**1. Introduction**

In all electronic devices, an electrical connection to the real world is necessary. In the case of thin-film transistors (TFTs), the quality of this electrical connection may be the difference in having high or low performance devices. The connection is made by source/drain electrodes in contact with the active layer. These metal-semiconductor interfaces have not played an active role in the transistor operation. However, a low-quality interface can be responsible for a low performance operation. Indeed, this research topic is one of the bottlenecks on the development of thin-film transistor technologies.

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It is desirable to have the closest to an ideal metal-semiconductor interface or an ohmic contact with very low contact resistance; in other words, source/drain contacts with no barriers for the carrier flow in either positive or negative voltage polarization. Ideally, this occurs when metal and semiconductor work functions are of similar value and there are no interface states. However, having metal-semiconductor contacts without interface states is difficult and matching the semiconductor and metal work functions is nearly impossible. For these reasons, it is important to find alternatives to improve the metal-semiconductor interface in TFTs.

In metal-semiconductor interfaces, one may have several cases: ohmic contacts with low or high contact resistance and non-ohmic contacts with low or high contact resistance. Being this last, the most commonly obtained. The contact resistance can be extracted experimentally by the extrapolation of the width-normalized resistance (RW), obtained from the linear regime of the output characteristics *I* ds vs. *V*ds, for different channel lengths and gate voltages *V*gs, as indicate in **Figure 1** [1, 2].

The problem associated with a high contact resistance is that it induces a potential drop at the drain/source contacts, affecting the electrical performance of the device [1–4]. On short channel TFTs, as the channel length is reduced, the source/drain contact resistance may be higher

**Figure 1.** Width-normalized resistance (RW) (obtained from the linear regime of the output characteristics) for different channel lengths and gate voltages *V*gs.

than the channel resistance, as result the electrical behaviour may be governed by the contact resistance. This may induce several mechanisms: some of them reported the degradation of the transconductance, drop of carrier mobility, impact ionization, among others. Indeed, there is not a value of channel length to determine if short channel effects will be exhibited on the electrical characteristics of the TFT. Reported TFTs exhibited short channel effects at channel lengths *L* lower than 20 μm, meanwhile other TFTs (some with high contact resistance) did not exhibited short channel effects at *L* of 10 μm [1, 3–10]. Moreover, a TFT may exhibit high contact resistance effects at considered long channel values. These effects can be presented as superlinear behaviour or current crowding in output characteristics [1, 11, 12]. It is always desirable to have a low contact resistance; however, the channel resistance must be dominant.
