Polyera Corporation

Introduction

A transistor is an electronic valve or switch, where the current flow between the source and drain electrodes is controlled by the magnitude of the or electric field applied at the gate, known as the gate bias. The charge flow in the transistor channel can be dominated by holes (positive charges) or electrons (negative charges), which define whether the semiconductor is p-type or n-type, respectively.

The most common type of transistor is the field-effect transistor (FET). An FET relies on an electric field to control the conductivity of a "channel" of one type of charge carrier in a semiconductor material. A thin-film transistor (TFT) is a special kind of FET made by depositing thin films of materials in a layered configuration known as the stack.

Architecture

A TFT is a three-terminal device, composed of a source, drain and gate electrodes, a dielectric (insulating) layer, and a semiconducting layer:

Currently a common substrate on which to build OTFTs is glass, since the primary application of OTFTs is in liquid crystal displays. With the increasing demand for flexible electronics, plastic substrates such as PEN are becoming more common. This differs from the conventional FET, where the semiconductor material such as silicon is the substrate.

In OTFTs, there are four different configurations or architectures as determined by the locations of the gate as well as the source and drain within the material stack:

• Top-Gate, Top-Contact (TGTC) TFT. Here the gate is farthest away from the substrate (i.e. the last layer to be applied), and the source and drain are placed at the top of the dielectric-semiconductor interface.

Top Gate, Top-Contact

• Top-Gate, Bottom-Contact (TGBC) TFT. Here the gate is still the outer-most layer, but the source and drain are now adjacent to the semiconductor-substrate interface.

Top Gate, Bottom Contact

• Bottom-Gate, Bottom-Contact (BGBC) TFT. Here the gate is embedded within the dielectric atop the substrate, and the source and drain are at the semiconductor-dielectric interface.

Bottom Gate, Bottom Contact

• Bottom-Gate, Top-Contact (BGTC) TFT. Here the gate is is embedded within the dielectric atop the substrate, but the source and drain are now applied on top of the semiconductor layer.

Bottom Gate, Top Contact

With each architecture having its pros and cons, the choice of device architecture often has real consequences. For example, TGBC devices offer a large injection-face (i.e. the surface from which the charge-carriers leave the source and enter the semiconductor). However, processing-wise, BGBC may be preferred, since the devices with the semiconductor layer as the last one has historically been the easiest to produce. This is due to that the semiconductor layer does not have to be exposed to the potentially damaging chemicals needed to process subsequent layers.

Performance

For transistors, the two most important performance parameters are the charge carrier mobility (u - how fast holes or electrons moves) and the current on-off ratio (Ion:Ioff - how efficient the current can be modulated by the source-gate bias). Furthermore, to maximize the transistor speed, the carrier mobility should be as high as possible and the distance between the source and drain electrodes (channel length) should be as short as possible.

When evaluating the performance of an OTFT device, there are three main parameters to consider:

• Mobility (u). Measured in m²/Vs, this is a measure of how easily charge carriers (i.e. electrons or holes) move within a material when subjected to an electric field. Commonly reproduced mobilities are on the order of 10^-1 while leading academic groups and companies producing mobilities close to 1.

• On/Off Ratio (I_on:I_off). Unitless, this is a measure of the relative difference in the source-drain current at two fixed gate voltages (these voltages are usually 0V and 20V and are often specified). Varying widely, a ratio of 10⁶ is preferable for most applications.

• Turn-On Voltage (V_on). Measured in volts, this indicates at what gate voltage the transistor switches on (as defined by a rapid increase in the source-drain current). The ideal V_on is 0V, as then the device requires voltage bias (and thus no power) to keep it in an "off" state.

Applications

OTFTs can be used for a wide variety of applications, including display backplanes and integrated circuits for lighting, sensors, RFID tags, and any application where logic circuitry is used. Learn more.

CMOS Circuits

Presently, most OTFTs are made with p-type semiconductor materials, as p-types have far superior performance. However, with the recent advances in n-type semiconductors, it is now possible to create circuits using both p- and n-type transistors using a technology known as CMOS. CMOS results in circuitry that is easier to design, cheaper to produce, and more energy-efficient. Learn more.

Challenges

Despite much progress made in OTFTs - especially the discovery of high-performing n-type materials to enable CMOS circuits - significant challenges still remain:

• Material Mobility. The carrier mobility of printed semiconductors is at least two orders of magnitude lower than crystalline inorganic materials.

• Feature Resolution. The resolution for the OFET channel length (L) in printed devices is larger than that in inorganic devices also by two orders of magnitude.

For these two reasons, OFET circuit speeds cannot compete with those based on silicon or GaAs. However, when the performance requirements are relaxed and/or there are the needs for additional device functions (eg, flexibility, easy integration, etc.) and/or to reduce costs, OFETs may become very competitive.

• Shelf and Operational Life. Even with substantial progress recently, the stability of organic materials both on the shelf and in performing devices is still not sufficient for many applications. Particularly challenging is in the area of OLED display backplanes, where even slight degradation of material performance can result in changes in pixel brightness easily detected by the human eye.

References