significantly improved when put under tension.

To demonstrate this effect, the researchers took a very common semiconductor, gallium arsenide, and created nanowires that were encased in an indium aluminium arsenide shell. The lattice structure of the two materials is slightly different, and this causes the outer indium shell to exert a large tensile force on the inner gallium arsenide nanowire. The resulting data from the nanowire showed that the electron mobility improved by 30%, which allows for faster switching speeds.

Measuring the nanowires electron mobility inside the indium aluminium arsenide structure required the researchers to use an optical probe that emits light of a specific wavelength. The chosen wavelength allowed the photons to pass through the outer shell virtually unstopped and only absorbed by the gallium arsenide nanowire. When these photons hit the electrons in the nanowire, it causes the electrons to oscillate, and this oscillation is dependent on electron mobility.

Another challenge that the researchers had to face was isolating individual nanowire structures. Their test material consisted of hundreds of thousands of nanowires, and nanowires that are close together behave as a thicker wire. The researchers applied several models and multiple measurements across nanowires with varying densities to get around this.

How can nanowires help with the future of transistor design?

Transistors built using nanowires will not only have significantly reduced energy requirements, but the use of nanowires under tensile forces will also help to increase transistor speed. This will, in turn, allow for the development of faster CPUs without the need for reducing the size of transistors or increasing the total transistor count on a circuit.

Of course, this all depends on if such nanowires can be fabricated en masse and if those wires can be easily manipulated to create circuits. Modern semiconductor manufacturing practices rely on the ability to deposit, etch, and layer various materials, but such nanostructures may not be as easy to grow in specific locations.

If the researchers can find a way to isolate individual nanowires and use them in an active device, then the use of nanowires in future transistors could become a reality. If, however, it turns out that nanowires cannot be practically manufactured, then they may never enter the commercial space.