Oxides make transistors smaller and faster

Conventional semiconductor materials replaced by crystals of oxides

Microscope image of an oxide layer sample © University of Augsburg
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Components in microelectronics are becoming smaller and more powerful. However, conventional transistors based on common semiconductors soon reach the limits of miniaturization. In the latest issue of Science Express, physicists are now introducing a new form of transistor that is not made of silicon or galium arsenide, but is composed of oxides. They could allow for even greater downsizing.

Physicists at the Center for Electronic Correlations and Magnetism (EKM), Collaborative Research Center 484 "Cooperative Phenomena in the Solid State" at the University of Augsburg, and Penn State University, Pennsylvania (USA) show that particularly fast transistors, so-called high-electron mobility Transistors (HEMTs), which are usually made of ordinary semiconductors such as silicon or gallium arsenide, can also be realized with oxides. The big advantage of oxides over semiconductors is that the oxides can be made with material properties, such as a particularly high density of electrons that can not be achieved with semiconductors.

If two layers of different oxides are combined, they can form a wafer-thin boundary layer between them, which consists of an electron gas cloud. In this boundary layer, which is only two nanometers thick, the electrons are in a quantum state that blocks the movement perpendicular to the layers. As a result, the current can flow there only parallel to the layers. The electrons thus form a two-dimensional electron gas. That's why they are very agile and fast.

The Augsburg physicists have now investigated such a boundary layer between the oxides strontium titanate and lanthanum aluminate. For this purpose, they produced double layers of these oxides by means of a high-power laser, the thickness of which could be set exactly on an atomic scale. The scientists found that the conductivity of the electron gas changes dramatically with the thickness of the upper oxide layer (lanthanum aluminate). After the researchers had applied one, two or three lanthanum aluminate crystal layers, a highly insulating boundary layer was formed. A crystalline layer is only 0.4 nanometers thick. However, if the thickness of the lanthanum aluminate layers is four crystal layers or more, the boundary layer becomes abruptly conductive, but then very well.

As the researchers suggest, this erratic behavior is a great way to build HEMTs. Since the electron gas in the crystals with the three layers is perfectly insulating, but nevertheless almost conductive, it can be easily switched into the conductive state by an electrical voltage applied perpendicular to the interface. Thus, the entire arrangement can be used as a transistor and thus serve as an amplifier and switch of electrical currents. display

Physicists were thus able to show that high-electron mobility transistors not only work with conventional semiconductor materials, such as gallium arsenide, but also with oxides. The oxide HEMTs offer completely new perspectives for miniaturization, since more electrons are present in the boundary layer between the layers and the switching to the conductive state is enhanced by a so-called quantum phase transition.

"With our experiments, we want to open up new perspectives in oxide electronics, " says Professor Jochen Mannhart, head of the Department of Experimental Physics VI at the University of Augsburg. "It may also make it possible to make transistors in microelectronics even smaller and more efficient than before."

(University of Augsburg, 01.09.2006 - NPO)