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Light-Emitting Particles Yield Faster Computing
schliz writes to tell us that researchers at the University of California San Diego are developing new transistors based on particles called 'excitons' in an attempt to speed up the interaction between computing and communications signals. "Excitons are formed by linking a negatively-charged electron with a positively-charged 'hole'. An exciton decays when the electron and hole combine, emitting a flash of light in the process. By joining exciton-based transistors to form several types of switches, the UCSD physicists were able to achieve switching times on the order of 200 picoseconds."
We're talking about a faster porn delivery system. This quote kind of says it all,
""Excitons are formed by linking a negatively-charged electron with a positively-charged 'hole'."
The original source for this particular experiment is this Science article. The submission was terrible. Press releases should be banned from any site which claims to have intelligent discussion.
An indirect exciton (what these guys are using) is made using three layers. In one layer, you have extra electrons (negative charges). In another layer, you have a lack of electrons (positive charges). In between those two is an insulating layer. If you tune the charge densities and some other parameters (temperature, for example), you can get the positive and negative charges in the two charged layers to align into pairs. Each pair is an exciton.
A normal exciton is a pair like this without the insulator between them. As you might imagine, they don't last very long and pretty much instantly combine. When an exciton combines, it gives off light at a very particular wavelength. Conversely, when light at that particular wavelength is adsorbed by the material, it creates an exciton.
You could imagine creating an exciton with light, making it an indirect exciton (so that it's stable), doing something with it, and then making it a normal exciton again and waiting the picosecond or so it takes for it to collapse and emit light. That's basically what they've done... but it's much harder than I've made it sound.
Disclaimer: I work in a lab that develops both transistors and photocells. I don't know exactly what they did, but based on the summary and the article, I'd surmise the following.
In an organic photocell an incoming photo will excite an electron. The positive and negative charges (electron and hole) will be "linked" together (i.e. they will move around together). In this state they are not useful. However, if you can separate them and draw them in different directions, then you'll get a current. They can only be drawn apart if you create a situation in which it is energetically favorable for them to separate, usually by attracting them to high and low work function contacts. Therefore, in a photocell of this type, you sandwich two materials together - one in which it is easy for holes to move, but difficult for electrons, and one in which it is easy for electrons to move, but difficult for holes (called the hole transport and electron transport layers). Then, you put a bias across the layers by using two dissimilar contact materials, one high work function and one low work function. Note that one contact needs to be transparent (ITO is most common) so photons can get to the middle layers.
Anyway, when an exciton is created it goes on a random walk through the material in which it is created, and will eventually collapse. The 'exciton diffusion length' is the distance over which your average exciton will move before collapsing. You want any created excitons to be within a diffusion length of the interface between the hole and electron transport layers. When the exciton hits an interface, it separates and the charges move towards their respective contacts. Put a load across the contacts, and you've got a working circuit (assuming excitons are being created).
This is a mildly simplified explanation, but it works.
Anyway, you can go the other way - imagine injecting an electron in one side and a hole into the other. You could choose your materials such that they would meet up at the interface and collapse together, emitting a photon. This is an OLED, and is conceptually similar to the photocells I just described.
So now imagine that you make it so that either the hole or electron transport layer is semiconducting. You could set up your device such that a dielectric layer and then a 'gate' contact are touching the transport layers along an axis perpendicular to the nominal current flow through your device (like in a thin-film transistor). Then, the layers would only transport charge (like in a transistor) and hence emit light (like in an OLED) when a voltage is applied to the gate contact. Then you have a thin-film device across which you put a bias that only emits light (and draws current) when it is switched 'on' by the gate contact.
In other words, you've combined a TFT with an OLED. Very, very slick.