For the first time, researchers at TU Wien have successfully manufactured a silicon-germanium (SiGe) transistor using an alternative approach that will not only allow for smaller dimensions in the future, but will also be quicker, require less energy, and function at extremely low temperatures, which is critical for quantum chips. The results were reported in IEEE Electron Device Letters.
Walter Weber, Masiar Sistani and Andreas Fuchsberger. Image Credit: TU Wien
Electronic components are increasingly complicated to fabricate as they get smaller. For many years, the semiconductor industry has struggled with this.
The oxide layer that insulates the semiconductor is the crucial component; it is doped and creates a long-range effect that penetrates the semiconductor. Bergakademie Freiberg, JKU Linz, and TU Wien (Vienna) created the technology.
Doping: Contamination by Design
Previous electronic components were made from doped semiconductor materials. Elements such as silicon or germanium were employed, with a limited quantity of foreign atoms introduced in a controlled manner. Instead of a pure, regular crystal, the outcome was a crystal with foreign atoms placed at random points.
This fundamentally modifies the electronic characteristics of the material: the presence of foreign atoms, known as ‘doping,’ influences the mobility of electrically charged particles and hence the material’s electrical conductivity. This method, which has been continually optimized over decades, is one of the foundations of modern microelectronics.
However, with components in the nanometer range, this method is increasingly reaching its limits. The smaller the transistor, the greater the effect of random fluctuations in doping. Since microelectronics is based on the interconnection of many millions to billions of transistors, this leads to ever greater challenges.
Andreas Fuchsberger, PhD Student, Institute of Solid State Electronics, TU Wien
Temperature sensitivity is also an issue: electronic components must not grow too hot, but extremely low temperatures are also undesirable since charge carriers cannot travel well enough. This is especially important in quantum computing, where quantum bits often need to be cooled to near absolute zero. To operate effectively, these qubits must be paired with classical electronic transistors for control and readout. However, those transistors are also exposed to the same ultra-low temperatures, which presents unique technical challenges
Clean Crystal Covered with a Doped Oxide Layer
Our solution to these problems is a new form of doping – known as modulation acceptor doping. This involves adjusting the properties of the semiconductor by remote coupling.
Walter Weber, Professor, TU Wien
Instead of doping the semiconductor crystals, the oxide layer that protects the semiconductor material is doped.
“This allows the oxide layer to improve the conductivity of the semiconductor without having to incorporate foreign atoms into the crystal itself,” Weber explained.
Even if the semiconductor material is not doped, a change in the oxide layer can have a remote impact on it, just like a magnet can act through other materials.
Modulation acceptor doping (MAD) has already been used in experiments on so-called Group III-V compound semiconductors. In silicon, the research team at TU Wien, in collaboration with the Bergakademie Freiberg and Johannes Kepler University Linz, is the first to successfully show this effect on the crucial semiconductor silicon-germanium and, moreover, to create a functional SiGe transistor in this manner.
This is especially significant in industry, where efforts are being made to continually increase Ge content in transistors to obtain quicker switching times and reduced power consumption. Quantum information in quantum processors might also be processed more quickly and with less energy loss.
The measurement findings are quite encouraging.
We were able to show that MAD technology has over 4000 times higher conductivity, improved switch-on behavior, and lower energy consumption. This could pave the way for a new generation of versatile nanotransistors.
Dr. Masiar Sistani, Postdoctoral Researcher, TU Wien
Fit for Quantum Chips
The new approach is also extremely appealing for quantum chips.
Dr. Sistani added, “The relevance of quantum technologies is growing. However, they still require classical electronics, for example, to control or read out the quantum systems. This means that conventional transistors have to work in very close proximity to ultra-cold quantum components.”
“This is where conventional doping technology often fails – this is referred to as ‘freezing out’ of the charge carriers. Our technology circumvents these problems. The doping of the oxide layer remains effective even at extremely low temperatures,” Dr. Sistani further added.
Journal Reference:
Fuchsberger, A. et.al. (2025) Modulation-Acceptor-Doped SiGe Schottky Barrier Field-Effect Transistors. IEEE Electron Device Letters. doi.org/10.1109/LED.2025.3577243