Researchers devise a way to a create graphene transistor

Jul 18, 2012 by report
Two different epitaxial graphene materials combined to a monolithic transistor. Image from Nature Communications 3, Article number: 957 doi:10.1038/ncomms1955

(Phys.org) -- Researchers in Germany appear to have found a way to create a monolithic (integrated) graphene transistor, using a lithographic process applied to silicon carbide, a breakthrough that could lead to computers based on graphene chips, rather than those that use silicon. This is significant because researchers are beginning to see the light at the end of the tunnel regarding the degree to which silicon can be used to make smaller and smaller chips. Using graphene wouldn’t necessarily allow for smaller chips, but because it conducts electricity faster, it would allow for faster chips without having to downsize. The German researchers working with another group from Sweden, describe the new process in their paper published in the journal Nature Communications.

By now everyone has heard that graphene is expected to take the world by storm over the next few years as ways are found to make use of its amazing properties (it’s just one carbon atom thick and is the fastest conductor ever found). The problem of course is in trying to work with such a thin material; it’s hard to connect to other metals such as electrodes and breaks easily. Another problem is that it’s not a natural semiconductor, which is a material that is conductive in one state and to not conductive in another. Semiconductors are what allow computers to store “1s” and “0s”. Thus, to use graphene in a computer, a way needs to be found to allow it to behave as a semiconductor so that can be fashioned. That way appears to have now been found.

The new research is based on earlier research that found that if the crystal, silicon carbide is baked just right, the silicon atoms on its surface are pushed out of it leaving just a single layer of carbon, i.e. graphene. The result is a material that suggests a transistor is possible due to the graphene layer remaining affixed to more layers of (which is a semiconductor) below it. To make a transistor, the team used a high energy beam of charged atoms to etch channels into the material to create the parts needed for a transistor to run; namely, gates, drains and sources. They also found that using oxygen gas during the etching of the middle channel converted it from a contact into a gate. The end result is a fully functioning transistor.

Because the researchers scaled up the transistor size to allow for easier research, it’s not yet known how much faster the new transistor actually is, or how fast those might be once they are scaled down. What is now known though, is that it can be done, and that is the breakthrough computer engineers have been waiting for.

Explore further: Using strong lasers, investigators observe frenzy of electrons in a new material

More information: Tailoring the graphene/silicon carbide interface for monolithic wafer-scale electronics, Nature Communications 3, Article number: 957 doi:10.1038/ncomms1955

Abstract
Graphene is an outstanding electronic material, predicted to have a role in post-silicon electronics. However, owing to the absence of an electronic bandgap, graphene switching devices with high on/off ratio are still lacking. Here in the search for a comprehensive concept for wafer-scale graphene electronics, we present a monolithic transistor that uses the entire material system epitaxial graphene on silicon carbide (0001). This system consists of the graphene layer with its vanishing energy gap, the underlying semiconductor and their common interface. The graphene/semiconductor interfaces are tailor-made for ohmic as well as for Schottky contacts side-by-side on the same chip. We demonstrate normally on and normally off operation of a single transistor with on/off ratios exceeding 104 and no damping at megahertz frequencies. In its simplest realization, the fabrication process requires only one lithography step to build transistors, diodes, resistors and eventually integrated circuits without the need of metallic interconnects.

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User comments : 17

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h20dr
4 / 5 (1) Jul 19, 2012
Wow, this is cool beans.
Code_Warrior
3.7 / 5 (3) Jul 19, 2012
It is really a MOSFET where the metal has been replaced with graphene, or a GOSFET. I wouldn't exactly call this a graphene transistor. If they found a way to dope graphene to form the semiconducting layer, then I might be inclined to agree that they created a graphene transistor. While the thickness of the gate material may have some effect on gate capacitance, the bulk of gate capacitance is a result of electrode area rather than its thickness. In addition, no current flows into the gate so it is unclear to me what advantage the graphene brings to the table as a gate electrode material. In addition, other than having graphene electrodes on the source and drain, the current still flows through the doped silicon carbide so I fail to see the advantage of the graphene. I wouldn't hold my breath for major performance improvements over a standard MOSFET. It seems to me that the novel process used to create the graphene layer is more interesting here.
antialias_physorg
3.8 / 5 (4) Jul 19, 2012
It is really a MOSFET where the metal has been replaced with graphene, or a GOSFET. I wouldn't exactly call this a graphene transistor. If they found a way to dope graphene to form the semiconducting layer, then I might be inclined to agree that they created a graphene transistor.

