Contact resistance extraction

For an ohmic contact resistance (Vc = RC1d) and d I, Eq. (16) reduces to Eq. (14). While the two previous analyses [35, 36] assumed constant contact resistance and gate voltage-dependent mobility, Street and coworkers make the assumption that the mobility is constant and find a non-ohmic contact resistance that varies with gate voltage. 

At this stage, a technique that would enable independent access to the channel and contact resistances is needed. Such a feature is offered by the transfer line method (TLM) [38-41, 89], a method adapted from a classical technique use to estimate contact resistance, and first developed for the amorphous silicon thin-film transistor [42]. The method consists of measuring the channel resistance for different channel lengths. The measured resistance is actually the sum of the channel and contact resistances. As long as the measurement is performed in the linear regime (small drain voltage) the channel resistance is proportional to L (see Eq. 1) and the width-normalized (Rx W) total resistance is given by  

An alternative method to TLM is the four-point probe, which consists of introducing into the conducting channel two additional electrodes [45, 46]. The current remains the same all along the channel and the voltage drop between these two additional electrodes is not affected by the contact resistance, thus giving access to the true channel resistance. Moreover, as shown in Fig. 1.14, the contact resistance at each side of the channel can now be estimated independently. 

An even more powerful technique makes use of an atomic force microscope (AFM) tip to probe the potential along the channel of the transistor [47, 48]. The technique and its results have been analyzed in detail by Burgi et al. [49]. A variety of semiconductors and metals have been studied. The main features can be summarized as follows  

As expected, the contact resistance strongly depends on the nature of the electrode, e.g. its work function  

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Contact resistance extraction cannot be performed from simple current-voltage curve. At this stage, specific techniques that allow an independent access to the... 

Necliudov, P. V. et al., Contact resistance extraction in pentacene thin film transistors, Solid-State Electron., 47, 259, 2003. 

Fig. 6.9. A model for contact resistance. A real device is modeled as an ideal device which exhibits no contact resistance and a contact resistance in series at the source and drain (a). If a longer but otherwise identical device is fabricated, the contact resistance should remain constant, while the ideal transistor scales (b). Comparison of different length devices is the basis for the transfer line method of transistor contact resistance extraction.

Judiciously selected grades provide good impact and fatigue resistance, good stiffness and impact balance, suitability for food contact, low extractables,good organoleptic properties, the possibility of autoclave and radiation sterilization in the presence of aqueous solutions. .. 

Quite often the mobilities determined for a given molecule, e.g. rabrene or pentacene, differ for different devices in different laboratories, reflecting the problems related to the extraction of the mobility data from the electrical characteristics. Typically, these characteristics are not only defined by the mobilities but also by the contact resistance and of course the presence of domain boundaries defects and impirrities in the organic semiconductor. The determination of the true intrinsic mobility of an organic semiconductor is still a challenge, which has only been overcome in a very few cases. 

The contact resistance is extracted by plotting the width-normalized resistance as a function of channel length. The extrapolation to zero length readily gives the contact resistance, while the slope of the curve can be used to extract device parameters. The method is exemplified in Figure 2.2.14. 

Top contact OFETs (Fignre 2.4.7a) generally exhibit the lowest contact resistances. This is likely becanse of the increased metal-semicondnctor contact area in this configuration. A major contribution to contact resistance in the top contact configuration is access resistance (see Figure 2.4.8a). Access resistance resnlts from the requirement that charge carriers must travel from the source contact on top of the film down to the accnmnlation layer (the channel) at the semicondnctor-insnlator interface and then back np to the drain contact to be extracted. 

Fig. 6.10. A schematic demonstrating extraction of the contact resistance from measurements of different length devices. The total chaimel resistance for a set of devices of different lengths are plotted, where Rtotai = Rc + L pchannei- At the intersection point (ideally L — 0, but a systematic offset in the length can cause this point to be elsewhere), Rtotai = Rc...

This technique can be applied at a range of gate biases and plotted as a function of channel charge density, gate voltage, or applied lateral electric field. Non-linear models have been proposed for the contact resistance as a function of the applied bias (see, for example, [ffS]), which generally assume a Schottky or other diode-like charge injection from the contact into the channel. The transfer line method can also be used to extract a purely empirical model. 

Fig. C.9. Several test structures for determining material parameters, (a) schematically shows a transfer line, which can be useful for extracting contact resistance, (b) shows a resistance test structure, and (c) shows a capacitor which can be used to determine the capacitance of the gate dielectric.

Due to the excellent redox properties and high conductivity of the conducting polymer nanocomposites, researchers have been extensively investigating them as electrode materials for batteries and supercapacitors [29,30]. Further, the soft porous conductive polymer matrix can efficiently buffer the severe volume changes of active electrode material during the ion intercalation and extraction process, hence improving cyclability of the electrode material and also acts as a conductive binder, decreasing the contact resistance between particles of active material [31]. In this section... 

Mixed-conducting lithium-ion-doped emeraldine polyaniline (PAni)-PEO blends have been developed in order to achieve optimal electronic-ionic conductivity balance in nano-tin composite anodes. They found that the SEI impedance of the composite anodes increases with a decrease in PEO content and is much lower in pressed than in cast electrodes. Nano-Sn, AlSi , and Li Sn powders were studied by EIS to determine the electrochemical kinetics and intrinsic resistance during initial lithium insertion-extraction. It was shown that the SEI formed on particle surfaces, together with particle pulverization are responsible for the high contact resistance. 

Extraction stability is an interesting example of the significance of possible side-effects, Fig. 3.3. In order to stabilize PE-HD in contact with extractive media such as water or oil, the antioxidant should retain its effect even after extended contact with the medium and not be extracted from the base polymer. Here, not only effectiveness is essentially demanded from the antioxidant, but its resistance to extraction, e.g., for coolant compensator reservoirs [510]. 

In the case of both motors and generators, current must be fed or extracted from the rotating coils of wire. This is accomplished by connecting the ends of the coil to insulated collector rings which rotate against stationary brushes (Fig. 10.8). Since it is important that electrical contact resistance and friction be low, collector rings are usually made of copper and brushes of graphite. 

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