Finally, it is also possible to find the contact resistance via capacitance-voltage measurements. This technique assumes a constant mobility and should be applied in the linear transport regime. In the perfect case with no contact resistance, this extrapolation aligns with the applied potential (i.e., V DS at the drain and 0 at the source) a difference of Δ V D or Δ V S is necessary in order to extract/collect charges between the electrode and semiconductor interfaces and is attributed to the contact resistance. The slope of the potential drop between these two probes is extrapolated to the contacts. The gated four-point-probe technique utilizes a device architecture with two small voltage probes ( V 1 and V 2) in the channel, placed a known distance from the source/drain electrodes ( Uemura et al., 2016 Natali and Caironi, 2012 Choi et al., 2018 Pesavento et al., 2004 Chen and Kanicki, 1997 Chesterfield et al., 2004 Hailey et al., 2015). This technique is very straightforward to apply, but it should be noted that the requirement for homogeneity is not always strictly met for thin-film devices, leading to errors in the results. (14.18) R Ch = ∂ V DS ∂ I D V DS → 0 = L μ C i W V GS − V T 14.18) extrapolation of the trend to a channel length of 0 yields the contact resistance (from Eq. The width-normalized on-resistance is measured in the low- V DS regime of the transport plots as a function of length (Eq.
The former requires a homogenous semiconductor with several devices of varying channel length fabricated on its surface, and it has been used successfully for both a single crystal and thin films ( Uemura et al., 2016 Gundlach et al., 2006 Ward et al., 2012, 2014b Hou et al., 2016 Zaumseil et al., 2003 Luan and Neudeck, 1992 Hamadani, 2004). Other methods include the gated-transmission-line model and gated four-point probe structures. This measurement should be accomplished under an inert atmosphere, as oxygen and water interact strongly with the exposed organic surface. This has allowed researchers to map the potential drop directly across a device in operation ( Pingree et al., 2009 Teague et al., 2008 Seshadri and Frisbie, 2001 Nichols et al., 2003 Bürgi et al., 2002 Mathijssen et al., 2007).
Scanning Kelvin-probe microscopy combines spatial resolution with a direct measurement of the contact potential difference between a metallic tip and a conductive surface. Fortunately, several measurement techniques allow its evaluation. Given the impact that contact resistance has on both an accurate understanding of OFET operation and on their scalability, intense effort is dedicated to understanding and minimizing its effects ( Baeg et al., 2013 Natali and Caironi, 2012). Reducing contact effects is recognized as one of the most stringent problems in present OSC research as, for example, OTFTs are operated in the linear regime in such applications as active matrix displays.
They can yield nonideal I-V characteristics, reduced (or artificially increased) mobility, increased threshold voltage, sometimes the lack of current saturation in the output characteristics, and current saturation in the transfer characteristics. The parasitic contact effects become more severe with decreasing channel length and for materials with higher intrinsic mobility, where the contact resistance can be comparable with the channel resistance. 14.6 or 14.7) is an effective device property, μ eff, rather than an intrinsic property, μ int, of the OSC material. This originates from the critical effect that the contact resistance has on the device properties, pointing to the fact that the mobility extracted from the OFET current-voltage characteristics (Eqs. Often, however, the linear mobility will appear to be less than the saturation mobility. 14.6 and 14.7, respectively) are the same. In an ideal device displaying ohmic contacts, the mobility values derived from the linear and saturation regimes (Eqs. Jurchescu, in Handbook of Organic Materials for Electronic and Photonic Devices (Second Edition), 2019 14.2.4 Contact effects
Conductivity measurements of organic materials using field-effect transistors (FETs) and space-charge-limited current (SCLC) techniques
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