What do you think the 'T' in MOSFET stands for?
In addition, no current flows into the gate

A FET does not require current to flow into the gate (that is why there is the 'FE' for 'field effect' in FET)

Graphene allows for higher frequencies of gate switching because the field effect mobility in graphene is much higher than in silicon - this (should) translate directly into faster transistors if they were to be manufactured at the same size.
Daxwax
4 / 5 (1) Jul 19, 2012
Graphene Valley, Germany? :)
Code_Warrior
3 / 5 (2) Jul 19, 2012
My point in stating that no current flows into the gate was to question how the conductivity of graphene improves the performance of the gate since there is no current flow that could possibly take advantage of the improved conductivity of the gate.
Graphene allows for higher frequencies of gate switching because the field effect mobility in graphene is much higher than in silicon - this (should) translate directly into faster transistors if they were to be manufactured at the same size.
The channel of their transistor is not formed in the graphene it is formed in the doped silicon carbide, look at their diagram. The graphene is only used as the electrodes that make the connection to the doped silicon carbide. Therefore, the greater electron mobility of the graphene does not contribute to the performance of the transistor. Since the graphene is only used to replace the metal electrodes the device is simply a silicon carbide GOSFET. That was the point of my first post.
Bewia
2.7 / 5 (3) Jul 19, 2012
Graphene allows for higher frequencies of gate switching because the field effect mobility in graphene is much higher

This was just the point of Code_Warrior - in this transistor the speed is limited with mobility of charge carriers in silicon carbide bellow graphene layer, not inside of graphene. Because what we can see is the SiC transistor with graphene electrodes, not graphene transistor as such. You shouldn't to underrate the IQ and qualification of the other posters so apparently - they understand their business better than you at many cases.
antialias_physorg
2.7 / 5 (3) Jul 20, 2012
My point in stating that no current flows into the gate was to question how the conductivity of graphene improves the performance

I don't understand why you would pose this question since the conductivity or lack thereof is nowhere an issue in the structure described in the article.
The advantage comes from the ability to switch the gate faster - not from an increased mobility of the charge carriers in the substrate.
Code_Warrior
3.5 / 5 (2) Jul 20, 2012
I don't understand why you would pose this question since the conductivity or lack thereof is nowhere an issue in the structure described in the article.
That is my point, the improved conductivity of the graphene electrode contributes nothing to the performance of this device.
The advantage comes from the ability to switch the gate faster - not from an increased mobility of the charge carriers in the substrate.
The channel is formed by attraction of charge carriers in the doped silicon carbide to the region under the insulating layer below the gate electrode. The speed with which the charge carriers can migrate to/from the channel region in the silicon carbide impacts the maximum switching frequency. Switching the gate faster doesn't help if the charge carriers can't migrate to/from the channel region fast enough to keep up with the gate. Consequently, I don't see the advantage of using graphene over metal in the electrodes of this device.
antialias_physorg
4 / 5 (1) Jul 20, 2012
That is my point, the improved conductivity of the graphene electrode contributes nothing to the performance of this device.

Yes. That's why I wonderd why you brought it up. The article doesn't make a mention of it. So why make a big deal about it?

The speed with which the charge carriers can migrate to/from the channel region in the silicon carbide impacts the maximum switching frequency

The way I remember it the limiting factor for MOSFET frequencies (non-power MOSFETs) isn't charge carrier mobility but the capactiance between source and drain - which is heavily influenced by the gate material.
At very low gate widths we're also not dealing with charge carriers being in thermal equilibrium with the substrate, so the mobility values become sort of iffy (variations due to scattering events become significant).

