Junctionless transistor could simplify chip making, say researchers

Junctionless transistor could simplify chip making, say researchers.

Peter Clarke

2/22/2010 4:48 AM EST

In a move that could revolutionize nanoelectronics manufacturing and the semiconductor industry, scientists at the Tyndall National Institute (Cork, Ireland) have designed and fabricated what they claim is the world’s first junctionless transistor. LONDON — In a move that could revolutionize nanoelectronics manufacturing and the semiconductor industry, scientists at the Tyndall National Institute (Cork, Ireland) have designed and fabricated what they claim is the world’s first junctionless transistor.

The breakthrough is based on the deployment of a control gate around a silicon wire that measures just a few dozen atoms across. The gate can be used to “squeeze” the electron channel to nothing without the use of junctions.

Professor Jean-Pierre Colinge
Tyndall National Institute

The junctionless transistor, otherwise known as the gated resistor, which could simplify manufacturing of transistors at around the 10-nanometer stage, was created a by a team led by Professor Jean-Pierre Colinge (shown left) and a paper on the development has been published in Nature Nanotechnology.

The structure simplifies the production of transistors and also produces a near-ideal sub-threshold slope, extremely low leakage currents and less degradation of mobility with gate voltage and temperature than classical transistors, the researchers have claimed. Nonetheless such device can be made to have CMOS compatibility.

Since their invention transistor- and diode-action has depended on controlling the flow of electrons across junctions giving rise to the familiar NPN and PNP notation for bipolar devices and p- and n-type FETs with sources and drains. Controlling the junction allows the current in the device to be turned on and off and it is the precise fabrication of this junction that determines the characteristics and quality of the transistor and is a major factor in the cost of production. However, as a consequence of the repeated miniaturization predicted by Moore’s Law transistors at the leading edge are becoming so small that conventional transistor architectures are becoming exceedingly difficult to fabricate.

“We have designed and fabricated the world s first junctionless transistor that significantly reduces power consumption and greatly simplifies the fabrication process of silicon chips,” declared Tyndall’s Professor Colinge, in a statement.

Cross section of a silicon wire with wrap-around insulator and overlaid gate

Control gate like a wedding ring
“The current flows in a very thin silicon wire and the flow of current is perfectly controlled by a “wedding ring” structure that electrically squeezes the silicon wire in the same way that you might stop the flow of water in a hose by squeezing it. These structures are easy to fabricate even on a miniature scale which leads to the major breakthrough in potential cost reduction,” explained Professor Colinge.

Professor Colinge’s team used commercial SOI wafers and electron-beam lithography to define silicon nanowires (or nanoribbons) approximately 30 nanometers across and 10-nm thick. After growing a 10-nm gate oxide, the nanowires were uniformly doped by ion implantation, using arsenic to dope the n-type devices and boron fluoride to dope p-type devices.

In the gated resistor, high doping is required to ensure a high current drive and good source and drain contact resistance. The wrap-around gate was formed by the deposition of a 50-nm layer of amorphous silicon. This is doped with an opposing dopant to the channel — so n-type for a p-channel and p-type for an n-channel — and annealed to activate the sites and transform the gate material to polycrystalline silicon.

Professor Colinge and his team also built a junctionless transistor on a silicon nanowire measuring about 10-nm by 10-nm.

Another key challenge for the semiconductor industry is reducing the power consumption of microchips. Minimising current leakage is one of the main challenges in today’s complex transistors. “The Tyndall junctionless devices have near ideal electrical properties and behave like the most perfect transistors. Moreover, they have the potential of operating faster and using less energy than the conventional transistors used in today s microprocessors,” said Professor Colinge.

He went on to say that the junctionless transistor resembles a semiconductor transistor structure, first proposed in 1925 — the so-called Lilienfield device, which was patented in Canada in 1925 by Austro-Hungarian physicist Julius Edgar Lilienfield. But to-date, no-one had been able to fabricate it. Professor Colinge attributed the successful fabrication at Tyndall to the skill and expertise of researchers who were able to fabricate silicon nanowire with a diameter of a few dozen atoms using electron-beam writing techniquesl.

“We are very excited by the outstanding results that Jean-Pierre has achieved,” commented Tyndall CEO, Professor Roger Whatmore. “We are beginning to talk about these results with some of the world’s leading semiconductor companies and are receiving a lot of interest in further development and possible licensing of the technology. These results could not have been achieved without the expertise of Jean-Pierre and his colleagues together with the state-of-the art facilities that we have at Tyndall.”

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