Printed Transistors

Each printed TFT element is composed of nanoscale conducting, dielectric, or semiconducting particles. The electrical performance of the printed transistors and printed integrated circuits is dependent on the uniformity of the... 

This chapter summarizes some of our recent work in printing techniques and plastic electronics. It also presents new data from printed transistors that use several different organic semiconductors in a variety of device geometries. In all cases, we observed good performance. pCP for the source/drain electrodes is attractive because it provides a simple and potentially low-cost route to high resolution (i.e. small channel lengths, L) structures that can be used to build transistors which... 

The rectifier in an HF RFID application needs to be able to rectify at 13.56 MHz, and current standards require the clock be divided from this same frequency as well. The former appears to be possible with printed electronics. The latter is problematic however it is possible to generate a kHz clock locally using an oscillator, i.e., without dividing down from 13.56 MHz. This introduces more variability and noise into the tag, but, for low data rates, the reader can screen this out. As a result, it is hkely that a 13.56 MHz printed RFID tag is realizable, albeit with a local clock and a low data rate. This appears to be usable for many apphcations including authentication, anti-coimterfeiting, etc., and therefore, there is substantial industrial and research institute activity in this regard. The driver, of course, as in displays, is the development of printed transistors. 

An area of recent interest for printed electronics is printed sensors. The general concept is to develop simple sensors for product quality monitoring, etc., that use small numbers of transistors with very low performance. This is a diffuse area that is still developing, and therefore will not be discussed here. However, it is important to note that many of the concerns that apply to displays and RFID apply here as well, and therefore, the main technology driver for most areas of printed electronics continues to be the printed transistor. 

As discussed above, the driver for many printed electronics applications is the printed transistor. The printed transistor is essentially a thin film transistor fabricated using printable materials. In other words, in its ultimate implementation, all three major material components of the printed transistor, i.e., the conductive electrodes, the insulating gate dielectric, and the semiconducting channel material, are printed. 

Fig. 6. Cross-section of an archetypal printed transistor. The specific configuration shown is a bottom-gate configuration, with the gate below the channel.

Overall, therefore, printed transistors impose nmnerous constraints on both materials and printing, which will be considered herein. 

Fig. 7. Conceptual process flow for fabricating a top-gated printed transistor.

Fig. 8. Conceptual process flow used to fabricate a bottom-gated printed transistor.

Critical Process Integration Issues in Printed Transistor... 

At this point, it is appropriate to review the various process integration issues that must be considered during printed transistor fabrication. These issues in turn impact material selection, which is covered in a separate section. 

Layer roughness — Printed transistors are inherently multilayer complex devices. Roughness of individual layers is therefore... 

Step coverage — From the process flow schematics shown previously, it is apparent that printed transistors inherently have substantial topology within their cross-sectional structure. As a consequence, step coverage becomes an important parameter in process optimization. Given the large steps (typically several tens of nm or more) and the use of relatively thin subsequent layers, it is important that the layers cover each other adequately liquids must be able to coat the vertical sidewaUs of steps during a multilayer print process. This places constraints on fluid viscosity, evaporation rate, wetting, etc. 

Just as polymers may be used to form printable semiconductors, so they may be used to form dielectrics as well. Indeed, polymer dielectrics are in widespread use in conventional microelectronics as well. For printed electronics applications, polymer dielectrics are therefore a natural choice for use in printed transistors. Several families of polymer dielectrics have been studied and used in printed transistors. These include various polyimides and other polymer dielectrics such as pol)rvinylphenol (PVP). In general, these dielectrics are characterized by the following properties ... 

Several polymer conductors are commercially available, and have been used in the demonstration of printed transistors. These include PEDOT PSS, which is a commercially available polymer conductor, as well as various versions of polyaniline. The latter is typically doped with an acid or salt to increase conductivity. Both of these material systems are water soluble and easily printable. They also typically form good interfaces to organic semiconductors, making them attractive for use in printed transistors. As with polymer dielectrics, however, it is important to note that their usability with inorganic semiconductors is questionable, of course. 

As discussed previously, roughness is a significant concern for printed electronic devices in general and printed transistors in... 

This large overlap, of course, results in degraded performance of printed transistors. Given that the performance of printed transistors is marginal for many of the intended apphcations, this is a serious concern and is an area of intensive research. 

Fig. 14. Optical micrograph of a printed transistor showing large overlap between gate and source/drain. This overlap is necessary to account for large variability in drop placement accuracy...

Figure 1.2 illustrates the typical design of a printed transistor the source and drain electrodes are mounted on a polyester foil, followed by the semiconducting layer of polymer (i.e. polythiophenes), the insulating layer of polymer insulators is on top and, as the final layer, the gate electrode. 

Several other BTBT and DNTT derivatives were prepared in the year 2014 (14ACR1493). Representative samples are given below. Several of these materials have been used in all-printed transistor arrays, flexible circuits, and in medical applications underscoring their promise as practical semiconductors for electronic device applications. 

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