Effect of Trap Density on Carrier Propagation in Organic Field-Effect Transistors Investigated by Impedance Spectroscopy (original) (raw)
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Effect of Traps on Carrier Injection and Transport in Organic Field-effect Transistor
IEEJ Transactions on Electrical and Electronic Engineering, 2010
This study illustrates effect of traps on the charge injection and transport in the organic field-effect transistor (OFET). Here are included silicon nanoparticles (NPs) on a semiconductor-gate insulator interface, which work as trapping centers of charge carriers. Charge transport and injection phenomena are investigated by electrical measurements in presence of traps with designed densities. We find that OFETs with a low concentration of intrinsic carriers, such as a pentacene, are extremely sensitive to the internal electric fields. A significant threshold voltage shift due to trapped charge is observed, with a possibility to tune it by controlling the NP density. We demonstrate that the NP film can serve to design the amount of the accumulated charge in OFET and thus change the space-charge-limited conditions to the injection-limited conditions. A detailed analysis of pentacene OFET based on dielectric properties and the Maxwell-Wagner model reveals the internal electric field created by NPs. Additionally, the effect of NPs is discussed with respect to effective mobility, and its decrease is related to deceleration of carrier propagation by the trapping effect as well as low injection due to the increase of the carrier injection barrier by the internal field.
Impact of the interfacial traps on the charge accumulation in organic transistors
Journal of Experimental Nanoscience, 2013
A silver nanoparticles (NPs) self-assembled monolayer (SAM) was introduced in the pentacene organic field-effect transistors using modified Langmuir-Sch€ affer (horizontal lifting) technique, and its effect was being evaluated electrically using current-voltage (IÀV) and impedance spectroscopy (IS) measurements. The IÀV results showed a significant negative threshold voltage shift indicating that hole trapping phenomena exist inside the devices with about two orders higher in the contact resistance and an order lower in the effective mobility when a SAM was introduced. The IS measurements with the simulation of Maxwell-Wagner equivalent circuit revealed that the incorporation of the NPs SAM had a comparable negative voltage shift as in I-V measurements with a higher trapping time based on the simulation results.
Japanese Journal of Applied Physics, 2011
Transient measurements of impedance spectroscopy and electrical time-of-flight (TOF) techniques were used for the evaluation of carrier propagation dependence on applied potentials in a pentacene organic field effect transistor (OFET). These techniques are based on carrier propagation, thus isolates the effect of charge density. The intrinsic mobility which is free from contact resistance effects was obtained by measurement of various channel lengths. The obtained intrinsic mobility shows good correspondence with steady-state current-voltage measurement's saturation mobility. However, their power law relations on mobility vs applied potential resulted in different exponents, suggesting different carrier propagation mechanisms, which is attributable to filling of traps or space charge field in the channel region. The hypothesis was verified by a modified electrical TOF experiment which demonstrated how the accumulated charges in the channel influence the effective mobility.
Journal of Applied Physics, 2010
A significant difference between the transient electric field profiles of the pentacene organic field-effect transistors ͑OFETs͒ with SiO 2 and poly͑methyl-methacrylate͒ ͑PMMA͒ insulators was found by the time-resolved microscopic optical second-harmonic generation ͑TRM-SHG͒ experiment. The profile of former device was broad and changed smoothly, while the latter one had a sharp peak. Particularly, the peak of the transient electric field in SiO 2-insulated devices moved much faster than that in the PMMA-insulated one. Based on several experimental evidences and computational simulations, we proposed that these differences might arise from a higher trapped carrier density in the conductive channel on the PMMA insulator. Simple approaches were developed to evaluate the trap density and define dynamic carrier mobility in terms of the transient electric field measured by the TRM-SHG technique. This mobility quantitatively depicts that the transient hole transport in the OFET with the PMMA insulator is trap controlled.
Applied Physics A: …, 2009
A dynamic method for quantifying the amount and mechanism of trapping in organic field effect transistors (OFETs) is proposed. It exploits transfer characteristics acquired upon application of a triangular waveform gate sweep V G . The analysis of the transfer characteristics at the turning point V G = −V max between forward and backward gate sweeps, viz. around the maximum gate voltage V max applied, provides a differential slope m which depends exclusively on trapping. Upon a systematic change of V max it is possible to extract the initial threshold voltage, equivalent to one of the observables of conventional stress measurements, and assess the mechanism of trapping via the functional dependence on the current. The analysis of the differential logarithmic derivative at the turning point yields the parameters of trapping, as the exponent β and the time scale of trapping τ . In the case of an ultra-thin pentacene OFET we extract β = 1 and τ = 10 2 -10 3 s, in agreement with an exponential distribution of traps. The analysis of the hysteresis parameter m is completely general and explores time scales much shorter than those involved in bias stress measurements, thus avoiding irreversible damage to the device.
