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Research centres on ferroelectric transistors

Posted: 27 Apr 2009     Print Version  Bookmark and Share

Keywords:ferroelectric transistor  titanate  silicon substrate  ferroelectric gate 

Electronic devices utilising ferroelectric transistors would turn on instantly without the need to boot from flash or hard disc memories. Such ferroelectric transistors would likely use a gate oxide with memory to create non-volatile circuitry that retains its state when power is turned off. Many research efforts have pursued ferroelectric transistors, but so far all have failed for lack of a suitable manufacturing process.

A solution may be near: strained strontium titanate deposited on standard silicon substrates, according to a team of industry, university and government researchers. Engineers from Cornell University, the University of Pittsburgh, the National Institute of Standards and Technology, Penn State University, Northwestern University, Motorola Corp., Ames Laboratory and Intel Corp. participated in the ferroelectric research.

"Our work is an important step on the way to a ferroelectric transistor on silicon," claimed Cornell materials scientist Darrell Schlom. "We showed that strontium titanate could be deposited on silicon—strained by 1.7 per cent in biaxial compression—and that it was indeed ferroelectric."

Ferroelectric gate oxides also would lower the energy required to turn a transistor on and off by virtue of a negative capacitance effect, thereby saving power and enhancing performance. Various non-volatile topologies are possible using ferroelectric materials, such as those used by Ramtron International Corp., a supplier of non-volatile ferroelectric RAM. But a ferroelectric transistor would make the state of all transistors in a device non-volatile, not just its memory cells.

Texas Instruments, Sharp Laboratories of America, Infineon Technologies and others have all patented different approaches to harnessing ferroelectric materials in transistor structures that retain their states. Nevertheless, commercialisation remains elusive.

Schlom acknowledged that the latest material breakthrough could still be hobbled by electronic traps at the interface or electrical leakage through the ferroelectric. Nevertheless, the Cornell-led team remains hopeful that it has found the key to manufacturing ferroelectric transistors by reducing the complexity of the materials required to fabricate its gate oxide.

"We have not made a ferroelectric transistor yet, but we have gotten rid of all of the intermediate layers," said Schlom.

Good candidate
Strontium titanate is a well known oxide, but it was not well known that straining it—compressing its atomic lattice to match that of silicon—could make it a good candidate for ferroelectric transistors. A related compound, strontium bismuth tantalate, is used in ferroelectric smart cards as is lead zirconium titanate. However, neither compound has proven to be good candidate as the gate dielectric for a transistor.

The research group claimed that strained deposition of strontium titanate on silicon is the answer. "By creating a ferroelectric directly on silicon, we are bringing the possibility of ferroelectric transistors closer to realisation," said Schlom.

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