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MIT Reports 400 GHz Graphene Transistor Possible With 'Negative Resistance'
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."
http://xkcd.com/678/
And? We're at similiar Ghz to back then but not because we want to be.
"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.
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. CMOS won out because it scales much better. Graphene is a horrible material for traditional logic; it has no bandgap. Graphene switches have an on-off current ratio of ~3 (tiny and useless), whereas similar sized silicon-based MOSFETs have on-off current ratios of ~1000.
There is some interesting work on making a new kind of logic with graphene-based BiSFETs, but it's still not possible to actually fabricate them. In contrast, neuristors, which are another interesting form of nanoscale logic, have been fabricated by HP labs from Mott-insulator based memristors. If I had to put my money on a replacement for CMOS, it would be these neuristors. However, there are still huge engineering challenges that lay ahead. Nonetheless, the Mott-insulator based memristors are already being commercially developed for high density, solid-state memory, with the hopes of eventually replacing flash memory.
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.
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.
>And what prevents silicon transistors from operating at frequencies over 400 GHz in theory?
http://en.wikipedia.org/wiki/Electron_mobility
Simply put, electrons (and holes, if you're looking at the other way) can only move so quickly through a given material.
Cogito, igitur comedam pizza.
Dont change your name, it serves you well.
Tomorrow is another day...
NAND? Don't leave us hanging.
(-1: Post disagrees with my already-settled worldview) is not a valid mod option.
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.
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.
A little-known example of negative differential resistance is the common electric arc. In an arc, as the current increases the arc gets "fatter" (wider), and so the voltage across the arc decreases. Increasing current with decreasing voltage is negative differential resistance. This enables oscillations, which were first encountered as audio noise in electric arc lighting in the mid-1800s. These led to William Duddell's "Singing Arc", in which Duddell added a tuned circuit to the negative resistance, creating a stable audio tone. The next step was obvious; he wired a keyboard to the arc and made the first electronic music.
Danish physicist Valdemar Poulsen took Duddell's audio oscillator and, by placing the arc in a transverse magnetic field, and in a hydrogen atmosphere (and somehow not getting blown up in the process), moved the frequency of oscillation up into the low radio range, around 500 kHz or so. This was the arc radio transmitter. It differed from the more common spark transmitter in that the arc's output oscillation was continuous, while that of the spark transmitter was a damped (decaying) oscillation.
The arc transmitter caught the attention of Cyril Elwell, of Palo Alto, California, who arranged to obtain the rights to the arc from Poulsen, and started commercial production of it with his company, the Federal Telegraph Company. The arc transmitter became a big success in World War One, when transmitters as large as 1 MW (one million watts) output were installed by 1918.
Much as the Fairchild Semiconductor Company spawned several successful companies in Silicon Valley in the 1960s, Federal did so, too, 50 years earlier; refugees from Federal formed well-known companies like Magnavox and Litton Industries.