Modeling of static electrical properties in organic field-effect transistors
Journal of Applied Physics, 2011
A modeling of organic field-effect transistors' (OFETs') electrical characteristics is presented. This model is based on a one-dimensional (1-D) Poisson's equation solution that solves the potential profile in the organic semiconducting film. Most importantly, it demonstrates that, due to the common open-surface configuration used in organic transistors, the conduction occurs in the film volume below threshold. This is because the potential at the free surface is not fixed to zero but rather rises also with the gate bias. The tail of carrier concentration at the free surface is therefore significantly modulated by the gate bias, which partially explains the gate-voltage dependent contact resistance. At the same time in the so-called subthreshold region, we observe a clear charge trapping from the difference between C-V and I-V measurements; hence a traps study by numerical simulation is also performed. By combining the analytical modeling and the traps analysis, the questions on the C-V and I-V characteristics are answered. Finally, the combined results obtained with traps fit well the experimental data in both pentacene and bis(triisopropylsilylethynyl)pentacene OFETs.
Journal of Materials Science: Materials in Electronics, 2017
field-effect transistors (OFETs) and sensors [4] have been fabricated. In recent years, Organic field-effect transistors show a considerable development because of their low manufacturing costs, their power consumption and the possibility to fabricate electronic devices performing simple operations or functions such as inverter and ring oscillators [5]. On the other hand, the variety of synthetic organic chemistry has enabled the engineering of both n-and p-type semiconductors, giving rise to many potential candidates for circuit designs based on CMOS technology [6]. The study of the charge transport in these materials is therefore very important for the development of organic devices and the optimizing of their characteristics. It is therefore critical that the research in this area is necessarily be coupled with theoretical perspective, in order to resolve several of the important issues, which will help in further enhancing the electrical properties of these devices. The basic idea of this paper is defended by simulation and comparison between two stable n-and p-type materials, pentacene and Polyera™ N2200, respectively. We analyse the differences between the two types of transistors, in terms of the electrical properties and the influences caused by structural changes. For this, we use a 2D drift-diffusion simulator Integrated System Engineering-Technology Computer Aided Design (ISE-TCAD®). 1.1 Structures and operating principal We address the influence of mobility on the OTFTs electrical characteristics and the interplay between bulk traps states and fixed charges at the oxide/semiconductor Interface. As a reference, we will compare the simulated characteristics of both types of OTFTs to those realized by our group. The dependence of the transfer characteristic on Abstract Pentacene and Polyera™ N2200 based-organic field effect transistors (OFETs), have been fabricated and simulated in a bottom-gate/back contacts configuration. The simulations were processed with the help of 2D driftdiffusion model. Comparison and analysis of the electrical characteristics of both n-and p-channel OFETs have been investigated. The study was centred on the electrical performance of every structure in term of mobility, fixed charge at the oxide/semiconductor interface and bulk traps density and its related energy. The dependence of the transfer characteristic on the grain boundaries traps state, in the case of pentacene, has been also outlined. Comparison between the simulation results and our experimental data show a good agreement. We finally present how our model can be applied to different devices with different channel length and we analyse their relationship with extracted electrical parameters.
Trap-limited electrical properties of organic semiconductor devices
arXiv (Cornell University), 2023
We investigated the electrical properties of a unipolar organic device with traps that were intentionally inserted into a particular position in the device. Depending on their inserted position, the traps significantly alter the charge distribution and the resulting electric field as well as the charge transport behavior in the device. In particular, as the traps are situated closer to a charge-injection electrode, the band bending of a trap-containing organic layer occurs more strongly so that it effectively imposes a higher charge injection barrier. We propose an electrical model that fully accounts for the observed change in the electrical properties of the device with respect to the trap position.
IETI Transactions on Engineering Research and Practice, 2018
In this paper, new-improved carriers mobility model of OFET (Organic Field Effect Transistor) structures is presented. It is proposed to introduce two new factors: traps concentration ratio and electric field degradation factor, in carriers mobility model. The imact of OFET geometry is also considered. The above-mentioned model includes also carriers mobility dependence on temperature and electric field. Proposed model is incorporated in current-voltage characteristics of OFET.
Exploiting diffusion currents at Ohmic contacts for trap characterization in organic semiconductors
Organic Electronics, 2014
Studying space-charge limited currents enables fundamental insight into the properties of charge carrier transport. However, in unipolar devices with Ohmic contacts, diffusion of charge carriers from the contacts into the intrinsic layer can dominate the current-voltage (J-V) characteristics, especially when the devices are thin as in organic electronic devices ($100 nm). Thus, the common approximation of drift-only trap-limited currents (J $ V lþ1 ) caused by an exponential distribution of traps is not applicable for determination of the trap distribution. Here, we show by numerical drift-diffusion simulations of unipolar devices with p-doped injection layers (p-i-p devices), how diffusion currents affect the J-V power law depending on the intrinsic layer thickness for typical transport parameters of organic semiconductors. As the thickness dependence of the power law is characteristic of the trap distribution, the distribution can be determined from a simple variation of the device thickness.