Remember “Fast, cheap, good – pick any two?” How about “Fast, small, low power – pick any two?” Doesn’t ring a bell? This ‘perfect trio’ applies to transistors. Typically, if a transistor is fast, it uses energy like crazy. If it’s really small, it gets very hot. Fast and small often go together, but at a cost – heat. That’s the current status of the most common method for creating transistors – CMOS (complementary metal-oxide-semiconductor). CMOS is approaching its theoretical limit of smallness at about 16 nanometers. It’s very fast, up to 100 GHz, but it’s a ‘gas guzzler’ at those speeds.
Technicians would like even smaller transistors, and fast…and oh-by-the-way using very little power. Probably not going to happen (for a while anyway). But there is something that could be a reasonable trade-off: Transistors made from nanowire.
Published in the journal Nature February 10, 2011 [Programmable nanowire circuits for nanoprocessors] a research team led by Charles Lieber at Harvard University (Massachusetts, USA) have taken a design using silicon-clad germanium nanowires, which they first introduced in 2006, and now have made practical to manufacture in quantity.
Transistors made in the CMOS process are usually etched (actually using acid in the photo-lithographic process). The circuits can’t be any smaller than ‘mask’ that guides the etching, which it is theorized can’t be any smaller than 16 nanometers. Now that’s pretty small compared to some early technologies, and CMOS was also considered low power – but as electronics progresses ‘small’ and ‘low power’ become ever more important – and CMOS doesn’t look like it can go where engineers need it to go.
Instead of etching, nanowire transistors are made by laying down patterns of nanowire. The nanowire is manufactured first and in the case of this research is made of germanium and coated with silicon. This ‘wire,’ which is about 12 nanometers thick (one nanometer is 1/100,000 the thickness of human hair), is used to create ‘logic tiles,’ an arrangement with up to eight logic gates (gates are the ‘on/off’ switches of digital technology). Each of the individual transistors is only 2 nanometers in size, which handily beats the CMOS 16 nanometers. So nanowire transistors can be much smaller.
The innovation by the research team is a new covering for the nanowires, a three-layer dialectric of aluminum oxide, zirconium oxide, and then another layer of aluminum oxide, which allows the transistors to trap a charge and act as nonvolatile memory. After several years of experimentation, this combination of materials and corresponding processes make it possible to produce the ‘logic tiles’ relatively easily. Logic tiles can be linked together to produce complex electronic components.
The catch? Mainly, it’s speed. A CMOS transistor can attain operational speeds (the speed at which gates can open or close, for example) above the gigahertz level (that’s 10,000,000,000 cycles per second). A nanowire transistor can achieve between 10 and 100 megahertz, a whole order of magnitude slower.
Ah, but remember the ‘perfect trio?’ A nanowire transistor uses a lot less power – about 1 nanowatt per transistor versus 10 to 100 nanowatts in CMOS technology. In practical terms, a nanowire transistor won’t compete with CMOS to run the fastest, nastiest game machines on the planet. But it’s combination of very small, very low power and only relatively slow, make it ideal for many kinds of embedded applications, for example sensors embedded in the human body (biosensors).
There is still much work to be done on nanowire transistors before they are ready for commercial application. In particular the researchers need to fine-tune the manufacturing process for the ‘logic tiles’ so that they can properly work together in larger units. Lest anyone gets overly excited about the prospects, this ‘tuning’ could take a year or two. Nevertheless, along with other new transistor technologies (for example, spintronics and photoelectronics), a new generation of digital processors are coming in roughly a five year horizon.