**2. Experimental section**

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

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

ds vs. *V*ds, for different channel lengths and gate voltages *V*gs, as

of the output characteristics *I*

104 Different Types of Field-Effect Transistors - Theory and Applications

channel lengths and gate voltages *V*gs.

indicate in **Figure 1** [1, 2].

The high quality SiO2 film was obtained by spin-coating of SOG (SOG700B Filmtronics) diluted with deionized water and cured at 200°C. The a-SiGe:H active layer was deposited using low frequency (110 kHz) plasma-enhanced chemical vapor deposition (PECVD) at 200°C, pressure of 0.6 Torr and an RF power of 300 W. The a-SiGe:H films were deposited from SiH<sup>4</sup> and GeH<sup>4</sup> feed gases with H2 dilution. The flow rate of SiH<sup>4</sup> (10% H<sup>2</sup> ) and H<sup>2</sup> was 45 sccm and 1000 sccm, respectively, and the GeH<sup>4</sup> (90% H<sup>2</sup> ) flow rate was 105 sccm. The n+ a-Ge:H film was deposited using low frequency PECVD at 200°C with a pressure of 0.6 Torr and RF power of 300 W, with a GeH<sup>4</sup> (90% H<sup>2</sup> ) flow of 250 sccm, H<sup>2</sup> flow of 3500 sccm and PH<sup>3</sup> (99% H<sup>2</sup> ) flow of 20 sccm. The aluminium was e-gun evaporated.
