Nanotech gets ready to take on transistors

For half a century, electronics has been synonymous with transistors. These ubiquitous electronic switches have changed beyond recognition since the first walnut-sized hand built devices: you can now buy billions in a single chip for a few pounds. Yet although the process of smaller, cheaper, faster still has some way to go, the search is on for the next fundamental electronic device that will replace transistors to fuel another half century of advances, in the same way that transistors replaced 50 years of valve technology.

One of the more exciting possibilities is barely electronic at all. Roxatane devices are more mechanical than anything else: a combination of a circular molecule wrapped around another that acts as a spindle, with end stops that prevent the whole thing drifting apart. Unlike normal chemical compounds, the structure is kept together by purely mechanical means rather than the usual shared electron bonds, so it can change state rapidly without the sort of energies normally involved in breaking apart and creating compounds.

Roxatanes -- and their close relatives, the linked ring catenates -- have been known about since the early 1980s, but they were produced in very small quantities by chance during other reactions and seen as exotic curiosities of no real use. Advances in mechanical chemistry have changed that: they can now be mass-produced cheaply and reliably by reacting chemicals together on templates that hold the components in the right alignment to create the finished article.

With the right structures, roxatane molecules can be built to be bistable -- like a light switch, they can remain in one of two stable states. You only need energy to switch between them. If one of the states has a higher electrical resistance than the other, the molecule becomes a memory device that can store a one or a zero that can be read by electronic circuits. By arranging the molecule so that it can flip between the states when it gets the right electrical pulse, a complete device can be built that's only a few hundred atoms big.

This is where HP Labs and others get excited. Create two sets of parallel wires -- one running north-south, the other east-west -- and put roxatane molecules at each cross-over, and you have a memory architecture that can theoretically shrink much further than anything depending on transistors -- achieving a feature size of two to three nanometres. It should also be less power hungry and faster. The semiconductor physics underpinning transistor design has limits on how small voltages can be before they stop being effective: different and more advantageous rules apply for molecular switches.

It's also possible to configure the switches to work as logic gates, so that they change state depending on a combination of inputs. Creating a circuit that detects when either or both of two inputs are present is a simple matter of arranging switches in the right order with some extra components: it's been far harder to make an inverter that's got an output of one when the input is zero and vice-versa. Called a NOT gate, this is essential for all computer logic -- and it is this that HP recently announced.

It's an involved process, turning a signal upside-down with just switches. In effect, a high signal sets a switch that couples a separate low signal to the output, whereas a low signal doesn't change a switch that's set to high, but to make this happen properly in a grid of switches which share their inputs and outputs a series of clock signals have to switch between components in the right sequence. A beneficial side effect of this is that the output signals are restored to full strength, just as long as the inputs are good enough to change the switches -- this regeneration process is essential in complex circuits with many stages.

At this point, you have all the building blocks needed to duplicate existing processor designs in the new technology. All that remains is making the new designs economic to produce and reliable enough to work commercially.

That is a huge task, and one that nobody active in the field underplays. There is considerable interaction between wires carrying different signals a couple of nanometres apart, and when you combine thousands or millions of such wires into a small space it can be nightmarish telling wanted signals from crosstalk. HP's approach is to subdivide the big grids of wires into isolated patches by introducing changes in the chemistry of the wires at the boundaries. Producing the circuits may be as simple as stamping them out using a version of contact printing: without the need to align tens of layers of careful chemistry, the potential is there for even greater economies in production than transistor-based silicon enjoys.

The first practical devices using these techniques will probably be hybrids, much as there was a 25-year period where transistors and valves coexisted, often in the same circuits. Work is underway to modify existing CMOS design, test and production processes with nanotechnology capabilities; a memory circuit might use existing transistor engineering for its interface to the outside world and to control a core array of roxatane storage devices. There are other chemicals that can be used in crossbar architectures too; they don't have the full potential range of benefits roxatanes enjoy, but may be more amenable in the short term. Still others may be better all round. Work continues.

There is no guarantee that any of these ideas will successfully see full production, but as the researchers and engineers conquer each new problem their confidence is growing. New ideas impracticable in silicon -- such as folding layers over on top of each other -- are helping to feed that excitement. Stan Williams, director of HP's Quantum Science Research group, is on record as saying "I think we've picked a winner -- something that will allow Moore's Law to continue for another half century. I used to think that was impossible. Now I think it is inevitable."