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New technique opens a gap in graphene



Researchers in Germany and Switzerland have developed a new way to make extremely narrow graphene ribbons with specific widths and electronic bandgaps. The ribbons also have smooth edges, something that is crucial for making electronic devices out of graphene.
Graphene is a flat sheet of carbon just one atom thick – with the carbon atoms arranged in a honeycomb lattice. Since the material was first created in 2004, its unique electronic and mechanical properties have amazed researchers who say that it could be used in a host of device applications. Indeed, graphene might even replace silicon as the electronic material of choice in the future.
However, unlike the semiconductor silicon, graphene has no gap between its valence and conduction bands. Such a bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off. One way of introducing a bandgap into graphene is to make extremely narrow ribbons of the material.

Cutting or unzipping
Until now, these graphene nanoribbons were made using top-down approaches, such as "cutting" the ribbons from larger graphene sheets or "unzipping" carbon nanotubes. Such methods produce ribbons that are relatively wide (more than 10 nm across) with rough edges. For high-efficiency electronics devices, the ribbons need to be much smaller than 10 nm wide and, importantly, their edges need to be smooth because even minute deviations from the ideal edge shapes, "armchair" and "zigzag", seriously degrade graphene's electronic properties.
The new technique, developed by a team led by Roman Fasel of the Swiss Federal Laboratories for Materials Science and Technology (Empa) and Klaus Müllen from the Max Planck Institute for Polymer Research in Germany, is a simple, surface-based bottom-up chemical process. It involves first spreading specially designed halogen-substituted bianthryl monomers onto gold and silver surfaces under a high vacuum. Next, the monomers are made to link up to form polyphenylene chains.

'Important first step'
Fasel and colleagues then remove hydrogen atoms from the polymers by heating up the ensemble. This leads to the polymer chains interconnecting to form planar, aromatic graphene ribbons that are just one atom thick, 1 nm wide and up to 50 nm long. The ribbons are narrow enough to have an electronic bandgap and thus switching properties. "Such switching is an important first step for the shift from silicon microelectronics to graphene nanoelectronics," say the researchers.
And that is not all: the edges of the graphene ribbons are smooth and armchair-shaped, and the ribbons themselves are either straight or zigzagged, depending on the monomers used to make them. The smooth edges will be important for studying fundamental experimental physics too, says the team – for example, observing how magnetic properties of the ribbons change with different edge structures. Until now, previous methods to make graphene nanoribbons always produced rough edges that were difficult to study
The new technique could also be used to dope the graphene ribbons by using monomers containing nitrogen or boron atoms. And monomers with additional functionalities should allow the researchers to create positively and negatively doped ribbons – for making p–n junctions in transistors, for instance.

Solar cells
Going further still, a combination of various monomers might even allow heterojunctions (interfaces between different types of graphene nanoribbon, such as those with large or small bandgaps) to be created. Such structures could be used in applications like solar cells or high-frequency devices. Fasel and colleagues have already shown that this technique is viable by connecting three separate graphene ribbons together using two suitable monomers.
The team, which includes scientists from ETH Zürich and the Universities of Zürich and Bern, is now working on creating the nanoribbons on semiconductor surfaces, rather than on just metallic substrates as in this work. This will be critical for making real-world electronic devices.

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power portable electronics Nanofibres New transistor breaks speed

A new kind of miniature energy harvesting device that generates electricity using nanometre-sized fibres has been unveiled in the US. The nanogenerator could harvest energy from human or other motion to power wireless sensors, personal electronics and even medical implants, claim its inventors at Stevens Institute of Technology and Princeton University.

"We are particularly excited about using the nanofibre-based generators in bio-compatible situations, like embedding the devices in shoes and clothing to harvest energy from the motion of the human body to charge personal electronics such as iPod batteries and cell phones," says team leader Yong Shi, who is a mechanical engineer at Stevens.

The new high power output devices are based on lead zirconate titanate (PZT) nanofibres. PZT has a high piezoelectric voltage and dielectric constants – ideal properties for converting mechanical energy into electrical energy. Unlike bulk thin films or microfibres, PZT nanofibres prepared by electrospinning processes are also highly bendable and mechanically strong.

Embedded in a soft polymer

Shi's team made the nanogenerator by depositing electrospun PZT nanofibres on preformed arrays of electrodes on a silicon substrate. The nanofibres are around 60 nm in diameter and they were embedded in a soft polymer (polydimethylsiloxane, PDMS) matrix. The finished device can be released from the silicon substrate or prepared on flexible substrates, depending on the application desired.

