InGaAs - keeping Moore’s Law alive

Moore’s Law - the prediction by Intel founder Gordon Moore that the number of transistors on microchips will double every two years - has been under threat as the speed of operation of nanometre-scale silicon transistors is being limited by the amount of current that can be produced by the devices.

To keep Moore’s Law alive, researchers have, for some time, 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 fibre-optic communication and radar technologies and is known to have extremely good electrical properties. But 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.

MIT’s Microsystems Technology Laboratories has now built the smallest transistor ever from silicon’s main rival, indium gallium arsenide.

The compound transistor performs well despite being just 22 nm long. This makes it a promising candidate to eventually replace silicon in computing devices, says co-developer Jesús del Alamo, the Donner Professor of Science in MIT’s Department of Electrical Engineering and Computer Science (EECS), who built the transistor with EECS graduate student Jianqian Lin and Dimitri Antoniadis, the Ray and Maria Stata Professor of Electrical Engineering.

Now del Alamo, Antoniadis and Lin have shown it is possible to build a nanometre-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 says.

A cross-section transmission electron micrograph of the fabricated transistor. The central inverted V is the gate. The two molybdenum contacts on either side are the source and drain of the transistor. The channel is the indium gallium arsenide light colour layer under the source, drain and gate. Image courtesy of the researchers.

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. The team then deposits 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 (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, del Alamo says. “Through a combination of etching and deposition, we can get the gate nestled [between the electrodes] with tiny gaps around it,” he says.

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 fibre-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,” del Alamo says.

The team’s 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, they will attempt to further shrink the device, with the ultimate aim of reducing the size of their transistor to below 10 nm in gate length.

Matthias Passlack, of Taiwanese semiconductor manufacturer TSMC, says del Alamo’s work has been a milestone in semiconductor research. “He and his team have experimentally proven that indium arsenide channels outperform silicon at small-device dimensions,” he says. “This pioneering work has stimulated and facilitated the development of CMOS-compatible, III-V-based-technology research and development worldwide.”