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Investigation of the potential of organic circuit for RFID tags Qintao Zhang and Sha Li University of California Berkeley Abstract-In recent years, RFID technology is one of the attractive technologies in service industries. Till now, the core of RFID technology is based on the single crystal silicon IC technology. In order to reduce the cost of RFID tags furthermore to less than 1 cent, organic transistor technology is introduced as one of the most promising alternative techniques. In this project, RFID and organic transistor technologies are reviewed and different organic circuit schemes used in RFID are simulated based on an amorphous silicon spice model. The speed of organic circuit is going to be investigated to tell the capability of the organic circuit on high frequency RFID application and the power consumption of the device will also be evaluated. I. Introduction In recent years, RFID (Radio Frequency Identification), which is a means of identifying a person or object using a radio frequency transmission, has gained a lot of attentions from both the academia and industries due to its great potential on detecting and tracking capability. RFID can store more information about the object, like its unique serial number, manufacture date and composition for the commercial product, besides being identification like the barcode and the personal and medical information for the human user. In RFID, communication takes place between a reader and a transponder (often called a tag), shown in figure 1[1]. Inside the tag, which is the core of the RFID technology, the information of the products is stored and sent back to the reader or modified according to the command from the reader. Currently, there are two kinds of Tags, 1) active tags, powered by a battery and 2) passive ones, powered by the coupling magnetic field from the reader to the antenna of the tag. Figure 1, a typical RFID system Figure 2 is the schematic diagram of a passive RFID tag, which is more favorable than an active tag for the future market due to its simple architecture. The commands from the antenna, where not only the signal but also the power is coupled from the reader through a metal circle, are intercepted by the decoder and stored into the sequencer, which is basically a register. According to the command, the sequencer reads the necessary information from EPROM and sends it to the encoder and the modulator to modulate the signal into the corresponding radio frequency and broadcast it back to the reader. In this process, the rectification of the AC is done by the AC/DC converter and a power control module is embedded to switch the DC voltage from the converter to the working voltage of the chip. The working frequency of the device is determined by an on-chip frequency generator, which is not shown in the figure. Figure 2 the basic tag IC architecture [1] Currently the application of the RFID is only confined to the business of pallets and shipping containers, and the reason is mainly because of the high cost of tags. All the commercial tags are made from the silicon process, leading to the cost per chip 25 cents typically and no less than 5 cents. Compared to the goal, under 1 cent per chip, for item level tracking, this conventional fabrication technique can’t satisfy the requirement. Therefore, a new technique which uses inherent organic semiconductor and soluble metal nanoparticles to print circuits and antennas on flexible substrates or to fabricate chips and print circuitry on them is under investigation. The organic tags would not only cut the cost to 1 cent per piece or less eventually after commercialized but also provide a flexible substrate, which makes tags easy to use on all kinds of surface or versatile products. Organic circuits have been under research for more than a decade now. Instead of using silicon as the semiconductor material, organic transistors use semiconductor polymer as the active material, which gives these transistors the great properties like the flexibility and low processing temperature. Therefore, it can be fabricated on versatile substrates, such as plastic and paper at very low temperature, generally at room temperature or around 100 oC. And because most of Organic materials are soluble in particular solvents, such as P3HT (Poly3hexylthiophone) in Chloroform, a cheap fabrication method, inkjet printing, could be used for the potential organics circuits. This maskless process can reduce the cost dramatically. According to the current research, organic cicuits show great potential on the RFID application, however, the inherent properties of the organic semiconductors set some restrictions on the final circuit. Therefore, a study on the potential performance and design of the organic circuits for RFID is necessary. Figure 4 is the first demonstrated passive organic RFID tag [4]. In the