Impact of the Capacitance of the Dielectric on the Contact Resistance of Organic Thin-Film Transistors

Many advanced materials have been developed for organic field-effect transistors (OFETs) or thin-film transistors (TFTs) based on organic and organic hybrid materials. However, although many new OFETs exhibit superior characteristic parameters (such as high mobility), most of them show nonideal performances that have strongly limited progress in the design of molecules, the understanding of transport mechanisms, and the circuit applications of OFETs. In this review, the device physics of ideal and nonideal OFETs is discussed first to understand the factors that limit effective mobility in semiconducting channels, distort the potential distribution, or reduce the drift electric field. Then, recent advances in optimizing the material combinations, device structures, and fabrications of OFETs toward ideal transistors are discussed. Based on the good control of materials and interfaces, some new and novel concepts to utilize the nonideal properties of OFETs to build low-power circuits and integrated sensors are also discussed.

Section: Optimization Of Nonideal Ofetsmentioning

confidence: 88%

Understanding, Optimizing, and Utilizing Nonideal Transistors Based on Organic or Organic Hybrid Semiconductors

Yang

Dai

et al. 2019

“…The current lowest measured contact resistance for an OFET was reported by Borchert et al as R C = 29 Ω cm for bottom gate, bottom contact devices with an ultra‐thin (5.3 nm) gate dielectric . This work is the culmination of a series of reports that identified the importance of the gate dielectric layer on the OFET contact resistance, combined with chemically modifying the contacts using a self‐assembled monolayer of pentafluorobenzenethiol (PFBT). It should be noted that the devices were operated at gate–source and drain–source voltages ( V GS and V DS , respectively) less than 3 V, and produced mobilities of 5 cm 2 V −1 s −1 , record subthreshold swings of 62 mV decade −1 , on/off ratios of 10 9 , and ring oscillator stage delays of down to 138 ns, all enhanced by low R C .…”

Section: Introductionmentioning

confidence: 90%

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Waldrip

Jurchescu

Gundlach

et al. 2019

238

261

Organic semiconductors have sparked interest as flexible, solution processable, and chemically tunable electronic materials. Improvements in charge carrier mobility put organic semiconductors in a competitive position for incorporation in a variety of (opto-)electronic applications. One example is the organic field-effect transistor (OFET), which is the fundamental building block of many applications based on organic semiconductors. While the semiconductor performance improvements opened up the possibilities for applying organic materials as active components in fast switching electrical devices, the ability to make good electrical contact hinders further development of deployable electronics. Additionally, inefficient contacts represent serious bottlenecks in identifying new electronic materials by inhibiting access to their intrinsic properties or providing misleading information. Recent work focused on the relationships of contact resistance with device architecture, applied voltage, metal and dielectric interfaces, has led to a steady reduction in contact resistance in OFETs. While impressive progress was made, contact resistance is still above the limits necessary to drive devices at the speed required for many active electronic components. Here, the origins of contact resistance and recent improvement in organic transistors are presented, with emphasis on the electric field and geometric considerations of charge injection in OFETs.semiconductors with superior intrinsic charge transport properties, there is a stringent need to improve the device properties in order to allow organic devices to fulfill their technological prospectives. In the organic semiconductor community, contact resistance (R C ) has come under increased scrutiny as it is apparent that the final device performance is often domi nated by carrier injection, rather than the transport through the semiconductor layer. Contact resistance can impact OFET development in multiple ways. First, if not accounted for, a high contact resistance may lead to inaccurate extraction of the device parameters. Second, any inaccuracies in parameter extraction can have significant consequences on material development, from generating incorrect structure-property relationships to discarding organic semiconductors whose efficient intrinsic electrical properties have been masked by inefficient contacts. Beyond OFET and semiconductor characterization, analog, low power, and high frequency applications require a greater level of device parameter control than has been obtained in typical DC, high-voltage OFETs. For analog applications, e.g., integration of conditioning circuits such as local sense amps for distributed flexible sensor arrays, nonlinearity of the transistor response inhibits proper functioning and limits applicability of common models used to design circuitry. Low power device development for novel materials are hindered by the high voltage turn-on and current suppression in devices with large R C . Considering high-frequency applications, Klauk suggest...

“…Second, the contact resistance is independent of the thickness of the semiconductor layer, which is beneficial whenever precise control of this thickness is difficult, as in the case of vacuum‐deposited small‐molecule semiconductors. Third, the contact resistance in coplanar TFTs depends strongly on the thickness of the gate dielectric and can thus, in principle, be reduced in a direct and systematic manner. For coplanar organic TFTs with a gate‐dielectric thickness of 5.3 nm, a contact resistance of 30 Ω cm was recently reported, resulting in an equivalent frequency of 3.5 MHz at 1.6 V (2.2 MHz V −1 ) in ring oscillators based on TFTs with a channel length of 1 μm and gate overlaps of 2 μm, and a transit frequency of 10.4 MHz at 3 V (3.5 MHz V −1 ) obtained from S ‐parameter measurements on TFTs with a channel length of 0.85 μm and gate overlaps of 5 μm (see Figure ).…”

Section: Resultsmentioning

confidence: 99%

“…Figure b shows the transit frequency at a drain–source voltage of 3 V calculated using Equation and plotted as a function of the width‐normalized contact resistance for the dimensions of the TFT in Figure ( L = 0.85 μm, L ov = 5 μm) and for the minimum dimensions achievable by stencil lithography ( L = 0.3 μm, L ov = 2 μm). Figure b indicates that the highest transit frequency that can realistically be expected for organic TFTs fabricated by stencil lithography is ≈30 MHz at 3 V for a contact resistance of 30 Ω cm and ≈100 MHz at 3 V in the event that the contact resistance can be reduced to about 5 Ω cm, e.g., by functionalizing the contact surfaces with better‐performing molecules or by reducing the gate‐dielectric thickness …”

Section: Resultsmentioning

confidence: 99%

Roadmap to Gigahertz Organic Transistors

Zschieschang

Borchert

Giorgio

et al. 2019

Despite the large body of research conducted on organic transistors, the transit frequency of organic field-effect transistors has seen virtually no improvement for a decade and remains far below 1 GHz. One reason is that most of the research is still focused on improving the charge-carrier mobility, a parameter that has little influence on the transit frequency of short-channel transistors. By examining the fundamental equations for the transit frequency of field-effect transistors and by extrapolating recent progress on the relevant device parameters, a roadmap to gigahertz organic transistors is derived.