OFETs models

In Fig. 13.7 the experimental drain currents are compared with modeled characteristics. Below, the simple nine-term OFET model that is used here will be outlined. Three regimes of operation are distinguished ... 

Fig. 3 Scheme of an OFET model based on a resistor-capacitance network circuit... 

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... 

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. 

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. 

The above discussion demonstrates that high field effects included in the FDTO model modify the OFET characteristics very significantly. The FDTO model is in very good agreement with the experimental results. The model has been compared with two sets of experiments [158], The experimental data for -6T [157] is compared with the FDTO... 

In addition to its n-eleetron system, PTCDA possesses a seeond functionality, namely the carboxylic oxygen atoms in the four eomers of the molecule. Their existence creates a quadrupole moment in the molecule, allows hydrogen bonding between molecules and offers an additional interaction channel with any substrate. The interplay of all these factors makes PTCDA an interesting model molecule in the context of molecular adsorbate layers. As a consequence, PTCDA is still a fruit fly , although it is not used any more to fabricate electronic devices such as OFETs, in spite of early reports of an organic heterojunction device based on PTCDA [23,24]. 

As pointed out before, interfaee stmeture in OFETs is important for both charge injection (metal-semiconductor interface) and charge transport (semi-conductor-dielectric interface). The discussion of the previous section demonstrates that even for the small number of model systems considered here a large variety of sfructures is observed. Because many factors influence the interface and film sfructures, it is difficult to establish general mles. Flowever, a few important observations regarding the stmcture-forming factors can be kept hold of ... 

In prineiple, all of these effeets eould also be present in real deviees and have an impaet on their performanee. In this sense, experiments on model systems offer the unique opportunity to unearth potential eontributions to the physies of OFETs that are hidden behind the average behaviour of teehnieal, often non-ideal, deviees. But as OFETs are getting smaller, some of these interesting effeets may even beeome deeisive for their performanee. 

The investigation of the interface of a real device contact with siuface sensitive techniques is generally not possible, since the interface is buried and these techniques penetrate only a few atomic layers. However, an interface analysis with, e.g., electron spectroscopic techniques would be highly desirable since these methods provide valuable (semi-) quantitative information on the electronic structure and ehemical interaction in the contact regime. To tackle this problem, i.e. to aecess the buried interface, we have applied a lift-off technique, whieh is illustrated in Figure 14.8a and was previously developed by our group [30]. Therefore, we prepared a sample sandwich which consists of the aetual model OFET, i.e. a gold film deposited on a DIP film that itself... 

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