When mechanical pressure is applied on the top surface of the ensemble, it is transferred to the nanofibres via the PDMS matrix. This results in electrical charge being generated thanks to the combined tensile and bending stresses in the nanofibres as they move. This results in a voltage between two adjacent electrodes.

The researchers say that, for a given volume of nanogenerator, the nanofibre device generates much higher voltages and power than devices made from semiconductor piezoelectric nanowires for the same energy input. In theory, the maximum output power from a piezoelectric nanogenerator depends on the properties of the active materials, so the higher the piezoelectric voltage constant of the material between two electrodes, the higher the output voltage and power. What is more, varying the length of the active materials between the two electrodes will also vary the voltage output and current at the same time, explains Shi.

The devices could be used to power wireless sensors, personal electronics and, in the future, biosensors and bioactuators that are directly injected into the human body.

Powered by blood flow

Arthur Ritter – who is director of biomedical engineering at Stevens and was not involved in the research – said, "One of the major limitations of current active implantable biomedical devices is that they are battery powered. This means that they either have to be recharged or replaced periodically. Shi's group has demonstrated a technology that will allow implantable devices to recover some of the mechanical energy in flowing blood or peristaltic fluid movement in the gastro-intestinal tract to power smart implantable biomedical devices."

And, because the technology is based on nanostructures, it could provide power to nanorobots in the blood stream for extended periods of time, he adds. Such robots could transmit diagnostic data, take biopsy samples and/or send wireless images directly to an external database for analysis.

The team now plans to optimize the structure of its nanodevice and simplify the fabrication process. "We are also working hard on implantable bio-applications," revealed Shi.

YOSEPH BUITRAGO  C.I. 18257871   EES   SECCION2

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A pair of physicists in the US has built the fastest ever transistor: one that can operate at a frequency of over 600 gigahertz. Developed by Walid Hafez and Milton Feng at the University of Illinois at Urbana-Champaign, the device is made from the semiconductors indium phosphide and indium gallium arsenide (Appl. Phys. Lett. 86 152101). The work demonstrates the feasibility of making transistors that can operate at frequencies of several terahertz, which could be used in ultrafast communications, high-speed computing, medical imaging and sensors.
The new device is a so-called bipolar transistor, which is very different from the more well-known field-effect transistor. In it, electrons are injected from the "emitter" terminal, travel towards the "base" and are then received by the "collector", an arrangement that allows the device to work faster than a field-effect transistor.
Hafez and Feng have previously built a high-frequency bipolar transistor, but this earlier work focused on reducing the time it takes electrons to pass through the device by minimizing the device's vertical thickness. Their new research further increases electron speeds through the device by slightly varying, or "grading", the composition of the semiconductor layers. This, say the researchers, lowers the band gap in selected areas of the transistor and makes it easier for electrons to travel across the device.
The two physicists have shown their transistor can operate at a frequency of 604 gigahertz, a new record. However, according to Hafez, what is more important is that they have developed a technology that could be used to build transistors operating in the terahertz range. "Projections from our earlier high-frequency devices indicated that in order to create a transistor with a cutoff frequency of 1 terahertz, the devices would have to operate above 10,000 degrees C," he says. "By introducing the grading into the layer structure of the device, we have been able to lower the potential operating temperature for a terahertz transistor to within an acceptable range."
Devices operating at terahertz frequencies (the far infrared) could be used in communications applications or as sensors to detect toxic gases. They could also be used for medical imaging, since the radiation is long enough to Penetrate skin and image What lies underneath.
The Researchers' next step is to show That Can Be Their devices assembled Into circuits.


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record

RTL is the acronym for resistor transistor logic or logic-transistor resistance. It was the first logic family to appear before the integration technology. Belongs to the category of bipolar logic families, or involve the existence of two types of carriers: electrons and holes.

This type of network, presents the phenomenon called power grab that occurs when multiple transistors are coupled directly and input characteristics differ slightly from each other. Then one of them lead before the others placed in parallel (monopolize the flow), preventing the proper functioning of the rest.

Figure 1, is represented, by way of example, a NOR logic gate and its corresponding electronic circuit in RTL logic.

It can be seen as in series with the base of each transistor is placed a compensation resistor (Rc) of a high enough value for the current distribution is more equal as possible and not occurrence of the phenomenon described above.

This circuit arrangement has the disadvantage that with the addition of resistance Rc increases the switching delay, having to be loaded and unloaded through the same input capacitance of transistors but on the other hand, has the advantage of increased output factor (fan-out). Therefore, in the design of these circuits is necessary compromise between output and factor switching delay. Normal values \u200b\u200bare output by a factor of 4 or 5, with a switching delay of 50 nanoseconds.

