This is the abstract of the work done on the topic Paper transistor. Here the history, advantages and its application in various electronics devices are discussed.
Paper transistor is one of the most important inventions in modern times. It revolutionized electronics and unlocked new and much, much smaller circuits. Portuguese researchers have produced the first paper-based transistors. To be more precise, they have made the first field effect transistors (FET) with a paper inter strate layer. According to the research team, these new transistors offer the same level of performance as state-of-the-art oxide based thin film transistors (TFTs) produced on glass or crystalline silicon substrates.
A common paper sheet is used on both sides in the fabrication of the paper transistor. It means paper is used instead of silicon which was invented by a Portuguese team and is manufactured at ambient temperature. This way, the paper acts simultaneously as the electric insulator and as the substrate. Furthermore, electric characterization of devices showed that the hybrid FETs' performance outpace those of amorphous silicon TFTs, and rival with the actual state of the art of oxide thin film transistors.
There is an increased interest in the use of biopolymers for low-cost electronic applications. Since cellulose is the Earth's major biopolymer, some international teams have reported using paper as the physical support (substrate) of electronic devices but no one had used paper as an interstrate component of a FET.
The cellulose not only used as substrate but also act as electric insulator by fabricating the device on both sides of the paper. Add to that the paper transistor outperforms the amorphous silicon thin-film transistor used in modern LCD displays and is up to par with the very latest oxide thin-film transistors, which are still a rare sight.
Since paper is a flexible and a biomaterial it would open up new possibilities for bendable displays, bio-labeling, small and cheap displays that could be used for labeling of varies things and more. Our only concern so far is the degradability, i.e. the lifespan of displays made from paper.
These results suggest promising new disposable electronics devices, like paper displays, smart labels, smart packaging, bio-applications, etc.
These transistors have been created at the Center of Materials Research CENTRO DE INVESTIGACAO EM MATERIAS or CENIMAT at the Faculty of Technology of the University of Lisboa (UNL), Portugal. The research team was led by Elvira Maria Fortunato, Associate Professor, and Rodrigo Martins, Professor at UNL.
Fortunato managed to produce transistors without using the (now traditional) silicon or silicium. In the “Invisible” project, her team uses ceramic materials, for example part of cements or bricks, as semiconductors in transistors. The conductor oxides can be spread on a glass or transparent surface - they have the optical qualities of a glass but also the electrical properties of a metal, so you can see leds and lights but no wires connected to them. One of the applications of this technology is related to the development of transparent PC and laptop screens, or transparent mobile phones.
The research team fabricated the devices on both sides of the paper sheet. This way, the paper acts simultaneously as the electric insulator and as the substrate.Furthermore, electric characterization of devices showed that the hybrid FETs' performance outpace those of amorphous silicon TFTs, and rival with the actual state of the art of oxide thin film transistors. Submicron nonvolatile memory transistors were fabricated by exposing silicon-on-insulator (SOI) buried oxides to hydrogen at elevated temperatures to generate mobile protons in the buried oxides. By switching the polarity of the bias to the SOI substrate, the mobile protons in the buried oxide were transported to either the top or bottom Si-buried oxide interface, switching the leakage current of top gate transistors from an ON to an OFF state.
There is also news that there is another research going on in Korea. Researchers at the INHA University in South Korea have developed a transistor made of a kind of cellulose paper, instead of a semiconductor alternative. The group was trying to come up with a way to produce an electronic component which is more environmentally-friendly than those which use semiconductors such as silicon, which is toxic and brittle.
The transistor was made by adding a carbon nanotube to the cellulose paper, depositing electrodes to the top and bottom of the device in order to help it conduct electricity.
The performance of a selective floating gate (VGS) n-type non-volatile memory paper field-effect transistor is at par with the normal transistor even challenging the existence of normal BJT. The paper dielectric exhibits a spontaneous polarization of about 1 mCm-2 and GIZO and IZO amorphous oxides are used respectively as the channel and the gate layers. The drain and source regions are based in continuous conductive thin films that promote the integration of fibres coated with the active semiconductor. The floating memory transistor writes, reads and erases the stored information with retention times above 14500 h, and is selective (for VGS > 5 ± 0.1 V). That is, to erase stored information a symmetric pulse to the one used to write must be utilized, allowing to store in the same space different information.
The applications of this technology is related to the development of transparent PC and laptop screens, or transparent mobile phones.The direct application is in the area of screens and communications, as the transistors to which the discoveries apply are the thin film transistors(TFT, used in PDA's, computer and television thin screens).
During the last SID Conference, in Los Angeles, Samsung presented their new generation of mobile phone LCD (Liquid Crystal Displays) and OLED (Organic Light Emitting Diodes) screens, supported by the discoveries ofthe researchers in the field of transparent electronics. The transparent transistors can be produced in large-scale, at room temperature, with lower fabrication costs than ever before, have a final performance which is much superior to the current TFT screens, are environment-friendly and biodegradable.
Other fields of application include projection systems, supercompact Xeroxes, intelligent windows, automobile windows and navigation systems, interactive and dynamic advertisements, toys and game. Having these opportunities in mind, HP(USA) and Fiat(Italy) are quickly adapting/or trying to adapt the technology in order to be counted as one of the first company to produce it for the consumers/market. Processor making giant, INTEL, recently unveiled a static random access memory chip which contains nearly three billion transistors and measures only 22 nanometres in size.
The paper transistors are also related to this area of research. As the materials are made at room temperature, the temperature does not affect the paper material, which can be used by the team. Furthermore, the paper is an insulating material, from the electrical point of view - the paper becomes not only the physical support of the transistor, but part of it as well. Cellulose is the main biopolymer on our planet, and the costs of its use for the production of electronic components are very low. To produce the famous thin-film paper transistors, the team used… greaseproof paper and photocopy paper sheets.
