Organic Thin-film Transistor Technology Research Articles

(Invited) Numerical Exploration of Spiking Neuron Circuits in Organic Potft Technology

Spike-based neural networks have been shown to hold more computational power than other architectures and their integration into mainstream computing is projected to herald a new age for information technology. Implementation using a flexible Thin Organic Large Area Electronics TOLAE process is a promising low cost way to integrate neuromorphic circuits into sensors, however its feasibility has never been demonstrated. Here we simulate spiking neuron circuits based on the characteristics of a gravure printed scalable organic thin film transistor technology. We then explore the variability of the parameters to show how much variation the circuits can support. Our results demonstrate the feasibility of OTFT technologies for spiking neuromorphic hardware. Figure 1

Low-Voltage, Flexible, and Self-Encapsulated Ultracompact Organic Thin-Film Transistors Based on Nanomembranes.

Organic thin-film transistors (OTFTs) are an ever-growing subject of research, powering recent technologies such as flexible and wearable electronics. Currently, many studies are being carried out to push forward the state-of-the-art OTFT technology to achieve characteristics that include high carrier mobility, low power consumption, flexibility, and the ability to operate under harsh conditions. Here, we tackle this task by proposing a novel OTFT architecture exploring the so-called rolled-up nanomembrane technology to fabricate low-voltage (<2 V), ultracompact OTFTs. As the OTFT gate electrode, we use strained nanomembranes, which allows all transistor components to be rolled-up and confined into a tubular-shaped tridimensional device structure with reduced footprint (ca. 90% of their planar counterpart), without any lossof electrical performance. Such an innovative architecture endows the OTFTs high mechanical flexibility (bending radius of <30 μm) and robustness-the devices can be reversibly deformed, withstanding more than 500 radial compression/decompression cycles. Additionally, the tubular device design possesses an inherent self-encapsulation characteristic that protects the OTFT active region from degradation by UV-light and hazardous vapors. The reported strategy is also shown to be compatible with different organic semiconductor materials. All of these characteristics contribute to further extending the potentialities of OTFTs, mainly toward rugged electronics.

A digital library for a flexible low-voltage organic thin-film transistor technology

This paper presents the design, fabrication and characterization of digital logic gates, flip-flops and shift registers based on low-voltage organic thin-film transistors (TFTs) on flexible plastic substrates. The organic transistors are based on the p-channel organic semiconductor dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and have channel lengths as short as 5 μm and gate-to-contact overlaps of 20 μm. The organic TFT is modeled which allows us to simulate different logic gate architectures prior to the fabrication process. In this study, the zero-VGS, biased-load and pseudo-CMOS logic families are investigated, where their static and dynamic operations are modeled and measured. The inverter and NAND gates use channel length of 5 μm and operate with a supply voltage of 3 V. Static and dynamic master-slave flip-flops based on biased-load and pseudo-CMOS logic are designed, fabricated and characterized. A new design for biased-load dynamic flip-flops is proposed, where transmission gate switches are implemented using only p-channel transistors. 1-stage shift registers based on the new design and fabricated using TFTs with a channel length of 20 μm operate with a maximum frequency of about 3 kHz.

Current Status and Opportunities of Organic Thin-Film Transistor Technologies

Attributed to its advantages of super mechanical flexibility, very low-temperature processing, and compatibility with low cost and high throughput manufacturing, organic thin-film transistor (OTFT) technology is able to bring electrical, mechanical, and industrial benefits to a wide range of new applications by activating nonflat surfaces with flexible displays, sensors, and other electronic functions. Despite both strong application demand and these significant technological advances, there is still a gap to be filled for OTFT technology to be widely commercially adopted. This paper provides a comprehensive review of the current status of OTFT technologies ranging from material, device, process, and integration, to design and system applications, and clarifies the real challenges behind to be addressed.

