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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'."
What a dopey idea ;-)
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Hmmm.... 200 ps switch times.
A modern processor operating at 2GHz has one clock cycle every 500ps. A signal leaving a flop and travelling to another flop typically goes through about 20 gate delays, yielding a switch time of 500/20=25ps.
Tell me again how this is faster?
Because the one in the submission was fairly content free. You can come to your own conclusions about what its unattributed original source is.
Exciton-based circuits eliminate a 'speed trap' between computing and communication signals
A wikipedia article, but still better than the submission
I'm still scratching my head, but at least it's not drawing blood anymore.
How does a "hole" differ from a positron?
A positron is a real particle with real mass. It is made up of quarks and has the same characteristics as an electron, except that it's charge is reversed.
A "hole", on the other hand, is essentially the absence of an electron where one should be. It's like those sliding puzzles with the 15 tiles that you have to arrange in numerical order. There should be 16 tiles, but one is "missing" creating a hole. This hole moves around by sliding tiles into it.
A similar thing happens in the silicon of a semiconductor. Ideal silicon is a regular grid of molecules that have exactly enough electrons to fill each other's electron shells. With P-type semiconductors, a small chemical impurity is introduced into the silicon grid. This impurity does not have enough electrons to share with the surrounding atoms. So, like the sliding tile puzzle, there is a "hole", a place where an electron could fit. By applying a voltage to the silicon, the hole can be made to move along the grid.
N-type semiconductors are built the same way, except that the impurity that is added to the silicon has an extra electron. This roam around the silicon grid looking for a spot to settle, much like the last kid in a game of musical chairs. The electron can also be moved around by applying a voltage to the silicon.
Now, if you have a mix of P-type and N-type, what happens is that the extra electron in the N-type eventually settles into the "hole" of the P-type. In doing that the electron loses a certain amount of energy, which is emitted as a photon. However, in doing so, it has induced a charge in the semiconductor. The P-type now has more electrons than protons (they were balanced before, despite the presense of the "hole"), and the N-type now has less electrons than protons (it too was balanced before, despite the "extra" electron). This charge imbalance makes it easy for a photon to come along and pop the electron out of the hole and back to the N-type.
By varying the quantities of impurites, and where, and how thick the transitions between P-Type and N-Type silicon, the clever engineers can make all sorts of semiconductor devices.
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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.
Excitons are _not_ supposed to be faster switchers (it even says this in the article).
The value proposition is that they can switch at the same rate as electronic circuits, but where normal electronic circuits have slower interconnect, excitons based switching transistors can use faster interconnect.
Basically electrons traveling down wires travel only about 50-75% the speed of light (as I recall that's some phonon-limit). In addition, there with current MOSTFET transistor technology, the gates are voltage sensitive so you need to charge up the capacitance as well. If a exiton transitor emits a photons, the photon has the potential to travel faster (given the right interconnect medium up to near the speed of light) to the next switch resulting in overall faster computing.
In the short term, this could make some things easier transiting things from one chip to another chip (say a processor to a memory chip), between chips in a multi-chip module (using some inter-die optical interconnect layer), or even from one side of a chip to another (which takes longer than a clock cycle in todays advanced high-speed chips).
In the longer term, these types of breakthroughs may actually make computing faster. For example, if your computation involved no feedback, in principle, it would be limited by switching speed (and many circuit design techniques try to do this today by pipelining clocks with data in the same direction), but with feedback, you eventually become limited by circuit-to-circuit propagation delay (so-called wire-delay). This is probably what they are thinking about it helping, but that type of development is probably much further away.
Electrons and positrons aren't made of quarks. They're fundamental particles.
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