Disposable electronics, of large-scale consumption and low cost, intelligent cards, smart packaging, smart labels, posters and flyers, paper displays, biological applications, interactive newspapers, RFID tags are among some of the applications of paper transistor technology. In practical terms this will mean, for example, smart, changeable and/or interactive packaging for things as simple as milk and juice packs or cereal boxes, intelligent tags for clothes, posters and newspapers which don't need to be replaced nor bought every day. In the following paragraphs the various application of paper transistors are discussed in detail topic wise.
A. In TFT.
The best known application of paper transistors is in TFT. These transistors are used in LCDs, an implementation technology. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability. At present , many color LCD TVs and monitors use this technology. TFT panels are heavily used in digital radiography applications in general radiography. It is used in both direct and indirect capture as a base for the image receptor in medical radiography. The most beneficial aspect of TFT technology is a separate transistor for each pixel on the display. As each transistor is small, the amount of charge needed to control it is also small. This allows for very fast re-drawing of the display
Organic thin film transistors TFT alllow the fabrication of electronic circuits and active-matrix displays on a variety of subtrates, such as silicon, glass, polyamide, polyethylene naphthalate PEN, polyethylene terephthalate PET, and polycarbonate. This wide range of substrates reacts the inherent simplicity and the relatively low thermal budget of the organic TFT manufacturing process. It also increases the likelihood that organic thin- film electronics will penetrate a variety of applications since different application often denamd different subtrates. For example, backlit crystal displays require,transparent substrate, such as glass, PEN or PET. Light emitting and reactive displays allow the use of opaque or not fully transparent substrates, such as silicon or polyimide. Flexible electronics, conformable image sensors, and foldable displays call for the mechanical exibility provided by polymeric film or steel foil. Finally, commercal products targetting loe cost, large volume applications may require the use of particularly substrates, such as PEN, PET, or paper.
B. In OLED
OLED stands for organic light emitting diode. It is a light-emitting diode (LED) whose emissive electroluminescent layer is composed of a film of organic compounds. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited. They are deposited in rows and columns onto a flat carrier by a simple "printing" process. The resulting matrix of pixels can emit light of different colors. Such systems can be used in television screens, computer monitors, small, portable system screens such as cell phones and PDAs, advertising, information and indication. OLEDs can also be used in light sources for general space illumination, and large-area light-emitting elements. OLEDs typically emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point-light sources.
B. 1. Working of OLED
OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are made of organic molecules that conduct electricity. The layers have conductivity levels ranging from insulators to conductors, so OLEDs are considered organic semiconductors. The first, most basic OLEDs consisted of a single organic layer, for example the first light-emitting polymer device synthesised by Burroughs corporation, involved a single layer of p-phenylene vinylene.
Multilayer OLEDs can have more than two layers to improve device efficiency. As well as conductive properties, layers may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile,or block a charge from reaching the opposite electrode and being wasted. Voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode. Thus, the cathode gives electrons to the emissive layer and the anode withdraws electrons from the conductive layer; in other words, the anode gives electron holes to the conductive layer. Soon, the emissive layer becomes negatively charged, while the conductive layer becomes rich in positively charged holes. Electrostatic forces bring the electrons and the holes towards each other and they recombine. This happens closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons. The recombination causes a drop in the energy levels of electrons, accompanied by an emission of radiation whose frequency is in the visible region. That is why this layer is called emissive. The device does not work when the anode is put at a negative potential with respect to the cathode. In this condition, holes move to the anode and electrons to the cathode, so they are moving away from each other and do not recombine . Indium tin oxide is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer. Metals such as aluminum and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the polymer layer.
The above description is of passive OLED. There is another type of OLED called active-matrix organic light emitting diode (AMOLED) first manufactured by Samsung Corporation. AMOLED require a thin-film transistor backplane to switch the individual pixel on or off, and can make higher resolution and larger size displays possible.
AMOLED (Active Matrix Organic Light-Emitting Diode ) has many advantages such as high-resolution, low power consumption, a very thin layer, etc. as next-generation display after LCD (liquid crystal display) and PDP (plasma display panel) and is referred to as the display of a dream.
C. In Paper Memory Device
Paper transistor is being researched for use as a device for storing memory, hence a memory out of a paper. Here an integrated foam composed of fibers are set up that increases the capability of storing carriers or charges in paper. These charge allow the paper to display information that is also erasable and the paper is rewritable so that additional information can be stored.
One of the interesting features of this memory paper is that is is manufactured in room temperature, which means that it doesnot need any special condition on temperature front fot its manufacturing. To create the paper long fibers from pine and polyester were mixed together and put into an iconic resin matrix. The fibers are than coated with Galium Indium Zinc Oxide using magnetron sputtering. The descrete fibers are integrated and contacts are applied at the extreme of the channel region to allow the induced carrier to move. Thus electrons move aong the fibers. Hence information gateways.
Another interesting features of this memory paper is the ability to store data in multiple layers. Which means that by applying different voltages at the same point layers of data can be stored, for e.g. if a +5v of charge is applied at the end to write a data -5v of potential difference is required to erase the data at the same time it is also possible to store more data at the same area by applying different voltage say 10volts to store more data and forming a multilayer.
From the above discussion as it can be clearly stated that the invention of paper transistor is a boon for the electronics community. The material used in the manufacturing of this products viz. Cellulose is one of the most abundable biodegradable substance in the earth. It is non toxic, and recyclable against silicon based semiconductor which is a source of health hazards for living beings. Although, as it can be seen as a important inventions this technology will take some time to be commercialized as research has revealed that it doesnot stores data for a unlimited time and also since it is soluble reaesarch in the field of paper transistor is a interesting work keeping in mind its value and importance in application in electronice devices in near future.
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