Dimensional scaling of high-speed printed organic transistors enabling high-frequency operation

Printed electronics has promised to deliver low-cost, large-area and flexible electronics for mass-market applications for some time; however, so far one limiting factor has been device performance. Over the last decade, great progress has been made in terms of materials, processing and printing resolution for printed transistors. In this article, we review dimensional scaling of printed organic thin-film transistors, which has enabled high-frequency operation. We review different device architectures that require different dimensions to be scaled with accompanying tradeoffs in performance and complexity. Various printing methods have been used to print scaled transistors. Inkjet and gravure printing have seen the greatest improvements. We will focus on gravure printing here as it not only enables high-resolution features but also high-speed printing for low-cost manufacturing. Operating voltage has been scaled down less aggressively due to difficulties with scaling down the thickness of printed gate dielectrics. The performance of organic semiconductor materials has also improved substantially. When processing the semiconductor, the scaling of other device dimensions needs to be considered to optimize performance. Based on these advances, transistor switching frequency has increased dramatically over the last decade with several reports of high-speed printed inverters operating at high kHz to low MHz frequencies, which are promising results for emerging applications of printed electronics.

Contact Effects in Organic Transistors

Frederik Sebastian Ante

Organic thin-film transistors (TFTs) are of interest for applications that require electronic functionality with low or medium complexity distributed over large areas on unconventional substrates, such as flexible polymeric films. Generally these are applications in which the use of silicon devices and circuits is technically or economically not feasible. Examples include flexible displays and large-area sensor arrays. The static and dynamic performance of state-of-the-art organic p-channel transistors is already sufficient for applications in which the transistors operate with frequencies of a few tens of kilohertz. Strategies to improve the performance of organic TFTs include improvements in the field-effect mobility of the charge carriers in the organic semiconductor layer and more aggressive scaling of the transistor dimensions. Additional advances are also required in the environmental and operational stability of organic TFTs and in the application of controlled contact doping for the reduction of the contact resistance of organic TFTs. In this thesis, high-resolution silicon stencil masks are introduced to pattern top-contact organic TFTs with lateral dimensions as small as 0.8 µm. Taking advantage of the recently synthesized high-mobility air-stable organic semiconductor dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]thiophene (DNTT), polycrystalline organic p-channel TFTs with excellent air-stability and an intrinsic mobility of 3.4 cm2/Vs were fabricated. The transistors employ an ultra-thin gate dielectric based on a plasma-grown oxide layer in combination with an organic self-assembled monolayer (gate capacitance ∼1 µF/cm2), allowing the TFTs to operate with low voltages of about 3 V. Transistors with dimensions of less than 100 nm were manufactured using polymer resist shadow masks that have been fabricated by electron-beam lithography. Thanks to the 5 nm thick gate dielectric the TFTs show no short-channel effects. The subthreshold swing, the on/off current ratio, and the threshold voltage are comparable to long-channel TFTs with lateral dimensions of ∼100 µm. However, organic transistors with reduced dimensions are often limited by the energy barrier at the interface between the semiconductor and the source/drain contacts. One approach to reduce this energy barrier is the introduction of a strong electron acceptor into the contact area that acts as a dopant. With contact doping, the contact resistance is reduced and the effective mobility of short-channel TFTs increases by more than 200%. Transistors (without doping) with lateral dimensions of ∼1 µm have a transconductance of about 1 S/m, so that the TFTs operate at frequencies of several megahertz. The influence of strongly reduced lateral dimensions on the contact resistance and the maximum operation frequency is investigated and scaling limitations are discussed.