paper, Pentacene, a p-type organic semiconductor is used. Figure 4, a real organic RFID circuit based on pentacene devices. Figure 3, basic structure of organic transistors; top contact (a) and bottom contact (b). [5] Two typical structures of organic transistors is shown in figure 3, a) top contact and b) bottom contact. The organic transistor works in the accumulation mode. Instead of forming inversion layer in the channel, the accumulation layer is forced by the gate voltage, and the drain current reaches to the saturation region when the source drain voltage is high enough. Organic transistors show enhanced characteristics in the past ten years [5], mobility has been improved from 10-5cm2/VS to 1.5cm2/VS, which is already same as the mobility of amorphous silicon transistors. The Ion/Ioff ratio of 106 can be very easily obtained too. The improvement on the organic semiconductor material and fabrication make the whole-organic circuit possible. The first all-inkjet-print organic transistor and organic transistors on fiber have been reported in organic research group in Berkeley. II. Motivation III. Proposed work The main digital circuit inside the RFID would be the memory, decoder, buffer and the oscillator. In this project, we will first examine the properties of a single MOSFET of the organic transistor and then expand the discussion to the inverter and the oscillator. Eventually, we are going to design a memory cell and corresponding decoder. For the organic transistors, both of the most popular materials being used and studied, Pentance and Polythiophene, which has high mobility, are p-type semiconductors. So all-pmos circuit is inevitable for organic RFID if no comparable n-type organic material emerges. So in the project, an all-pmos circuit is designed first and the trade off of this design will be discussed. However, people are still researching for n-type organic transistors and soluble inorganic n-type transistors, such as ZnO nanoparticles done in Berkeley. The nmos transistor will then be added into the circuit in order to extract the maximum speed of the organic circuit for the investigation of the performance. The communication frequencies used by RFID ranges from 125KHz to 2.45 GHz, depending on applications. In US, 13.56MHz is the most attractive frequency because of the FCC rules and the trade-off between power and detection range. The speed of the organic RFID will be compared with this frequency range. n-type organic and soluble inorganic are on their beginning, n-type parameters are going be not real number. Actually, the mismatch of the n and p mos is another problem for organic transistor design. Power will be another issue for the RFID especially the passive one because the power collected from the antenna is decreased quadratic to the distance so the working region of RFID is determined by the power dissipation directly. We will discuss the basic power consumption for the circuit. On the other hand, the leakage is a big concern for the current organic RFID because of the ohmic contact between the source-drain contact and the channel and the high working voltage, higher than 10V, due to the high threshold voltage generates quite high gate leakage. But for these phenomena, there are no good model for them now so we will only qualitatively discuss the influence of the leakage. Summary The spice model of organic transistor is not ready to be used, so in this project, we are going to use the AIMSPICE amorphous silicon transistor model to simulate the organic circuits. Amorphous silicon transistors show same mobility as the organic transistors, and because they are all amorphous crystal, they show similar interface trapping mechanism between crystalline. Thus it’s reasonable to use amorphous silicon crystal model to simulate the circuit, although it can’t simulate correctly the power consuming. Spice parameters of pentacene are going to be extracted and optimized based on data from Professor Vivek Subramanian’s group. Because the In this project, we are going to investigate the speed of the organic transistors and tell the feasibility of organic RFID. Because organic RFID has not been simulated before, some circuit diagram is going to be proposed and suggested. Reference: [1] “A Basic Introduction to RFID Technology and Its Use in the Supply Chain,” Laran RFID white paper [2] Patrick A. Toensmeier, “As RFID applications increase, suppliers look to lower it cost,” 12 Plastics Engineering, February 2005. [3] K. Finkenzeller, “RFID handbook: Fundamentals and applications in contactless smart cards and identification,” [4] P. F. Baude, D. A. Ender, M. A. Haase, and et al, “Pentacene-based radio frequency identification circuitry”, Applied Physics Letters, Vol 82, number 22, 2003. [5] C. D. Dimitrakopoulos, and D. J. Mascaro, “Organic Thin-Film Transistors: A Review of Recent Advances,” IBM J. Res. & Dev., Vol. 45, No. 1, Jan, 2001