TRANSISTOR STRUCTURE

Introduction to Organic Transistors
Recent advances in organic electronics research open new perspectives on developing much cheaper technologies for manufacturing electronic circuits and devices. Key to these new developments is the performance organic transistor characteristics. Though still vastly inferior to silicon- based technologies, they have become appropriate for electronic circuits and display technology. Since a great variety of products need very low-cost and relatively low performance, many researchers and firms are focused on developing this new opportunity of using organic materials. Major electronics firms such as Philips and Pioneer, and smaller companies such as Cambridge Display Technology, Universal Display, and Uniax, are developing organic electronic and optoelectronic devices. They predict a tremendous opportunity for the low-cost organic technologies.
Some limited applications of organic electronics have already entered the commercial market. Organic multicolor automobile stereo displays are now available from Philips and Pioneer. For the most part, however, organic semiconductors and organic thin film circuit components remain in the world of research, as they have been for many years. Only recently have the performance of organic transistors become attractive for consideration in commercial applications.

Organic Materials
There are two main categories of organic semiconductors that can be used to build organic thin-film transistors: small-molecules and polymers. Polymers can generally be applied at room temperature and atmospheric pressure, opening the possibility of “printing” electronic circuits utilizing ink jets or printing press methods. Small-molecule transistors have a high mobility, but generally require deposition environments of temperature and/or pressure that are not at room temperature. These films are generally evaporated on a substrate through a shadow mask. Either way, the cost of processing of these organic materials even at this stage of technology is lower than can be achieved through silicon processing.

Top Contact and Bottom Contact Structures
Organic thin-film transistors [Fig. 1] can contain either a molecular or polymeric channel connecting the source and drain contacts. The gate is first deposited onto an insulating substrate such as glass or plastic, followed by deposition of the gate insulator, which can consist of either an organic or inorganic dielectric film. Source and drain electrodes are deposited onto the gate dielectric, and that step is followed by the deposition of the thin-film channel layer [Fig 1b]. Alternatively, the thin-film channel layer can be deposited before the source/drain electrodes [Fig 1a]. However, this sequence often restricts the avenues available for source/drain patterning to shadow masking since the organic thin-film channel layer is frequently susceptible to damage during the etch process.

Figure 1. OTFT device configuration: (a) Top-contact device, with source and drain electrodes evaporated onto the organic semiconducting layer; (b) Bottom-contact device, with the organic semiconductor deposited onto prefabricated source and drain electrodes.

Top Gate Structure
Another structure is called the “bottom gate structure” (see picture below). The operation of this transistor is just the same, but offers the advantage that the layer which requires the smallest definition (the metal contact layer) is the first layer to be printed on the substrate. The first layer is generally the easiest to pattern since the topology of the layers need not be contended with. On the process integration front, however, this structure poses an additional challenge since the dielectric/semiconductor, critical in the design of any MOS transistor, is generally superior when the semiconductor is deposited on the dielectric rather than vice versa.

Building An RFID Tag with Today’s Organic Technology
Devices based on organic electronics are still in development and certain parameters such as charge-carrier mobility, threshold voltage uniformity, and dark currents are far inferior to those of silicon. Based on preliminary data and simulation results, OrganicID believes that current performance of today’s organic semiconductor technology is sufficient to develop low-cost circuits for the applications that require only low performance. Specifically, OrganicID is developing an integrated RFID tag including circuit and antenna based on organic semiconductors. The size of the organic RFID tag could be about the same size as a bar code is today, thereby eliminating the need for tight design rules. With clever design and appropriate trade-offs, the cost of organic RFID tags could be low enough to be competitive with the cost of bar codes while providing many of the advantages offered by silicon-based RFID technology.

The OrganicID approach to building an end-to-end organic process line
Rather than emphasizing material development, OrganicID is focused on developing an end-to-end integrated organic process, the design of the RFID tag, and the print patterning techniques. In the last year, OrganicID has evaluated printable organic semiconductor materials (n-type and p-type), printable conductors, printable dielectric, and polymer-based non-volatile memories to find the set of compatible materials that result in the highest performing process. OrgancID has numerous partners to supply these materials, and joint development relationships to develop the printing techniques. With this approach, OrganicID is focused on product development, and not material research and development, accelerating the schedule to bring the product to market.

The Organic RFID Tag
Conservative design estimates are that this design will consist of about 4000 transistors, and that the size of the circuit utilizing presently defined design rules is a bit under 90 mm2. This will be the most complex circuit ever to have been built in an organic electronic process. The antenna will be printed directly on the substrate to interface with the circuit as shown in the diagram below.

All the power used to transmit data from the tag is supplied by the charge on the capacitor. Therefore, it is vital to the design of the RFID that careful consideration be given to power consumption. Consequently, leakage currents in the organic device components are extremely important to understand and model accurately.

Finally, the organic transistors must meet the performance necessitated by an RFID tag. Most of the RFID Tag circuitry can comfortably meet the requirements. Since all the power must be supplied by a charge of a capacitor, the standards are design to necessitate relatively slow circuitry even in silicon. The logic of an RFID tag operates in the vicinity of 100kHz. However, an RFID tag has a front end that must handle rectification, frequency division, and the sub-modulation of a 13.56 MHz signal. It is this front end that is thought to be the primary challenge in using organic transistors. OrganicID has developed a proprietary patented circuit designs that utilizes low performing transistors. These design techniques are vital to achieving the performance required to build a standardized 13.56MHz tag.