But in summary: they should just build the damn thing an see what it can do. Wouldn't be the first surprise transitor physics has thrown up.
FastEddy
1 / 5 (1) Jul 20, 2012
... it would allow for faster chips without having to downsize ..." AND a whole lot cooler. Faster electron migration (transport) through the device with lower resistance would dramatically reduce temperature problems and thus eventually smaller devices. If all of the on-chip power pathways were Graphene as well, well ...
FastEddy
1 / 5 (1) Jul 20, 2012
The arguments above should not be limited to whether or not this is a good idea, because it obviously is a very good idea. And when it comes to size, the larger versions may prove to be more practical for the real world ... More efficient Power MOSFETs being as or more important than a need for ever smaller transistors.
FastEddy
1 / 5 (1) Jul 20, 2012
" ... Another problem is that its not a natural semiconductor ..." Correct, Graphene is a "natural" and nearly perfect conductor, thus the FET scenario becomes a more perfect gate valve for current flow through the device, ... a non-linear, analog switch rather than a purely digital one, too.
Code_Warrior
not rated yet Jul 21, 2012
AND a whole lot cooler. Faster electron migration (transport) through the device with lower resistance would dramatically reduce temperature problems and thus eventually smaller devices...when it comes to size, the larger versions may prove to be more practical for the real world ... More efficient Power MOSFETs being as or more important than a need for ever smaller transistors.

Most of the device's heat is generated in the active channel region. The channel region is NOT made from graphene it is made from silicon carbide so the advantages you are touting for current flow through graphene do not exist in the channel region of this device. Silicon carbide has better thermal conductivity than pure silicon, but defect free silicon carbide crystals are very difficult to produce and have limited its use to Power devices like MOSFETS and LEDs, and small scale ICs for demanding thermal applications, so we are already taking advantage of silicon carbide in these applications.
Code_Warrior
not rated yet Jul 21, 2012
The arguments above should not be limited to whether or not this is a good idea, because it obviously is a very good idea.
My initial and subsequent posts are more related to the idea that this is some kind of graphene transistor. It is not a graphene transistor. That does not mean that I think the research is not any good. As I have already stated, I am more intrigued by the novel approach used to create the graphene electrodes. Since we are unable to inexpensively produce sufficiently defect free silicon carbide crystals for use with large scale integrated circuit mfg I am wondering if the process used to make the electrodes can be scaled up to produce high quality sheets of graphene on the scale of large wafers. Will the process produce wafer scale sheets of high quality graphene where defects are limited to those present in the silicon carbide substrate? If so, then this could be more valuable than any device related performance improvements.
sirchick
not rated yet Jul 21, 2012
How many times greater is the performance of graphene if it can replace silicon in computers?
Cave_Man
not rated yet Jul 21, 2012
Consequently, I don't see the advantage of using graphene over metal in the electrodes of this device.


I think the point is that pretty soon all of the connected 'tissue' in the computer brain can already theoretically be replaced by carbon nano structures (wires/tubes/sheets) and the last big part was creating a carbon nanostructure using materials that improve it's ability to integrate into aforementioned 'tissue' that can function as a transistor.

At least that is my understanding. I am not an expert and have very little hands on experience, except with consumer tech which everyone should be versed in. If you don't know how to build a computer and at least the basics behind each part then you dont really know anything at all.

Funny thing is this all seems to be leading to the evolution of a new species. I almost don't want to use organic terms with these tech concepts but they fit so well. We're even using carbon, soon the machines will be as alive as you or I. Be prepared
eachus
not rated yet Jul 22, 2012
Cool it guys. Yes, this is a GOSFET, similar to a MOSFET. But the huge improvement is not in the gate area, it is in the source and drain connections. Since the source and drain are graphene, at least the first level metalization becomes all graphene. This will both conduct more heat out of the gate area, and generate less heat due to resistance losses. Finally, designing the connections right will reduce the capacitance (and reactance) substantially.

Promise a transistor designer lower capacitance, and they might give you their first born. Add in lower resistance, and they will add in their wives. ;-) This is huge, it is practical, and it actually will have significantly lower manufacturing costs than current bleeding edge transistors. If all it does is make practical 28 nm CPUs that run at 10 GHz, great. (AMD has gotten over 8 GHz, but with LN2 or LHe cooling.) Push it to 15 nm, and what's not to like?

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