Speed limit for transistors

Smaller and smaller, faster and faster—these have been the demands placed on microelectronics for decades. But the limit for miniaturization is fixed. A transistor cannot be smaller than an atom. But speed is a different matter. The speed of light marks the end of all cosmic acceleration attempts, even if in the quantum world “spooky action at a distance” promises synchronicity under certain conditions. But neither is relevant for signal transmission in microchips. This is why physicists at Ludwig Maximilian University of Munich, the Max Planck Institute of Quantum Optics and the Vienna and Graz Universities of Technology explored the actual—although not practical—top speeds.

And, ultimately, these depend on the clock speed of the transistors. By way of comparison: The fastest mechanical relays are capable of about one thousand switching operations per second, current microprocessors manage millions.

High-frequency light as a speed booster

If you keep increasing the switching frequency, you inevitably end up with light. For instance, in optoelectronics—according to the quantum mechanical model for describing electronic energy states—this raises, exactly like an applied electric voltage, the charge carrier (electrons, holes) from the valence band (bound state) to the conduction band (movable state) in order to change a semiconductor from an insulated to a conducting state. The excitation energy determines the specific band gap of the semiconductor material—in other words, the energetic gap between the valence and conduction band. It is approximately in the frequency range of infrared light. Ultimately, this is also the maximum achievable speed in these materials.

Dielectric materials, such as glass or ceramics, overcome this limit because, compared to semiconductors, they need a lot more energy to be conductive. This circumstance allows the use of high-frequency light. When light gives up its energy to matter, it is “packet-shaped” in the form of photons whose energy quantity is calculated according to the formula E=hf. Here, “h” is a universal physical constant (Planck’s Quantum) and “f” is the frequency of light. The higher the frequency of light, the greater the energy of the photons.

High speed with ultrashort pulses

Specifically, the physicists now fired ultrashort laser pulses in the extreme UV range (EUV) at lithium fluoride samples. Lithium fluoride is dielectric and has the largest band gap of all known materials.

The ultrashort laser pulse put the electrons in the lithium fluoride into an excited state. They were able to move freely and the material quickly became an electric conductor. A second, slightly longer laser pulse controlled the excited electrons in the desired direction. This created electric current, which was detected with the electrodes on both sides of the material.

The measurements provided answers to the questions as to how fast the material responded to the ultrashort laser pulse, how long the signal lasted and when the material was ready again for the next signal. It was shown that a theoretical upper limit for controlled optoelectronic processes is around one petahertz (1 million GHz). In other words, optoelectronics will not become faster in the near future.

Time or energy?

In this experiment, the scientists also faced a classic uncertainty dilemma, as is often the case in quantum physics. Sufficiently fast free charge carriers occur only with extremely short UV laser pulses. But this means that you do not transfer a very precisely defined energy to the electrons, but that the electrons absorb very different energies. You can say exactly when the free charge carriers are created but you cannot say which energy state they have afterwards. But this determines their reaction to the electric field. In other words, if their exact energy is unknown, they cannot be precisely controlled and the electric signal is falsified, especially with high laser intensities.

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