LAMP
Laboratory for Advanced Molecular Processing

Organic Thin Film Transistors

Organic thin film transistors (OTFTs)

Transistors based on organic semiconductors as active layer to control current flow are known as organic thin film transistors (OTFTs). For the past ten years, organic polymers and oligomers have been extensively investigated as a semiconducting active layer in the thin film transistor (TFT) configuration. OTFTs will find use in a number of low-cost, large area electronic applications such as liquid crystal flat panel displays, active matrix all organic emissive flat panel displays, imagers, smart cards, smart price and inventory tags, large-area sensor arrays and complementary thin film integrated circuits. OTFTs have the advantage of light weight with bendable electronic feature and use of an organic semiconductor layer allows for inexpensive and low temperature processing of the devices. OTFTs fabricated at low temperatures allow the use of flexible plastic substrates and the organic layer applied by spin coating allows fast and inexpensive coverage of large area. These devices are not expected to compete with silicon technology in the production of high-end products but they can be components of lower resolution, mass produced items, such as identification tags, smart cards, and pixel drivers for displays.
High Performance Devices
Organic Thin Film Transistor Mobility

Organic semiconductors

Organic semiconductor materials primarily consist of carbon, hydrogen, and oxygen. Four classes of organic semiconductors have been used in organic FET (OFET) applications
1. "Small" molecules based on (hetero) aromatic rings
2. Conjugated polymers
3. Hybrid organic-inorganic structures
4. Molecular semiconductors such as nanotube-based semiconductors
A unique characteristic of small molecule organic semiconductors is their ability to form high-quality polycrystalline organic films using vacuum deposition to achieve high field-effect device mobilities. Among small molecule organic semiconductor materials, pentacene is the most extensively studied due to its commercial availability and relatively well-understood processing techniques, which result in polycrystalline films with enhanced device performance. The reported field-effect mobility of vacuum deposited pentacene-based thin film transistors has been regularly quoted in the range of 0.5-1.5 cm2/Vs which is comparable to devices using alpha-Si as the semiconductor material. The reported on/off current ratio of such devices was around 6 orders of magnitude.

Dielectrics

The dielectric is one of the most critical, but sometimes underappreciated, materials for organic transistor performance. Requirements of dielectric films are
1. Low trapping density at the surface
2. Low surface roughness
3. Low impurity concentration
4. Compatibility with organic semiconductors
Two categories of dielectrics are commonly used in organic transistors: inorganic and organic. Examples of inorganic dielectrics include silicon dioxide and silicon nitride, which are also often used for the gate dielectric in traditional silicon integrated circuits and amorphous silicon display backplanes. The mature manufacturing processes, chemical vapor deposition, and physical vapor deposition, are capable of depositing a pinhole-free gate dielectric layer with a thickness of a few hundred angstroms. The well-known dielectric characteristics of these materials significantly reduce process variability. However, the non-solution processable nature of these materials has prompted researchers to investigate solution processable organic dielectrics, Poly-vinylephenyle and polymethylmethacrylate (PMMA). Note that the performance of organic devices using solution processable organic dielectrics depend significantly on the semiconductor material used and quality of dielectric deposition process. Spin coating has been successfully used to deposit thin and pin-hole free polymer gate dielectrics.

Conductors

Evaporated gold and platinum are commonly used as electrodes for organic transistor fabrication. The minimal process control required can produce flawless thin metal films for device electrodes. The environmentally stable nature of Au and Pt also ensures low contact resistance at the source-drain and semiconductor interfaces. Conductive polymers poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT/PSS) and polyaniline (PANI) have also been used as organic transistor electrodes which are both commercially available. The conductive polymer inks can be printed, screen printed, ink jetted, or spun coated. The interface at the source-drain and organic semiconductor requires low contact resistance, which is a function of both parasitic resistance and the resistance from the energy barrier at the electrode/semiconductor interface. Low energy barriers are a result of matching the electrode work function with semiconductor ionization potential. This low energy barrier is required to promote hole injection at the interface.
Polyaniline PEDOT-PASS

Contact resistance

Despite the considerable progress made in recent years in improving the performance of organic TFTs, many of the design, material, and process parameters impacting organic TFT performance are still poorly understood and poorly controlled. One such parameter is contact resistance. Unlike in field-effect transistors based on single-crystalline silicon, polycrystalline silicon, or hydrogenated amorphous silicon, the source and drain contacts in organic TFTs are not easily optimized by conventional processes, such as semiconductor doping or metal alloying. Consequently, organic TFT performance typically suffers from large contact resistance, perhaps to the point where the speed of organic integrated circuits may not be limited by the intrinsic carrier mobility of the organic semiconductor, but by the contact resistance of the TFTs.
It is well known that the formation of the metal/organic interface is one of the most important factors in determining the device performance, since the charge injection into the semiconducting layer via the interface depends on the quality of the contact formed. Based on the mechanism of the OTFT, the source-drain current, IDS, will show a linear behavior at the small bias, VDS along with the conductivity of the film when the metal-organic interface has an ohmic contact. However, the non-ohmic contact property makes the behavior of Ids nonlinear at low VDS. To measure contact resistance, Transmission Line Method (TLM method) was used and to improve contact properties between organic semiconductors and metal electrodes, modification of the drain and source contacts using conductive polymer, O2 plasma treatment, UV treatment and charge transfer compounds by a self-assembled approach have been applied.
Higher Hole Injection Barrier -  Higher Contact Resistance
Contact Resistance