On the other hand, has a relatively poor noise immunity. The noise margin voltage to the voltage logic 0 threshold is about 0.5 volts, but voltage logic 1 to the threshold voltage is only about 0.2 volts.

may improve travel time by adding a capacitor in parallel with each of the resistors Rc, which would get a new logic family, to be called RCTL. However, the large number of resistors and capacitors difficult to integrate so much the technique, such as RTL, not used in modern designs but can still be found in very old equipment.

The emergence of DTL circuits, with their greater speed and noise immunity meant the end of the RTL circuit.

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Resistor-transistor logic Emitter-coupled logic technology

Emitter Coupled Logic (emitter coupled logic) belongs to the family of MSI circuits implemented with bipolar technology, is the most fastest available in the circuits of type MSI.

History:

Doors with ECL designs have been implemented even with vacuum tubes, and of course with discrete transistors. And the first family with ECL design, the ECL I, appeared in 62 with the first family of integrated circuits. Even at that time the family was faster (a typical propagation delay of 8ns.), And was already the most dissipated.

Today it may seem that much when 8ns CMOS circuits that consume very low (mostly static) far outweigh this benefit, but in reality the ECL technology has also evolved in design and manufacture, and currently get the ns delays significantly lower, with heavy drinking but not exorbitant.

Introduction:

Despite its limited use, this is one of the most rooted logic families, and noble ancestry, in digital technologies. You could even say that in general electronics, because the differential pair, in which the family is based, largely dominates analog integrated circuits. Bipolar family

As it is, the noise margin is not good. In this case not only reduced low margin, but also the margin at high level. This is a consequence of reduced trip logic. And the reason is that for speed should vary little voltage values.

The guiding principle of the family is trying to avoid at all costs that make up the circuit transistors enter saturation. As the switches are between court (or almost cut) and driving. Therefore we will always have driving transistors, so that consumption is continuous. Ie not only are current peaks in the transitions, but we always have a significant consumption in the circuit. On the other hand, the presence of significant currents in the circuit at all times, makes the fan-out is good.

is the fastest form of logic, since the active devices manage to work out of saturation. Also becomes even more making quick changes are even less logical signal (Dt = 800mV), that makes the time for loading and unloading of cargo and parasitic C are even lower ...

ECL circuit is based on the use of a current direction switch, which can be constructed with a differential pair that is polarized with a voltage and current Vr I cte both. the differential nature of the circuit makes it less susceptible to pick up noise.

There are 2 known forms, the ECL and ECL 10K 100k, the 100k is faster but consumes more power.

Structure:

ECL structure is based on a differential pair (Q1-Q2 and Q3) in which a branch is connected to a reference voltage, which determines the threshold HI / LO and another branch with n transistors in parallel to the n inputs. Differential can be obtained simultaneously two with the departure and negated output and very low jitter between them. These outputs are, finally, to respective emitter follower to provide current income and fan-out right, which in many cases can be fed directly 50Ω lines. It is common to have separate power pins for the latter transistors because, unlike the differential pair, the current varies with the signal if the two transistors are not connected to impedances equal. Feeding separately prevents these changes reach the differential pair.

This structure produces output simultaneously OR / NOR: any high level input causes the Q5 emitter pass high level and high level of Q6. By comparison, the structure function only produces TTL NAND.

Unlike other technologies (TTL, NMOS, CMOS), the ECL is supplied to the positive (Vcc) connected to ground, with the power between 0 and -5 '2 V, usually. Some families allow sea-5V VEE, to share food with TTL circuits.

Applications:

addition to the ECL logic families I, II ECL, ECL III and ECL100K ECL10K, ECL technology has been used in LSI circuits: Logic Arrays

Reports ( Motorola, Fairchild)

Microprocessors (Motorola, Ferranti F100)

To improve the performance of CMOS technology, the ECL is incorporated in certain critical functions in CMOS circuits, increasing speed while maintaining low total consumption.

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TTL is the abbreviation for transistor-transistor logic, ie "transistor-transistor logic." Is a logic family or what is the same, a construction technology of digital electronic circuits. In components manufactured with the technology TTL input and output elements of the device are bipolar transistors.

Features:

• Its characteristic voltage is comprised between 4.75 v and 5.25 V (as seen a very narrow range.)
• logic levels are defined by the voltage range between 0.2 V and 0.8 V for the state L (low) and 2.4 V and Vcc for the state H (high).
• transmission speed between logic states is the best base, but this feature makes increasing their consumption and its main enemy. Why different versions have appeared as FAST TTL, LS, S, etc and lately the CMOS: HC, HCT and HCTLS. In some cases it may achieve little more than the 250 MHz
• TTL output signals degrade quickly if not transmitted through additional transmission circuit (can not travel more than 2 m cable without serious losses).

