One petahertz is the end – there is a physical speed limit for optoelectronic transistors

More is not possible: scientists have for the first time determined where the physically determined maximum speed for optoelectronic circuits in computer chips is a value that cannot be exceeded even with optimized technology. Hence, this speed limit is approximately one petahertz. This is equivalent to one million gigahertz and is about 100,000 times faster than today’s best transistors, the team reports in the journal Nature Communications.

Our increasingly digitized world requires smaller and more powerful computer chips. Chip components such as transistors are now only a few nanometers in size. However, the miniaturization of classic, purely electronic silicon-based circuits is gradually reaching its limits. Optoelectronics, which uses light as a signal transmitter, offer an alternative.

Laser pulses are used in optoelectronic transistors to excite electrons in a semiconductor material. In this way, the material is brought from the insulating state to the conductive state and thus switched from zero to one. The speed of operation of such an optical system depends on the wavelength of the light and material used. For example, silicon-based optoelectronic chips are best switched by means of infrared laser pulses.

Speed ​​test with UV laser and dielectric

But there is a better way: “The faster you want to get, the higher the frequency of the electromagnetic signal must be,” explains senior author Martin Schultze of the Technical University of Graz. At the same time, however, a material is needed in which the distance between the non-conductive neutral and the excited, conductive state is large enough to respond to this higher energy.

In his speed tests, Schultze, first author Marcus Ossiander of the Max Planck and their colleagues used the fastest possible combination of laser and material: instead they used ultra-short laser pulses in the UV range, and the dielectric material – lithium fluoride of a silicon semiconductor. With a band gap of 13.6 electron volts, it is the natural dielectric material with the greatest distance between the valence and electron conductivity bands.

In an experiment and complementary models, scientists determined where the maximum achievable speed of an optoelectronic chip was.

More than one petahertz is not possible

Result: The physical speed limit of the signal is about one petahertz – about a million gigahertz. As Ossiander and his colleagues report, optoelectronic transistors cannot be faster. The reason for this limitation: with even faster signal pulses, the distances become too small and the transmitted energy can no longer be precisely defined enough.

The electrons no longer reflect cleanly between the two desired states. “Solids have different permitted energy bands, and with short laser pulses, many are inevitably populated simultaneously by free charge carriers,” explains study co-author Christoph Lemell of the Vienna University of Technology. This distorts the current signal that is produced last – one can no longer distinguish between ones and zeros.

How close can you get to the border?

However, the speed limit of one petahertz also means that there is still room for improvement for today’s popular transistors: the maximum that can be achieved is about 100,000 times faster. It is questionable, however, whether computer chips with this maximum capacity could ever be produced: according to scientists, the realistically possible technical upper limits are likely well below the absolute speed limit.

In any case, one thing is for sure: optoelectronics will not be any faster than research has shown – and it remains to be seen how close to this limit future technologies will come. (Nature Communications, 2022; doi: 10.1038 / s41467-022-29252-1)

Source: Graz University of Technology, Vienna University of Technology

Leave a Comment