MIT Reports 400 GHz Graphene Transistor Possible With 'Negative Resistance' 123
An anonymous reader writes "The idea is to take a standard graphene field-effect transistor and find the circumstances in which it demonstrates negative resistance (or negative differential resistance, as they call it). They then use the dip in voltage, like a kind of switch, to perform logic. They show how several graphene field-effect transistors can be combined and manipulated in a way that produces conventional logic gates. Graphene-based circuit can match patterns and it has several important advantages over silicon-based versions. Liu and co can build elementary XOR gates out of only three graphene field-effect transistors compared to the eight or more required using silicon. That translates into a significantly smaller area on a chip. What's more, graphene transistors can operate at speeds of over 400 GHz."
Not negative resistance (Score:5, Informative)
"in which it demonstrates negative resistance (or negative differential resistance, as they call it)"
Negative resistance and negative differential resistance are not the same thing. Negative resistance would mean the current flows against the voltage. Negative differential resistance just means that the current goes down when you increase voltage.
The first one is not possible (unless you've got an external energy source driving the current) because it would imply a perpetuum mobile. The second is unusual, but doesn't violate any fundamental laws of the universe.
Summary incorrect - Don't need 8 transistors in Si (Score:3, Informative)
Two NMOS transistors and a resistor can perform an XOR in Si. I remember interviewing at Intel in 1980, and every damn interview question was about XOR gates. First was an XOR gate in TTL, then an XOR gate in CMOS, and finally an XOR gate in NMOS. Apparently I passed all three questions, 'cause they offered me a job.
Not MIT's work (Score:4, Informative)
MIT may have reported it, but the research comes out of UC Riverside. Give credit where credit's due; interesting research isn't only done at MIT, Stanford, or Cambridge.
Re:How was the estimation of 400 GHz made? (Score:5, Informative)
>And what prevents silicon transistors from operating at frequencies over 400 GHz in theory?
http://en.wikipedia.org/wiki/Electron_mobility [wikipedia.org]
Simply put, electrons (and holes, if you're looking at the other way) can only move so quickly through a given material.
Silicon is already there (Score:5, Informative)
Silicon transistors with sub picosecond switching times were fabricated in 2002. That's in the THz range.
What holds back processors today is mostly the RC delay of metal wires.
Re:And again.. (Score:5, Informative)
You're cynicism is valid in this case. This is just rehashing research from 20 years ago on negative differential resistance (NDR) two-terminal devices.
Logic based on two terminal NDR devices has been around for more than 50 years (tunnel diodes, neon tubes, ...). Its big problem is input-output isolation: cascading elements is tricky. But these guys are using four terminal devices in a three terminal NDR mode, so they don't have that problem.
Graphene switches have an on-off current ratio of ~3 (tiny and useless),
Well, that depends. The ECL gates that Cray used for their early supercomputers had nearly constant current. Some specialized applications still use ECL. If you're changing state frequently, low static current may not actually save power. So, if this new technology ever becomes practical, you'll see it in fast clocking cores where essentially every gate and flip flop is busy all of the time. The surrounding support circuits will still be silicon.
Re:Silicon is already there (Score:2, Informative)
Stop being an arrogant asshole. The wikipedia article someone linked to him doesn't actually have any math relating charge carrier mobility to device switching speed. There's no way any reasonable person could expect him to accept it as an explanation in an informal context -- it's like getting angry at someone who asks why airplanes don't fall out of the sky, and isn't immediately satisfied by a link to a page full of basic fluid physics which doesn't explain (or even mention) lift.
As for your so-called contribution (which you're so angry at vovick for ignoring), all you did was bark at him and toss out some numbers which weren't ever going to be meaningful to him. You didn't do any math and you haven't come close to "proving" your point. You get to be rude when you have and someone is denying it in an offensive way. You don't get to be when you're utterly failing at communicating why you believe that silicon transistors can't hit 400 GHz.
Speaking of which, I for one suspect that you're completely full of shit. Plain silicon transistors hit 75 GHz in 1990:
http://www.nytimes.com/1990/03/15/business/company-news-ibm-researchers-increase-speed-of-silicon-transistors.html
And 7-ish years ago, SiGe devices hit 350 GHz at room temp, more at cryogenic temps:
http://www.eurekalert.org/pub_releases/2006-06/giot-gtt061706.php
So please, take your bullshit and shove it back up where it came from. It's not impossible for silicon (or material combinations involving silicon) transistors to switch at extremely high speeds, and 400 GHz at room temp is probably doable. You don't know as much as you think you know. You certainly haven't even lifted a finger to actually prove what you claim, despite all the noise you're generating. You are an example of Dunning-Kruger in action.