Mark Reed, from Yale, was another speaker at a meeting I was at in New Jersey last week. He gave a great talk about the promise and achievement of molecular electronics which I thought was both eloquent and well-judged.
The context for the talk is provided by the question marks hanging over Moore’s law, the well-known observation that the number of transistors per integrated circuit, and thus available computer power, has grown exponentially since 1965. There are strong indications that we are approaching the time when this dramatic increase, which has done so much to shape the way the world’s economy has changed recently, is coming to an end.
The semiconductor industry is approaching a “red brick wall”. This phrase comes from the International Technology Roadmap for Semiconductors, an industry consensus document which sets out the technical barriers that need to overcome in order to maintain the projected growth in computer power. In the technical tables, cells which describe technical problems with no known solution are coloured red, and by 2007-8 these red cells proliferate to the point of becoming continuous – hence the red brick wall.
A more graphic illustration of the problems the industry faces was provided in a plot that Reed showed of surface power density as a function of time. This rather entertaining plot showed that current devices have long surpassed the areal power density of a hot-plate, are not far away from the values for a nuclear reactor, and somewhere around the middle of the next decade will surpass the surface of the sun. Now I find the warm glow from my Powerbook quite comforting on my lap but carrying a small star around with me is going to prove limiting.
So the idea that molecular electronics might help overcome these difficulties is quite compelling. In this approach, individual molecules are used as the components of integrated circuits, as transistors or diodes, for example. This provides the ultimate in miniaturisation.
The good news is that (despite the Sch??n debacle) there are some exciting and solid results in the field. The simplest devices, like diodes, have two terminals, and there is no doubt that single molecule two-terminal devices have been convincingly demonstrated in the lab. Three terminal devices, like transistors, seem to be vital to make useful integrated circuits, though, and there progress has been slower. It’s difficult enough to wire up two connections to a single molecule, but gluing a third one on is even harder. This feat has been achieved for carbon nanotubes.
What’s the downside? The carbon nanotube transistors have a nasty and underpublicised secret – the connections between the nanotubes and the electrodes are not, in the jargon, Ohmic – that means that electrons have to be given an extra push to get them from the electrode into the nanotube. This makes it difficult to scale them down to the small sizes that would be needed to make them competitive with silicon. And the single molecule devices have the nasty feature that every one is different. Conventional microelectronics works because every one of the tens of millions of transistors on something like a Pentium are absolutely identical. If the characteristics of each of the components were to randomly vary the whole way we currently do computing would need to be rethought.
So it’s clear to me that molecular electronics remains a fascinating and potentially valuable research field, but it’s not going to deliver results in time to prevent a slow-down in the growth of computer power that’s going to begin in earnest towards the end of this decade. That’s going to have dramatic and far-reaching effects on the world economy, and it’s coming quite soon.