OFETs

Because most of the OFETs reported until now were grown on silicon wafers, the deposition of the semiconductor is the determining step of their fabrication. De-... 

Figure 14-12. Various types of OFETs. (a) Inverted coplanar on a highly doped Si wafer, (b) inverted coplanar on a neutral substrate, (c) inverted staggered oil a neutral substrate, (d) inverted staggered using the dielectric layer as the substrate.

OFETs constructed on a silicon wafer do not lake advantage of one of the main interest of organic materials, namely the possibility of building electronic devices on plastic substrates. A second important drawback of the silicon-based structure is the difficulty to individually address the gale of transistors built on the same wafer, which would prevent the achievement of integrated circuits. 

More recently. Gamier and coworkers used a printing technique to make OFETs on polymeric substrates [61]. Although printable field-effect transistors based on inorganic semiconductors have been reported as early as 1967 ]62], they did not come to any commercial development. We note, however, that in Gar-nier s device only the electrodes were actually printed. 

Figure 14-13. Evolution of the field-effect mobility of OFETs for five organic materials polythio-phenc (PT) and its derivatives, qualerthiophcne (4T), scxithio-phenc (6T), dihcxyl-sexithiophene (DH6T). and pcntaecnc.

Tablc 14-3. Performance of OFETs made with n-lypc organic semiconductors. 

The development of microelectronics cannot be envisaged without a comprehensive modeling of the devices. The modeling of OFETs is currently hampered by several features. First, charge transport in organic semiconductors is still not completely understood. The situation is clear at both ends of the scale. In high mobility materials (//>IOcnr V-1 s l), transport occurs within delocalized levels when temperature... 

Figure 14-22. Variation of the square root of the saturation current as a function of the gate voltage for a typical lightly doped DH6T OFET.

We Finally note that the MTR model is a priori more appropriate to disordered materials. It is not expected to give good results with single crystal OFET, especially when the mobility becomes temperature-independent (see Section 14.6.1.2). However, it has recently been invoked in the case of poly thiophene [112], the mobility of which is also thermally activated. 

Figure 14-27. Drain current-voltage charaeieristics of a doped DH6T OFET sliowiug both accumulation (V cO) and depletion (Fx>0) regimes.

Table 14-2. Performance of pcnlacene based OFETs. Mobility in cm2 V 1 s substrate temperature in °C. RT room temperature.

More recently, OFETs were made with a dimer of another condensed molecule. dithieno[3,2-/j 2, 3 -c/ lhiophene 100, which presents a very similar structure lo that of benzodithiophene, except that the central benzene ring is changed lo a... 

This section is divided in two parts. In the first one, we review the studies on the transport mechanism in materials used in OFETs, whereby temperature-depen-dent measurements are a very powerful tool. The study of the gate bias dependence has also been used by researchers. In the second part, we present the few analytical models of the organic FETs that have been developed until now. 

As all measurements were performed in air, il is mosl probably due lo oxygen doping, a feature that lias been already noticed in oligolhiophene 1101 and poly-thiophene 11281 OFETs. 

As a class of n-type organic semiconductors, PBI derivatives have received considerable attention for a variety of applications [312, 313], for example, for organic or polymer light-emitting diodes (OLEDs and PLEDs) [314, 315], thin-film organic field-effect transistors (OFETs) [316, 317], solar cells [318, 319], and liquid crystals [320]. They are also interesting candidates for single-molecule device applications, such as sensors [321], molecular wires [322], or transistors [141]. 

If amorphous Si TFTs and OFETs are adequate only for lower performance, lower cost applications, what are the options for more advanced applications To achieve the higher performance that is desired while keeping process technology consistent with the goals of macroelectronics, several different fabrica-... 

But, it is substantially better than the tens of kilohertz that classic amorphous silicon or OFETs can achieve. It is also in frequency ranges of interest for many mobile communication systems and radar detection bands and, therefore, would provide sufficient capability for a range of RF opportunities. 

---