Organic Thin-Film Transistors : Process Technology and Circuit Design (Organische dunnefilmtransistoren : procesontwikkeling en ontwerp van schakelingen)

Organische halfgeleiders liggen aan de basis van een relatief nieuw enuitgebreid onderzoeksdomein, nl. de organische elektronica. Debijzondere eigenschappen van deze materialen leiden tot een aantalspecifieke voordelen van een dergelijke technologie. Men verwacht dathet gebruik van organische halfgeleiders zal leiden tot een bredewaaier van toepassingen. Organische lichtemitterende diodes zijntegenwoordig al commercieel beschikbaar, en het ligt in de lijn van deverwachtingen dat ook organische dunnefilmtransistors gebruikt zullenworden in een aantal specifieke toepassingen.Deze thesis handelt over organische dunnefilmtransistors. Tijdensdit doctoraatswerk werden twee belangrijke onderwerpen onderzocht. Dehoofdbrok van deze thesis bestaat uit de technologische ontwikkelingvan een proces om organische dunnefilmtransistors en schakelingen tefabriceren. Verder is er ook onderzoek verricht op het gebied van hetontwerp van schakelingen op basis van deze transistors.Na een bondige inleiding tot de organische elektronica, beschrijvenwe de technologische ontwikkeling van organische dunnefilmtransistors,en leggen we zo verband tussen een aantal technologische aspecten en deelektrische performantie van de transistors. De verschillende soortenorganische transistors worden toegelicht. De depositie van pentaceen deorganische halfgeleider, de substraatbehandeling en de patronering vande halfgeleider worden besproken. Er wordt bijzondere aandacht besteedaan een nieuwe substraatbehandeling en aan een nieuwe methode om deorganische halfgeleider te patroneren.Vervolgens gaan we dieper in op het ontwerp en de realisatie vanschakelingen, gebaseerd op organische dunnefilmtransistors. Webeschrijven in detail een nieuwe methode om n- en p-typeorganischedunnefilmtransistors te patroneren op eenzelfde substraat. Het succesvan deze complementaire technologie wordt overtuigend aangetoond doorde excellente werking van eencomplementaire organische invertor.Verder hebben we ook een lichtemitterende organischeveldeffecttransistor gerealiseerd, waarin een pn-heterojunctie in hettransistorkanaal gepatroneerd werd.Tenslotte beschrijven we de relatie tussen de parameters vanorganische p-type transistors, en de ruismarge van een invertor diegebaseerd is op dergelijke componenten. Er wordt ook een statistischeverdeling van de drempelspanning geïntroduceerd om na te gaan welkeinvloed deze niet-uniformiteit heeft op op de werking van de invertor.Omdat de ruismarge van een invertor een maatstaf is voor de robuustewerking ervan, hebben we een direct verband kunnen leggentussen deuniformiteit van de drempelspanning van de transistors en de opbrengstvan organische schakelingen. Deze analyse geeft overduidelijk aan datde uniformiteit van de drempelspanning een cruciale parameter is omorganische schakelingen van enige complexiteit te fabriceren.status: publishe

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. © 2006 IEEE.status: publishe

Intra-grain and oligo-grain top-contact organic thin film transistors

The properties of intra-grain and oligo-grain transistors without influencing film growth and with little contact effects were studied. Intra-grain devices enabled to extract the intrinsic charge-carrier mobility. Results of organic thin film transistors (OTFT) showed that for small channel lengths, grain boundary barriers were found to dominate the output characteristics of OTFT.status: publishe

Pentacene organic field-effect phototransistor with memory-effect

Now organic thin film transistor technology gains growing maturity, high performance organic photodetectors are the missing link towards full organic photosensitive sensor arrays, needed for the realization of applications like organic scanners and organic cameras. In the borderline of the research in organic solar cells, research in organic photodetectors has mainly been limited to organic photodiodes. However, phototransistors offer the possibility to reach higher sensitivities, thanks to the internal current gain of a transistor structure. This document focuses on organic field-effect phototransistors (organic photoFETs) where illumination can be used as an optical gate signal in addition to the electrical gate signal. An experimental description of pentacene photoFETs during broadband illumination is given. A distinction between the direct photocurrent and the additional current enhancement by a thresholdshift can be made, and a relation for this thresholdshift in function of time is revealed. Consequently, the difficulty to compare different organic photoFETs without any information about the illumination time is highlighted and a comparison based on this power law is proposed. Subsequently it is pointed out that by the stability of this thresholdshift after illumination, the pentacene photoFET does not only act as a light detector but also as a memory element, capable to store information about illumination power and duration for an extended amount of time.status: publishe

