All That Matters

Intel's 3D tri-gate transistors have a feature size of only 22 nm. The thin structures are the silicon channels, the thicker ones are the gates and the contacts. Several gates can be used next to each other to enhance the efficiency of the switching. (c) Intel

Finally I am getting around to blog about the latest generation of transistors that Intel presented earlier this months. These transistors reach feature sizes of only 22 nanometres, down from 32 nm. To give you some perspective what this amazingly high integration means: 4,000 of those 22 nm structures fit across the width of a human hair, or similarly, 100 million of these transistors fit on the head of a pin.

Now how did they reduce transistor length scales down by almost a third? Well, even though Intel (and others) is in the business of shrinking transistor for more than 40 years, this time it’s a bit more than a mere scaling exercise. For the first time we have a commercial 3D transistor design on such a scale. In a typical ‘field-effect’ transistor, two electrical contacts are used to run an electric current through a silicon layer. The transistor is switched between an electrically conducting and an insulating state by a gate on top of the silicon. The voltage applied to that gate determines whether current can flow or not. Thereby the gate is able to set the digital ‘1’ and ‘0’ in a transistor.

A problem in shrinking transistors has been the fact that those three electric contacts need a certain minimum space of their own. Furthermore, as the gate has become smaller and smaller, it has been increasingly inefficient to switch the electric current in the silicon layer underneath. For smaller gates the electric fields from the gate just don’t reach that far down into the silicon layer. […]

Data transmission schemes: In TDM different channels are assigned individual time slots in the transmitted pulse. WDM transmits data across different wavelength channels. In OFDM, these wavelength channels are closer together, although it means that the signal needs to be filtered out later on by frequency analysis. Reprinted by permission from Macmillan Publishers Ltd. Nature Photonics, advance online publication (2011)

Here in the UK, the fastest broadband download speeds on offer for fibre optic broadband are 40 Mbit per second, which is much better than the 8 Mbit/s or so offered via conventional copper cables. But to those for which 40Mbit/s is not enough, fear not: In a Nature Photonics paper, Juerg Leuthold and colleagues from the Karlsruhe Institute of Technology have now demonstrated 26 Tbit/s optical data transmission – some 650,000 times faster than those 40Mbit/s.

There are different techniques to send large amounts of data through an optical fibre. For example, in Time Division Multiplexing (TDM), the signal transmission is divided into a succession of time slots, and each channel gets one bit in every time slot. This makes sense if you want to combine a number of slower channels (e.g. from individual households) into one fast fibre. The disadvantage of this scheme is, however, that this needs ultrafast lasers. And because short laser pulses are spectrally broader than longer pulses, TDM systems needs to be capable of dealing with these short laser pulses across a broad range of frequencies, which is demanding to realize.

Another scheme is to use longer laser pulses, which are spectrally much narrower, but then to distribute these across several wavelengths, so that data is transmitted in channels each using a different colour. The drawback here again is that you can only squeeze a limited number of those wavelength into your optical setup. Also, great care has to be taken that the laser pulses don’t overlap in wavelength, as otherwise the signals are mixed up between the channels.

To send even more data than possible with these schemes, modern systems use techniques such as orthogonal frequency-division multiplexing (OFDM). OFDM is already widely used for wifi as well as for 4G wireless networks. In comparison to WDM, the different wavelength channels can be squeezed closer together, which means more channels, and therefore much faster data transmission. […]

Hydrogen sensing at the nanoscale. Hydrogen molecules (red) are absorbed by a palladium nanoparticle (silver) and the resulting changes in optical properties amplified by a gold antenna. (c) Mario Hentschel, Na Liu, Harald Giessen

Sensing the presence of molecules in gases and liquids is a billion dollar business. Just think about all the carbon monoxide detectors in private homes, or blood glucose sensors. In particular for many technical and scientific applications, ultrasmall and precise sensors are desired. This includes sensors to measure gases in catalytic nanoreactors and fuel cells, or the monitoring of biochemical processes.

Laura Na Liu and Ming Tang from the group of Paul Alivisatos, director of Lawrence Berkeley Lab in the USA, and Mario Hentschel from Harald Giessen‘s group at the University of Stuttgart in Germany have now developed a new class of optical nanoscale sensors that are able to measure specific molecular concentrations down to single particles. This, says Alivisatos, “should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing.” Their paper is published this week in Nature Materials (declaration of interest: I was the handling editor of this paper, although I like to stress that I don’t benefit in my day job by blogging about this work). […]