Dual-&lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$V_{\rm th}$ &lt;/tex-math&gt;&lt;/inline-formula&gt; Low-Voltage Solution Processed Organic Thin-Film Transistors With a Thick Polymer Dielectric Layer

Threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ) control using different metal gates (aluminum and gold) was applied to realize dual-V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> low-voltage solution processed organic thin film transistors (OTFTs). In the devices, the low-operation voltage was realized based on a channel engineering approach instead of using large gate dielectric capacitance, therefore, a relatively thick and low-dielectric constant polymer dielectric layer can be used. The devices present a mobility about 1.0 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(V·s), ON/OFF ratio of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> and small subthreshold swing of 100 mV/decade with the maximum processing temperature not exceeding 100 °C. It was found that, even with a 400-nm thick polymer dielectric layer, since possible charge trapping effects was effectively suppressed by the crosslinking processes, well controlled V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> was achieved. Both unipolar single-V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> and dual-V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> inverters were fabricated, clearly showing the influence of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> on the circuit operation and improved performance with the dual-V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> OTFT technology.

S-Parameter Characterization of Submicrometer Low-Voltage Organic Thin-Film Transistors

This letter presents the first comprehensive experimental studies on the frequency response of staggered low-voltage organic thin-film transistors (OTFTs) using S-parameter measurements. The transistors utilize air-stable dinaphtho-thieno-thiophene as the organic semiconductor with various channel lengths and gate overlaps. A peak cutoff frequency of 3.7 MHz for a channel length of 0.6 μm, gate overlap of 5 μm, and a supply voltage of 3 V is achieved. In view of the low supply voltage and air-stability, this result marks a record achievement in OTFT technology. The channel length dependence of the cutoff frequency is described in a compact model and a close correspondence to the measured data of OTFTs with variable device dimensions is shown. Moreover, the cutoff frequencies at different gate biases are found to be proportional to the dc transconductance.

Analog Building Blocks for Organic Smart Sensor Systems in Organic Thin-Film Transistor Technology on Flexible Plastic Foil

In this work, we present the implementation and measurement results of all analog building blocks of an organic smart sensor system on foil. The presented building blocks are a 1D and a 2D 4 × 4 pixel flexible capacitive touch sensor with a sample rate of 1.5 kS/s, a DC-connected two-stage opamp with a 20 dB DC gain, a Dickson DC-DC up-converter with output bias voltages up to 60 V and down to - 40 V which are used as a bias voltage in the other building blocks, and a ΔΣ ADC with a 26.5 dB precision and a band width of 15.6 Hz. The sensors, the opamp, the DC-DC converter and the ADC respectively consume 6 μA, 15 μA, 1 μA and 100 μA from a 15 V power supply.

Enhancement of carrier mobility in all-inkjet-printed organic thin-film transistors using a blend of poly(3-hexylthiophene) and carbon nanoparticles

To enhance the carrier mobility of all-inkjet-printed organic thin film transistors, we fabricated devices that incorporated poly(3-hexylthiophene) (P3HT) and carbon nanoparticles (CNPs). The fabricated devices had an on/off ratio of 10 4, which is one order less than that of pristine organic thin-film transistors (OTFTs). The maximum carrier mobility as high as 0.053 cm 2/V-s was achieved for a CNP/P3HT weight–weight ratio of 7/100. This degree of mobility is 10 times greater than average mobility of pristine P3HT-OTFTs. X-ray diffraction and scanning electron microscopy images reveal that the carrier mobility was enhanced by reducing the injection barrier and enhancing the carrier injection. This work demonstrates the feasibility of all-inkjet-printed OTFT technology.

A Fully Integrated $\Delta \Sigma$ ADC in Organic Thin-Film Transistor Technology on Flexible Plastic Foil

In this work we present a fully integrated first order continuous-time ΔΣ ADC made in a pentacene-based dual-gate organic thin-film transistor technology on flexible plastic foil. The ADC achieves a precision of 26.5 dB at a clock speed of 500 Hz and draws 100 μA from a 15 V power supply. As sub-blocks of the ADC, we also present a Vt -insensitive single-stage differential amplifier with 10 kHz GBW, a 3-stage operational amplifier, an integrator, a comparator and a level shifter. The circuits are designed following a strict Vt -insensitive design strategy and use high-pass filters for offset cancellation. The active area is 13 × 20 mm2.