Bias stress in pentacene transistors measured by four probe transistor structures

This paper deals with operational lifetime measurements of organic field-effect transistors. The organic semiconductor under study is pentacene, the gate dielectric is thermal SiO2, treated with an OTS self-assembled monolayer, and the source and drain electrodes are gold, treated with thiols. The source and drain are defined by photolithography, and the channel length is 100 microns. We apply DC stress conditions on these structures, and monitor the output characteristics of the TFTs during stress as well as during recovery after stress. The transistor structures have been modified to incorporate two voltage-measurement probes in the channel in addition to the source and drain contacts. This results in a 4-probe configuration, that allows to measure the voltage drop in the intrinsic transistor channel separately from the voltage drop over the source and the drain contact regions. When applying a constant (DC) gate-source bias (VGS) corresponding to a field in the gate insulator of 1 MV/cm, we observe two degradation mechanisms: one part of the degradation is permanent, the other part recovers. The permanent degradation could be traced back to the drain side. It typically results in the current halving after 2 hours of stress at a vertical (gate insulator) field of 1 MV/cm. The degradation measured during stress includes both this permanent degradation and the recoverable pan. It is significantly larger in magnitude than the permanent degradation, resulting in typically a factor of 10 or more in drain current. The magnitude of the recoverable degradation not only depends on the gate voltage, but also on the drain voltage. It is therefore a consequence of the lateral field at the drain side of the channel. This phenomenological study is a first step towards a comprehensive model for degradation of bias stress in organic field-effect transistors. © 2004 IEEE.status: publishe

Patterning of organic thin film transistors by oxygen plasma etch

All applications of organic thin film transistors require patterning of the organic thin film to achieve a low off current and to prevent cross talk between neighboring transistors. A common method for patterning consists of using a protective layer and etching the uncovered small molecule film by oxygen plasma. One of the handicaps of this process is the observed degradation of the transistor characteristics. By varying the resist overlap, the authors show that the main cause of this performance degradation is, in fact, a far-reaching underetch of the oxygen plasma which can be overcome by choosing the right geometry of the resist pattern. © 2006 American Institute of Physics.status: publishe

Self-aligned surface treatment for thin-film organic transistors

For organic thin-film transistors where source-drain contacts are defined on the gate dielectric prior to the deposition of the semiconductor ("bottom-contact" configuration), the gate dielectric is often treated with a self-assembled molecular monolayer prior to deposition of the organic semiconductor. In this letter, we describe a method to apply an ultrathin solution-processed polymer layer as surface treatment. Our method is compatible with the use of the bottom-contact configuration, despite the fact that the polymeric surface treatment does not stand a photolithographic step. Furthermore, we show that our surface treatment results in superior transistor performance. © 2006 American Institute of Physics.status: publishe

Low voltage complementary organic inverters

We have developed a method for integrating n- and p-type organic thin-film transistors (OTFTs) on the same substrate. An integrated shadow mask was used for the n- and p-type semiconductor patterning. The integrated shadow mask can be aligned with submicron accuracy relative to the OTFT substrate. This allows for the integration at transistor level of n- and p-type OTFTs on the same substrate. A complementary inverter was fabricated, showing excellent performance while operating at a supply voltage of 2 V.status: publishe

Integrated shadow mask method for patterning small molecule organic semiconductors

We have developed a simple and efficient method for patterning small molecule semiconductors for applications in the field of organic electronics. In our approach, a profile is created using a single layer of photoresist, defining the regions where the organic semiconductor is to be deposited. Subsequent deposition of a small molecule semiconductor results in a discontinuity of the semiconductor film at the photoresist edge. The resulting transistor characteristics have an off current that is systematically below 1 pA. We demonstrate both p -type and n -type organic thin-film transistors using this method, using pentacene and copper hexadecafluorophthalocyanine (F16 CuPc), respectively. © 2006 American Institute of Physics.status: publishe