EPFL2011

Field-Effect Transistors Based on ZnO Nanowires

Daniel Dominik Kälblein

Semiconductor nanowires are an emerging class of materials with great potential for applications in future electronic devices. The small footprint and the large charge-carrier mobilities of nanowires make them potentially useful for applications with high-integration density, but also to replace thin-film transistors in large-area electronics. This thesis investigates the use of wet-chemically grown ZnO nanowires for the fabrication of high-performance, low-voltage field-effect transistors (FETs). The first part of this thesis addresses the electrical characteristics of the wet-chemically grown ZnO nanowires. The as-grown ZnO nanowires are highly conductive due to a large charge-carrier concentration caused by dopants unintentionally incorporated into the ZnO nanowires during the synthesis. This large charge-carrier concentration makes it difficult to modulate the conductivity of the nanowires by an external electric field, so that the as-grown wires are not suitable for FETs. A post-growth annealing step is employed to dramatically reduce the charge-carrier concentration of the nanowires which makes it possible to fabricate FETs with useful characteristics. ZnO-nanowire FETs in a global back-gate geometry based on annealed ZnO nanowires have a transconductance of 300 nS, an on/off current ratio of 107 and a subthrehold slope of 500 mV/decade. The maximum field-effect mobility of the annealed ZnO nanowires is around 50 cm2/Vs. An important requirement to obtain good FET characterisitcs is to minimize the influence of the source and drain contact resistance on the total device resistance. The material used for the source and drain contacts in this thesis is aluminum. It is demonstrated that the use of a plasma treatment in the contact regions prior to the evaporation of the aluminum contacts can greatly influence the contact properties between ZnO and aluminum. An argon-plasma treatment locally increases the charge-carrier concentration in the ZnO. It is shown that the doping effect of the argon-plasma treatment can be exploited to reduce the contact resistance between alumiunm and ZnO and improve the electrical performance of the FETs. In the second part of this thesis, the influence of the ambient air on the electrical characteristics of the ZnO-nanowire FETs is investigated. The electrical conductivity of the FETs is strongly affected by the distinct atmospheric conditions, which makes it difficult to obtain reliable FET characteristics. The potential of a self-assembled monolayer (SAM) based on fluoroalkylphosphonic acid molecules to passivate the ZnO nanowires against the undesirable effects of the ambient and to stabilize the electrical performance of ZnO-nanowire FETs is demonstrated. The stabilizing effect is attributed to the formation of a densely packed, hydrophobic SAM on the surface of ZnO and aluminum oxide. The quality of the hydrophobic SAMs is confirmed by means of water-contact-angle measurements on SAM-covered ZnO single crystals that show a contact angle of more than 110°. The third part of this thesis is dedicated to the fabrication of high-performance ZnO-nanowire FETs with patterned metal top-gate electrodes. A top-gate fabrication process is developed that uses a very thin gate dielectric consisting only of an alkylphosphonic acid SAM that can be deposited from solution. The insulating quality of the SAM is investigated for top-gate FETs that utilize either gold or aluminum for the top-gate electrode. Gold top-gate FETs show a transconductance of 1 µS, an on/off current ratio of 107, and a steep subthreshold slope of 90 mV/decade. The thin gate dielectric makes it possible to operate the gold top-gate FETs at low voltages of 1 V and simultaneously reduces the undesired gate current by more than three orders of magnitude compared to gold top-gate FETs that have been fabricated without a gate dielectric (MESFETs). The gate current of the gold top-gate FETs with SAM gate dielectric is as low as 1 pA for gate-source voltages between -1 V and 0.5 V. When aluminum is used for the top-gate electrode, a hybrid dielectric is formed at the interface between the SAM and the aluminum, consisting of the SAM and a spontaneously formed aluminum oxide layer. Owing to the hybrid gate dielectric, the aluminum top-gate FETs are operated with gate currents below 1 pA for voltages up to 3 V. The top-gate fabrication process does not require temperatures above 160 °C, which makes it possible to fabricate FETs on unconventional substrates that cannot tolerate high temperatures, such as glass or flexible plastics. Integrated circuits on single ZnO nanowires that are fabricated on glass and plastic substrates are demonstrated. The maximum switching frequency of the inverters is 1 MHz.

EPFL2011

Functionalisation of inkjet-printed Organic Field Effect Transistors with odorant binding proteins for the detection of gaseous analytes

Danick Briand, Nico de Rooij, Giorgio Mattana, Francisco Molina Lopez

Inkjet-printing of solution-processable organic materials is a well-known fabrication method utilised to produce electronic devices on flexible substrates. It is especially suitable to realise, with a simple, digital layout design process, large-area devices with limited production costs [1]. Among the several different types of electronic devices already fabricated, a particularly important role is played by Organic Field Effect Transistors (OFETs) [2]. OFETs have been already proved to be capable to detect chemical or biological analytes in gaseous environments [3, 4], essentially by exploiting the chemo-physical interaction between the gaseous species to be detected and the transistor’s semiconductive channel, but so far the integration of biological molecules into the transistors’ active layers to improve sensing performances has received little attention. In addition to that, it should be also pointed out the fact that, in most cases, gas-sensing OFETs reported in the literature were fabricated with expensive cleanroom fabrication techniques on rigid substrates. In this paper, we address these two different issues. First of all, we report on the fabrication of bottom-contact, top-gate OFETs on flexible, plastic substrates (PEN) realised by means of inkjet-printing. Conductive layers and electrodes were fabricated by utilising inkjet-printable silver nanoparticles based inks while commercial inkjet-printable TIPS-pentacene and N2200 were used as active layers for the realisation of p – type and n – type channel organic transistors, respectively; parylene-C was used as gate dielectric while the gate electrode was fabricated by using a conductive polymer solution. The printed OFETs structure is shown in Fig. 1. In order to detect gaseous analytes, we opted for a different approach than that usually reported in the literature, choosing to functionalise the gate electrode by inserting, into the solution used for its fabrication, a small amount of odorant binding proteins (OBPs). This approach is compatible with roll-to-roll, large area production techniques and, in combination with the top-gate architecture, allows to maximise the sensing active area. OBPs are low-molecular-weight biological molecules highly concentrated in the nasal mucus of vertebrates and in the sensillar lymph of insects; their strong chemical affinity towards odours and pheromones suggests that they may play a crucial role in olfactory perception [5]. By incorporating OBPs within the gate electrode, we attempted to use such functionalised OFETs as transducers, in order to detect some typologies of Volatile Organic Compounds (VOC) (in particular, menthone and linalool); an example of OFET’s response to such menthone (in terms of current increase when the transistor is exposed to the analyte plotted as a function of gate voltage) is depicted in Fig. 2. The preliminary results obtained, prove that OBPs may be successfully used to functionalise OFETs in order to make organic transistors able to detect VOCs.

2013

Contact Effects in Organic Transistors

Frederik Sebastian Ante

EPFL2011

Field-Effect Transistors Based on ZnO Nanowires

Daniel Dominik Kälblein

EPFL2011

Functionalisation of inkjet-printed Organic Field Effect Transistors with odorant binding proteins for the detection of gaseous analytes

Danick Briand, Nico de Rooij, Giorgio Mattana, Francisco Molina Lopez

2013