Reza Chaji

Backplane Requirements for Active Matrix Organic Light Emitting Diode Displays

Organic light emitting diode (OLED) displays are a serious competitor to liquid crystal displays in view of their superior picture quality, higher contrast, faster on/off response, thinner profile, and high power efficiency. For large area and/or high-resolution applications, an active matrix OLED (AMOLED) addressing scheme is vital. The active matrix backplane can be made with amorphous silicon (a-Si), polysilicon, or organic technology, all of which suffer from threshold voltage shift and/or mismatch problems, causing temporal or spatial variations in the OLED brightness. In addition, the efficiency of the OLED itself degrades over time. Despite these shortcomings, there has been considerable progress in development of AMOLED displays using circuit solutions engineered to provide stable and uniform brightness. Indeed the design of AMOLED pixel circuits, particularly in low-mobility TFT technologies such as a-Si, is challenging due to the stringent requirements of timing, current matching, and low voltage operation. While circuit solutions are necessary, they are not sufficient. Process improvements to enhance TFT performance are becoming inevitable. This paper will review pertinent material requirements of AMOLED backplanes along with design considerations that address pixel architecture, contact resistance, and more importantly, the threshold voltage stability and associated gate overdrive voltage. In particular, we address the question of whether conventional PECVD can be deployed for high mobility and high stability TFTs, and if micro-/nano-crystalline silicon could provide the solution.

46.1: Invited Paper: a‐Si for AMOLED — Meeting the Performance and Cost Demands of Display Applications (Cell Phone to HDTV)

Amorphous silicon AMOLED displays are almost ready for mass production and will be found in all display applications from high resolution cell phone to HDTV. But before commercialization can occur, the problems of lifetime and image sticking need to be overcome. When it comes to pixel circuit design, there is no one-size-fits-all solution. Several drive schemes and pixel circuits are presented, each with unique advantages, and each suited for a particular application. All leverage amorphous silicon manufacturing infrastructure which leads to the lowest cost position for AMOLEDs on glass.

An impedance-based integrated biosensor for suspended DNA characterization

Herein, we describe a novel integrated biosensor for performing dielectric spectroscopy to analyze biological samples. We analyzed biomolecule samples with different concentrations and demonstrated that the solutions impedance is highly correlated with the concentration, indicating that it may be possible to use this sensor as a concentration sensor. In contrast with standard spectrophotometers, this sensor offers a low-cost and purely electrical solution for the quantitative analysis of biomolecule solutions. In addition to determining concentrations, we found that the sample solution impedance is highly correlated with the length of the DNA fragments, indicating that the sizes of PCR products could be validated with an integrated chip-based, sample-friendly system within a few minutes. The system could be the basis of a rapid, low-cost platform for DNA characterization with broad applications in cancer and genetic disease research.

A Sub-μA fast-settling current-programmed pixel circuit for AMOLED displays

Current-programmed active matrix organic light emitting diode (AMOLED) displays have been valued for their immunity to spatial mismatch, differential aging, and temperature variation. However, the long settling time particularly at small current levels and large parasitic capacitance can be a constraint. This paper presents a new current-programmed pixel circuit that improves the settling time while preserving the stability of current programming. The pixel circuit was fabricated in amorphous silicon technology. The settling time of the new pixel circuit can be as low as 20 mus whereas the settling time of conventional current-programmed pixel is more than 2 ms.

Indium oxides by reactive ion beam assisted evaporation: From material study to device application

Indium oxides were deposited by reactive ion beam assisted e-beam evaporation at room temperature. A material study was conducted through a variety of material characterization including crystal structure, electrical properties, optical properties, and chemical composition, along with an investigation of material properties as a function of primary deposition parameters such as ion flux and deposition rate. Implementing the developed semiconducting indium oxide as a channel material, the authors further demonstrated high-performance indium oxide thin-film transistors (TFTs) with conventional silicon dioxide gate dielectric derived by plasma-enhanced chemical vapor deposition (PECVD). The n-channel TFT has a threshold voltage of ∼2.0 V, a field-effect mobility of 33 cm2/V s at a gate bias of 20 V, an ON/OFF current ratio of 108, and a subthreshold slope of 2.0 V/decade. The stability study displays a small threshold voltage shift of ∼0.6 V under a 60 h constant current stress condition. The TFT reported he...

Oxide thin film transistor technology: Capturing device-circuit interactions

This paper presents the current status of oxide semiconductor technology for applications ranging from interactive displays to imaging systems with a strong focus on device-circuit interaction to compensate for material weaknesses and issues related to non-uniformity, instability, and persistent photoconductivity.

Call for papers second international workshop on compact thin-film transistor (TFT) modeling for circuit simulation

In recent years, the increasing use of active matrix flat-panel displays and bio-medical imagers in commercial electronic products has drawn a significant attention to thin-film transistors (TFT) and technologies. TFTs on amorphous- and poly-silicon as well as newly emerging organic, transparent metal oxide and nano-composite semiconductor technologies are becoming increasingly common. For example, flat panel displays are finding widespread use in many products such as cellular phones, personal digital assistants (PDAs), camcorders, laptop personal computers (PCs), to name a few. The active matrix display is composed of a grid or matrix of picture elements called as “pixels”. Thousands or millions of these pixels together create an image on the display, in which the TFTs act as switches to individually turn each pixel. More increasingly TFTs are starting to be used as analog circuit elements for rudimentary signal conditioning. Therefore, physically-based compact modeling of TFTs for circuit simulation is crucial to accurately and reliably predict TFT behavior in the active matrix. A concentrated R&D effort is critical for developing physically-based compact TFT models for emerging thin-film technologies, and significant R&D efforts along these lines are underway world-wide.