Organic Transistors in Optical Displays and Microelectronic Applications

Organic thin-film transistors (OTFTs) offer unprecedented opportunities for implementation in a broad range of technological applications spanning from large-volume microelectronics and optical displays to chemical and biological sensors. In this Progress Report, we review the application of organic transistors in the fields of flexible optical displays and microelectronics. The advantages associated with the use of OTFT technology are discussed with primary emphasis on the latest developments in the area of active-matrix electrophoretic and organic light-emitting diode displays based on OTFT backplanes and on the application of organic transistors in microelectronics including digital and analog circuits.

Current Reduction Mechanism in Organic Thin Film Transistors

Hidden origin of current reduction mechanism in organic thin film transistor (OTFT) structure is investigated based on a device simulation, which makes it possible to derive current–voltage (I–V) characteristics, potential distribution and carrier concentration inside a device by solving Poisson's equation and continuity equation. It is made clear that the device performances improve more than twice to three hundred times only by inserting high impurity concentration layer under the source/drain contact region. The cause of the inferior characteristics of conventional OTFT devices is attributed to the deficiency of carriers in the channel region, resulting to a very high potential drop at the source–channel interface, thus leading to an apparent decrease of carrier mobility. These results clearly indicate the direction of OTFT technology development, i.e., development of high carrier concentration, or heavily doped organic semiconductor materials.

Effect of a SiNx insulator on device properties of pentacene-TFTs with a low-cost copper source/drain electrode

We fabricated different SiNx insulators at a 150–350 °C deposition temperature by plasma-enhanced chemical vapor deposition (PECVD) and investigated the effect of SiNx insulator on the device properties of pentacene-TFTs with a low-cost copper source/drain electrode. A pentacene-TFT with a SiNx insulator at a deposition temperature of 300 °C exhibited the highest saturation mobility of 0.29 cm2 (V s)−1, the lowest threshold voltage of −8.9 V and an on/off current ratio of 5.3 × 106. The best performance of the device with a SiNx insulator at a 300 °C deposition temperature can be attributed to the electrical properties and a relatively large water contact angle of insulator, smooth insulator surface and better crystallinity of the pentacene active layer. The present organic thin-film transistors (OTFTs) with the PECVD SiNx insulator and the low-cost Cu electrode could be a significant step toward the commercialization of OTFT technology.

Noise-Margin Analysis for Organic Thin-Film Complementary Technology

Parameter variation in organic thin-film transistor (OTFT) technology is known to limit the yield of digital circuits. It is expected that complementary OTFT technology (C-TFT) will reduce the sensitivity to parameter variations. In this paper, we quantify the dependence of yield on transistor parameter variations for C-TFT and compare it to unipolar logic. First, a basic inverter model is developed and fitted to measured transfer characteristics of organic complementary inverters. Next, the inverter model is used in numerical simulations to determine how the noise margin of the inverter, a measure for its reliable operation, changes as a function of transistor parameter variations. The noise margin is significantly improved with respect to p-type-only inverters with similar parameters. Finally, we perform circuit-level yield predictions as a function of parameter spread using the noise-margin simulations performed earlier.

Design of Organic TFT Pixel Electrode Circuit for Active-Matrix OLED Displays

A new current-programming pixel circuit for active-matrix organic light-emitting diode (AM-OLED) displays, composed of four organic thin-film transistors (OTFTs) and one capacitor, has been designed, simulated and evaluated. The most critical issue in realizing AMOLED displays with OTFTs is the variation and aging of the driving-TFTs and degradation of OLEDs. This can cause image sticking or degradation of image quality. These problems require compensation methods for high-quality display applications, and pixel level approach is considered to be one of the most important factors for improving display image quality. Our design shows that the current OLED and OTFT technology can be implemented for AMOLED displays, compensating the degradation of OTFT device characteristics.

Influence of transistor parameters on the noise margin of organic digital circuits

The concept of noise margin is crucial in the design and operation of digital logic circuits. Analytical expressions for the transfer curves of an inverter based on two depletion-mode p-type organic thin-film transistors (OTFTs) were calculated. Based on these expressions, the values for the noise margin of organic-based inverters were calculated. In this paper, the influence of the OTFT parameters on the noise margin is presented. Knowing that statistical variations of the transistor parameters are inherent to OTFT technology, these statistical variations are also taken into account. Finally, a circuit yield analysis is presented.