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| Walter Weber, Masiar Sistani and Andreas Fuchsberger Photo Credit: Technische Universität Wien |
The smaller electronic components become, the more complex their manufacture becomes. This has been a major problem for the chip industry for years. At TU Wien, researchers have now succeeded for the first time in manufacturing a silicon-germanium (SiGe) transistor using an alternative approach that will not only enable smaller dimensions in the future, but will also be faster, require less energy and function at extremely low temperatures, which is important for quantum chips.
The key trick lies in the oxide layer that insulates the semiconductor: it is doped and produces a long-range effect that extends into the semiconductor. The technology was developed by TU Wien (Vienna), JKU Linz and Bergakademie Freiberg.
A new kind of transistor
Image Credit: Technische Universität Wien
Doping: contamination by design
Previous electronic components were based on doped semiconductor materials. Elements such as silicon or germanium were used, and then a small amount of foreign atoms was added in a targeted manner. Instead of a pure, regular crystal, the result was a crystal in which foreign atoms were deposited at random locations. This completely changes the electronic properties of the material: the presence of the foreign atoms, known as ‘doping’, alters the mobility of electrically charged particles and thus the electrical conductivity of the material. This process, which has been continuously optimised over decades, is one of the cornerstones of modern microelectronics.
“However, with components in the nanometre range, this method is increasingly reaching its limits,” explains Andreas Fuchsberger, lead author of the new study from the Institute of Solid State Electronics at TU Wien. “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.”
Temperature sensitivity also becomes a problem: electronic components must not become too hot, but excessively cold temperatures are also bad because the charge carriers can no longer move well enough. This is critical in quantum computer applications, for example, where quantum bits, which often have to be cooled to almost absolute zero, must be combined with classical electronic transistors to control and read them, which then also become very cold.
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,” says Prof. Walter Weber, who heads the research group for nanoelectronic components at TU Wien. Instead of doping the semiconductor crystals themselves, the oxide layer that insulates 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,” explains Weber. Similar to how a magnet can act through other materials, a change in the oxide layer can also have a remote effect on the semiconductor material, even if this material itself is not doped.
Experiments with this modulation acceptor doping (MAD) have already been conducted in so-called Group III-V compound semiconductors and in silicon, the research group at TU Wien, in cooperation with the Bergakademie Freiberg and Johannes Kepler University Linz, is the first to successfully demonstrate this effect on the important semiconductor silicon-germanium and, moreover, to produce a functional SiGe transistor in this way.
This is particularly relevant to industry, as efforts are being made to continuously increase the Ge content in transistors in order to achieve faster switching times and lower power consumption. In quantum chips, quantum information could also be processed faster and with lower energy losses. The measurement results are extremely promising: “We were able to show that MAD technology has over 4000 times higher conductivity, improved switch-on behavior and lower energy consumption,” says Dr Masiar Sistani. “This could pave the way for a new generation of versatile nanotransistors.”
Fit for quantum chips
The new technology is also particularly interesting for quantum chips: “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,” says Dr Sistani. “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.”
Title: Modulation-Acceptor-Doped SiGe Schottky Barrier Field-Effect Transistors
Authors: Andreas Fuchsberger, Austria Kilian, Lukas Wind, Enrique Prado Navarrete, Johannes Aberl,Moritz Brehm, Ingmar Ratschinski, Daniel Hiller, Masiar Sistani, and Walter M. Weber
Source/Credit: Technische Universität Wien
Reference Number: tech092325_01
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