Short‐Channel Vertical Organic Field‐Effect Transistors with High On/Off Ratios

Abstract

requires a large transistor width and a short transistor channel length in order to excel in the mentioned attributes. [7][8][9][10] From the architectural perspective, in an integrated pixelated device, transistors should also have small footprints so that light-emitting/-sensing devices can occupy larger areas on each pixel. Yet for planar OFETs, the device area increases with the width and decreasing the channel length demands high-resolution patterning. A vertical geometry allows reduced area requirements by employing a submicrometer thick organic film as the transistor channel. [11,12] However, when the channel length of an OFET approaches the 100 nm regime, high electric fields result in large bulk current densities that cannot be modulated efficiently via a gate-field. [13][14][15] This phenomenon is known as the shortchannel effect and it restricts further improvement of vertical OFETs (VOFETs).In this report, a novel VOFET geometry is demonstrated addressing the short-channel effect, in particular by suppressing the undesired bulk current. It is based on a gate electrode consisting of massively parallel highly doped silicon nanopillars (Figure 1). Crucially different from other VOFETs, an insulating layer is deposited on top of the bottom contact in order to force injection of the charge carriers only from the sides of the bottom metal, and current flow within a thin layer close to the gate dielectric, minimizing space-charge limited current through the bulk semiconductor. Thanks to this unique geometry, ON/OFF ratios up to 10 6 can be realized, i.e., at least three orders of magnitude larger than in previously reported short-channel (100 nm) VOFETs. [11,15] In order to fabricate the nanopillars with a high areal density (≈10 6 pillars mm −2 ), optical resist dots (100 nm radius) arranged in a hexagonal lattice (250 nm lattice constant) are created using displacement Talbot lithography (DTL). It allows for fast wafer-scale patterning. [16] This resist dot pattern acts as an etch mask during etching of about 300 nm into the underlying silicon. Since the silicon is highly doped p-type (resistivity of 0.010-0.025 Ω cm), it can be directly used as a massively parallel nanopillar gate electrode for a single device (≈7 × 10 5 pillars per device of total size 0.7 µm 2 ). In our experiments, we use bulk silicon wafers, but silicon-on-insulator wafers can be used in order to electrically separate individual many-pillar devices. A stoichiometric low-pressure chemical-vapor-deposited (LPCVD) 45 nm silicon nitride (Si 3 N 4 ) isolates these A unique vertical organic field-effect transistor structure in which highly doped silicon nanopillars are utilized as a gate electrode is demonstrated. An additional dielectric layer, partly covering the source, suppresses bulk conduction and lowers the OFF current. Using a semiconducting polymer as active channel material, short-channel (100 nm) transistors with ON/OFF current ratios up to 10 6 are realized. The electronic behavior is explained using space-charge and contact-lim...

Edge-driven nanomembrane-based vertical organic transistors showing a multi-sensing capability

“…Such a geometry allows the fabrication of devices with nanoscale active channelsgiven by the thickness of the organic semiconductor (OSC) layer -having a large cross-sectional area. This assists in achieving higher J D at lower operating voltages [1][2][3][4] as compared to the traditional planar OFETs 5,6 . As a result, these devices are useful in applications that demand high current densities, such as lightemitting diode (LED) displays.…”

“…One of the primary objectives of fabricating OFETs in a vertical architecture has been the downscaling of transistor channel lengths to obtain higher current densities 3,17 . However, the main limitation for further shrinkage of channel length below 100 nm is related to the deposition of the top drain electrode using conventional evaporation techniques.…”

A Review of Vertical Organic Transistors

“…The group around Tamer Dogan recently proposed another vertical structure which we have denoted as an NP‐VOFET due to its similarity in operation to the VOFET design. In the NP‐VOFET, a pillar is formed by means of photolithography and etching, which defines a vertical edge along the direction the charge carrier transport occurs.…”

“…The red arrow represents the assumed current path. Reproduced with permission . Copyright 2019, Wiley‐VCH.…”

Optical Determination of Young’s Modulus of Nanoscale Organic Semiconductor Thin Films for Flexible Devices

“…In recent years, organic semiconductors have been extensively used in electronic, optoelectronic, and photovoltaic devices because of their unique photophysical properties such as tunable energy gap, high exciton binding energy, and thermal independence along with the mechanical flexibility and compatibility with large-scale continuous production processes such as roll-to-roll printing and solution processing. − In particular, in applications demanding mechanical flexibility and lightweight such as wearable devices, organic materials offer unique advantages over the inorganic counterparts. − However, because the molecules in organic semiconductors are held together by weak van der Waals forces, the elastic properties of an organic thin film can largely vary from those of its bulk form . To harness the full potential of organic materials for flexible thin-film nanoscale devices, a proper understanding and knowledge of the mechanical properties such as the elastic modulus are important.…”