Is silicon's crown under threat?

Posted by Thomas Anderson

A team of MIT researchers believe the semiconductor’s days as the king of microchips may be numbered - thanks to the development of the smallest transistor ever to be built from indium gallium arsenide.

The compound transistor, designed by a team in MIT’s Microsystems Technology Laboratories, reportedly performs well despite being just a mere 22 nanometers (billionths of a meter) in length.

According to Professor Jesús del Alamo, this makes it a rather promising candidate to eventually replace silicon in computing devices.

"To keep pace with our demand for ever-faster and smarter computing devices, the size of transistors is continually shrinking, allowing increasing numbers of them to be squeezed onto microchips," del Alamo explained.

"The more transistors you can pack on a chip, the more powerful the chip is going to be, and the more functions the chip is going to perform."

But as silicon transistors are reduced to the nanometer scale, the amount of current that can be produced by the devices is also shrinking, limiting their speed of operation. This has prompted concerns that Moore’s Law - the prediction by Intel founder Gordon Moore that the number of transistors on microchips will double every two years - could be about to come to an end.

To keep Moore’s Law alive and kicking, researchers have been investigating alternatives to silicon, which could potentially produce a larger current even when operating at these smaller scales. One such material is the compound indium gallium arsenide, which is already used in fiber-optic communication and radar technologies, and is known to have extremely good electrical properties.

However, despite recent advances in treating the material to allow it to be formed into a transistor in a similar way to silicon, nobody has yet been able to produce devices small enough to be packed in ever-greater numbers into tomorrow’s microchips.

Now del Alamo and his team have demonstrated that is possible to build a nanometer-sized metal-oxide semiconductor field-effect transistor (MOSFET) - the type most commonly used in logic applications such as microprocessors - using the material.

"We have shown that you can make extremely small indium gallium arsenide MOSFETs with excellent logic characteristics, which promises to take Moore’s Law beyond the reach of silicon," del Alamo confirmed.

Essentially, transistors consist of three electrodes: the gate, the source and the drain, with the gate controlling the flow of electrons between the other two. Since space in these tiny transistors is so tight, the three electrodes must be placed in extremely close proximity to each other, a level of precision that would be impossible for even sophisticated tools to achieve. Instead, the team allows the gate to "self-align" itself between the other two electrodes.

The researchers first grow a thin layer of the material using molecular beam epitaxy, a process widely used in the semiconductor industry in which evaporated atoms of indium, gallium and arsenic react with each other within a vacuum to form a single-crystal compound.

Researchers subsequently deposit a layer of molybdenum as the source and drain contact metal. They then "draw" an extremely fine pattern onto this substrate using a focused beam of electrons — another well-established fabrication technique known as electron beam lithography.

Unwanted areas of material are then etched away and the gate oxide is deposited onto the tiny gap. Finally, evaporated molybdenum is fired at the surface, where it forms the gate, tightly squeezed between the two other electrodes.

"Through a combination of etching and deposition we can get the gate nestled [between the electrodes] with tiny gaps around it," said del Alamo.

Although many of the techniques applied by the team are already used in silicon fabrication, they have only rarely been used to make compound semiconductor transistors. This is partly because in applications such as fiber-optic communication, space is less of an issue.

"But when you are talking about integrating billions of tiny transistors onto a chip, then we need to completely reformulate the fabrication technology of compound semiconductor transistors to look much more like that of silicon transistors," he added.

Their next step will be to work on further improving the electrical performance - and hence the speed - of the transistor by eliminating unwanted resistance within the device. Once they have achieved this, the MIT researchers will attempt to further shrink the device, with the ultimate aim of reducing the size of their transistor to below 10 nanometers in gate length.