Light-Emitting Particles Yield Faster Computing 65
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."
Here are some better articles (Score:5, Informative)
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 [physorg.com]
A wikipedia article, but still better than the submission [wikipedia.org]
I'm still scratching my head, but at least it's not drawing blood anymore.
Re:Holes vs. Positrons (Score:2, Informative)
a positron is the antiparticle of an electron, having spin 1/2 and charge +e, while a "hole" is exactly what you've described; a hole in a sea of electrons.
Re:Holes vs. Positrons (Score:3, Informative)
Re:Holes vs. Positrons (Score:4, Informative)
FYI: An exiton is just an imaginary particle. (Score:4, Informative)
An exiton is just an electron "bounced" out of its location, leaving a positively charged hole behind. The negative electron and the positive hole (the imaginary particle) pair forms an "exotic atom" state similar to a hydrogne atom, but with a much lower binding energy and a much larger size.
This behavior is the standard behavior semiconductors.
It appears the difference here is that when the electron/hole pair reunites, a photon is emitted.
This appear awfully close to what an LED is, and the article is void of any information to distinguish this component from the LED.
Re:Here are some better articles (Score:5, Informative)
The original source for this particular experiment is this [sciencemag.org] 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.
Re:Here are some better articles (Score:4, Informative)
Re:Holes vs. Positrons (Score:3, Informative)
An electron hole is a lack of positive-energy electrons. A positron in the Dirac model is a lack of a negative-energy electron.
Re:Holes vs. Positrons (Score:5, Informative)
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.
excitons are not supposed to be faster switchers (Score:5, Informative)
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.
Re:Holes vs. Positrons (Score:5, Informative)
Electrons and positrons aren't made of quarks. They're fundamental particles.
Re:excitons are not supposed to be faster switcher (Score:4, Informative)
Re:Holes vs. Positrons (Score:2, Informative)
You wouldn't have one in a quantum sea of them. The hole is basically a missing electron in an otherwise full valence band. You have to be talking about semiconductors or similar chemical contexts for the concept to make any sense.
A positron, on the other hand, is a particle that could exist on its own in a vacuum.