Beyond Silicon: A New Digital Switch

Though silicon is ubiquitous in our modern electronics, it possesses a number of drawbacks that leave scientists constantly searching for alternatives. The arrival of graphene a decade ago was thought to offer considerable promise in this area, but it too suffered from a fatal flaw. Now scientists report a means of combining graphene with nanotubes to yield a new generation of high speed electrical switches.

Silicon transistors are the fundamental building block of all electronic devices and one of the most influential discoveries of the modern age. First produced at Bell Labs in the 40s, it yielded its developers the Nobel Prize in Physics in 1956. Though the first transistor was decidedly macroscale, in the decades since we’ve been able to consistently reduce the size of these devices to keep pace with the famous Moore’s Law: the number of transistors per square inch on integrated circuits will continue to double every year for the foreseeable future. Moore made this observation in 1965 and it has been reasonably accurate ever since. The use of silicon, however, puts a mandatory end to this trend. Below a certain size this material simple cannot function properly, and we are fast approaching that limit.

The first transistor: this replica shows the scale of the first working transistor produced at Bell Labs. Imagine the size of a computer with billions of these!The first transistor: this replica shows the scale of the first working transistor produced at Bell Labs. Imagine the size of a computer with billions of these!

Scientists are well aware of silicon’s downsides and are always on the lookout for alternative solutions. The development of molecular electronics, in which single molecules can function as electronic devices, in recent years is the obvious extension of Moore’s Law. Unfortunately, these require considerable cost and expertise, cannot be integrated into the already existing electronics fabrication processes, and cannot be produced on the massive scale required.

The discovery of graphene over a decade ago was of great excitement to many, with those interested in ever-smaller transistors being no exception. At a single atom thick, graphene devices can be made incredibly tiny, but it can also be produced in large sheets that are compatible with all the machines previously developed for work with silicon. On top of that, electrons move in graphene at a pace far exceeding that in silicon. It seemed like a miracle material. But there’s always a “but” and in the case of graphene it was the absence of an energetic gap. Graphene is a semi-metal, not a semiconductor. Practically speaking, graphene has no true OFF state. And anyone who knows a thing or two about switches knows that they function best when they can be turned either ON or OFF. ON and slightly less ON is not nearly as useful.

Now scientists at Michigan Technical University have developed a brand new way to generate an OFF state in graphene: they’ve coupled it with an insulator – boron nitride nanotubes. As it turns out, semi-metal plus insulator equals semiconductor. This is a similar strategy to the first ever color-tunable graphene oxide LED recently reported. In this case, graphene sheets are produced and then perforated with tiny holes from which the nanotubes are then grown. The junctions where the nanotubes hit the graphene act as stopping points for electrons and give rise to the switching behavior. Explains lead study author Yoke Khin Yap, "imagine the electrons are like cars driving across a smooth track. They circle around and around, but then they come to a staircase and are forced to stop."

New graphene-nanotube switch: this computer generated image shows the structure of the switch. From the Michigan Technical University.New graphene-nanotube switch: this computer generated image shows the structure of the switch. From the Michigan Technical University.

This research is a great example of why you should never give up when faced with what appears to be an insurmountable challenge. A little ingenuity, this case in the form of a nanotube, can yield a remarkable outcome.

Via Phys.org, Michigan Technical University, and Scientific Reports.