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Graphene Won't Replace Silicon In CPUs, Says IBM
arcticstoat writes "IBM has revealed that graphene can't fully replace silicon inside CPUs, as a graphene transistor can't actually be completely switched off. In an interview, Yu-Ming Lin from IBM Research (Nanometer Scale Science and Technology) explained that 'graphene as it is will not replace the role of silicon in the digital computing regime.' Last year, IBM demonstrated a graphene transistor running at 100GHz, while researchers at UCLA produced a graphene transistor with a cut-off frequency of 300GHz, prompting predictions of silicon marching towards its demise, making way for a graphene-based future with 1THz CPUs. However, Lin says, 'there is an important distinction between the graphene transistors that we demonstrated and the transistors used in a CPU. Unlike silicon, graphene does not have an energy gap, and therefore, graphene cannot be "switched off," resulting in a small on/off ratio.' That said, Lin also pointed out that graphene 'may complement silicon in the form of a hybrid circuit to enrich the functionality of computer chips.' He gives the example of RF circuits, which aren't dependent on a large on/off ratio."
Can't be switched off? We finally found perpetual motion. It's huge!
Yes, but now I cannot stop posting. :(
So why not use it in a current-switching logic like ECL? Yeah, the power consumption might be a bit high, but you only need to make the cache and cores run faster.
That is all.
The thing that strikes me about this is that when something new comes out that has different strengths and limitations, at first we are at a loss for how to make it useful, but then we work around the limitations and are much better off.
The things I'm aware of that make the contrast between high and low important is measurement of those highs and lows and short term memory. So those are potential areas for improvement that could make the technology viable.
Just because they haven't worked out how to do it yet, doesn't mean it can't be done.
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100GHz? 300GHz? 1-fucking-THz?
I started drooling just a bit. Talk about a jump in speed. That's like going from floppies to thumb drives. Maybe even portable hard drives.
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Why they're right:
Graphene is a metal (or semimetal, whatever). Capacitive effects cause your current gain (ratio between input current and output current) to drop with frequency. The highest practical frequency of operation is where the gain is 1. IBM demonstrated a year ago a transistor whose current gain reached 1 at 100GHz (also known as fmax or unity gain frequency). However, that's just current gain. Digital circuits require a voltage gain as well. You can have high current gain but not have a high voltage gain by having a low output resistance.
Why they could be wrong
Without giving a crash course in electronics, computer (not all) transistors these days operate by raising/lowering a potential that either "gates" electrons and keeps them from passing through or allows them to go through freely. How big the gate is depends upon something called the band gap size, i.e, the energy required to move an electron from an atom (valence band) into participating in macro-scale conduction (conduction band). Graphene does not have a band gap.
However, you can artificially create a band gap by excluding the low energy, long wavelength electron states that exist at the bottom of the conduction and top of the valence bands. You do this by patterning your graphene into strips below about 30nm in width. In this way, no electron state with a wavelength longer than the width of the strip can exist. In a few years (right when graphene starts to hit its stride outside academia), such patterning will be possible (Intel is at 32nm now, if you recall).
As IBM breaks out the bad news, causing chip R&D departments of competitors to halt research into graphene, IBM releases a new 500GHz processor next year.
from 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
to 45 2F 6E 40 3C DF 10 71 4E 41 DF AA 25 7D 31 3F
Diamond, on the other hand, has a band gap of 5.5 eV, and has _excellent_ thermal properties.
Put my fist through my alarm clock with its ding-dong death inside my ear. - The Blackjacks.
Most of the freaking chip is cache. Have a look at the floorplan sometime.
Intel engineers sometimes joke that they're the biggest memory vendor that nobody heard of.
The fundamental problem with "doesn't turn off" is that leakage current (IDS(OFF)) is already a major component of chip dissipation, even when we use all sorts of tricks to reduce it. With graphene, that goes from "problem" to "useless." Except for analog, where the transistors don't turn off anyway.
Lacking <sarcasm> tags,
We have been using hafnium oxide instead of silicone to make chip for several years now. Silicone can't handle the tight lattice distances at such small scales anymore.
I called it a mighty Sperm Whale, she called it Finding Nemo.
it still losses its state when you remove power ... what it means is in a traditional transistor there is a fairly wide tween nearly off (you cant really switch them totally off either) and fully saturated (totally on) for the sake of simplicity its the difference tween ~microvolts and ~0.7 volts. This style of transistor that gap is very close together and transistors cause quite a bit of noise when zipping around even at low speeds and low quantities due to current being constantly switched on and off
bottom line is, currently it seems to be the equivalent of holding a gun with a hair trigger while your doing 60mph in a rock quarry, the rate of error is bound to be too high to be acceptable
though "wont" is a silly choice of words, currently impractical would be better, but that would be a "no shit" article or else we would be using the things already for this application
"Analog" is a bit of a simplifying assumption anyhow - if you look closely enough, everything is quantized.
Socialism: a lie told by totalitarians and believed by fools.
because after I seen the statement on gap, I searched and immediately got an article - graphene not only can be equipped with gap, but tunable one. just make double layer graphene: http://spectrum.ieee.org/semiconductors/materials/graphene-makes-transistors-tunable
This is only true if delta-sigma modulation is performed with infinite time-resolution. If, as in almost all digital systems, we're dealing with synchronous clock-strobed circuits, the signal's amplitude resolution is limited by the sampling rate.
DRM: Terminator crops for your mind!
Here's the electronics 101 version: Transistors have three ports. Electrons flow from one (the source/emitter) to another (the drain/collector). The amount of electrons that make it across depends on the mode the transistor is in, and the mode is controlled by the voltage applied to the third port, (the gate/base). There are three modes: off, linear, and saturation. In saturation, the electrons are flowing as fast as they can and small changes in the gate voltage don't matter. In linear mode, the current is directly proportional to the gate voltage - this mode is key to analog circuits. When the transistor is off, very little current gets across (on the order of femtoamps). When they say graphene transistors can't be completely turned off, they mean the amount of current that gets through when it is off is much larger than for normal transistors. It can still be "turned off" in the sense that if you take away all of the electricity, it loses its state, so there's no particular reason that it would be useful for storage.
As the article notes, a likely use would be in combination with more traditional transistors, wherein you could take advantage of graphene's speed, and then have a silicon "boot" to turn off the circuit when it's not in use by cutting off all of the power to that block.
The other problems with graphene mentioned aside, you start to run in to speed of light issues with extremely high frequencies. At high frequencies the wavelength is so short, that it can't travel across a chip in a single cycle. That has some real design issues. For example at the full speed of light a 5GHz signal has a wavelength of 6cm. Ok, not a problem. A core in a CPU is smaller than that so the signal can travel anywhere in a single clock, even taking in to account that wire runs could be longer. However at 50GHz, well then you are only talking 6mm. That's a potential problem. Current chips are larger than that, never mind the wire runs. Maybe if cores are kept small and simple it is fine, but it is getting problematic. At 1THz you are talking only 300 micrometers wavelength.
So even if graphene becomes practical, speeds that high may never make it in to CPUs. That a transistor can operate at those speeds doesn't mean a whole CPU can.