Historical Review:

TTL

While technology has its origins in studies of Sylvania, was Signetics the company that popularized by its greater speed and immunity to DTL noise than its predecessor, offered by Fairchild Semiconductor and Texas Instruments, mainly. Texas Instruments TTL immediately began to manufacture, 74XX and his family would become an industry standard.

Families

TTL:

technology

TTL circuits are normally prefixed with the number 74 (54 military and industrial series). Then a code of one or more digits representing the family and later one of 2 to 4 with the circuit model.

As families can be distinguished:

TTL: TTL Standard Series

-L (low power): A series of low

TTL-S (Schottky) FAST (using Schottky diodes) TTL-

AS (advanced schottky): Improved version of the previous series

TTL-LS (low power Schottky) Combining technologies L and S (the family is more widespread)

TTL-ALS (advanced low power schottky): Improved version of the AS series

TTL-F (FAST: Fairchild advanced Schottky) TTL-AF

( advanced FAST): Improved version of the F-HC

TTL (high speed CMOS): Actually it is not technology but CMOS TTL bipolar

-HCT TTL (high speed C-MOS): HC Series equipped with logic levels

TTL compatible TTL-G (GHz C-MOS) GHz (From PotatoSemi)

Versions:

A initial family 7400, or 74N, soon added a slower version but low consumption, and its counterpart 74L rapid, 74H, which was the basis of transistors doped with gold to produce recombination centers and reduce the average life of minority carriers in the base. But the problem of speed comes from a family that is saturated, ie cutting pass transistors into saturation. But a saturated transistor contains an excess of charge at the base to be removed before it starts to cut, extending their response time. The saturation state is characterized by the collector to less stress than the base. Then a diode between base and collector divert excess power by preventing the introduction of excessive loads on the base. Due to their low forward voltage using Schottky barrier diodes. So you have the 74S and 74LS families, Schottky and low power Schottky. The 74S and 74LS completely displaced 74L and 74H, because of its superior product delay · Consumption. Improvements in the manufacturing process led to the shrinking of transistors allowed the development of three new families: 74F (FAST: Fairchild Advanced Schottky Technology) Fairchild and 74AS (Advanced Schottky) and 74ALS (Advanced Low Power Schottky) of Texas Instruments. Subsequently, National Semiconductor 74F redefined for the case of buffers and interfaces, becoming 74F (r).

Technology:

TTL technology is characterized by three stages, the first of which he is appointed:

emitter input stage. Multiemisor transistor is used instead of the diode array DTL.

phase separator. It is a common emitter connected transistor which produces at its collector and emitter signals in counterphase.

Driver. It consists of several transistors, separate into two groups. The first is connected to the emitter of the phase separator and drain current to produce the low level output. The second group is connected to the collector phase splitter and produces the high level.

general

This configuration differs slightly between devices of each family, especially the output stage, which depends on whether they are buffers or not and if they are open collector, three states (ThreeState), etc. Major variations among the different families: 74N, 74L and 74H differ primarily in the value of polarization resistance, but most 74LS (not 74S) lack the characteristic multiemisor TTL transistor. Instead, they have a matrix of Schottky diodes (as DTL). This allows them to accept a wider range of input voltages up to 15V on some devices, for easy interface with CMOS. It is also quite common in circuits connected to the bus, place a pnp transistor to the input of each line, to reduce the input current and thus charge less for the bus. There are interface devices that integrate adaptation bus impedance to reduce reflections or increase speed.

Yoseph BUITRAGO CI 18257871 EES SECTION 2

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TTL CMOS Technology Semiconductor Technology

CMOS technology is an extension of MOS technology and is by far the most popular today. MOS technology is divided into two sub-technologies (PMOS and NMOS) that match each with the type of impurity used (P or N). As already mentioned the type of impurities used determines the type of transistor. It turns out that technology can only fabricate NMOS transistors N type impurities and PMOS technology only P-type impurities

Years ago there were design uses only one type of transistors but if you combine both designs are simpler.

This is where CMOS technology enables the manufacture of both types of transistors. Virtually everything is manufactured in CMOS technology with the exception of some transmitters used in optical communications with tens of GHz frequencies where the use of faster technology is a necessity.

As an extension of MOS technology, CMOS technology inherits the advantages of low consumption and high integration and therefore also the disadvantages of not so high operating speeds. The truth is that's what happened when technology was becoming popular but now after the huge investments made in this technology can claim to have overcome many disadvantages associated with their old mother MOS technology.

then presented as a curiosity sections es decir cortes de esquemas de tecnología CMOS:

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What happens to our planet?

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