G. Reza Chaji

Stable a-Si:H circuits based on short-term stress stability of amorphous silicon thin film transistors

Hydrogenated amorphous silicon (a-Si:H) technology is interesting for large-area active matrix structure due to its good uniformity over large-area, low-temperature, and low-cost fabrication, and its industrial accessibility. However, the circuits implemented in this technology suffer from the instability of the material under prolonged bias stress. To improve the circuit stability, we present a circuit design technique based on the stability of a-Si:H thin film transistors (TFTs) under short-term bias stress. Here, an a-Si:H local current source (LCS) is used to adjust the circuit current bias. Since the LCS circuit is under stress for a small fraction of operation time, its current remains stable. The measurement and analysis of the LCS circuit indicate that the a-Si:H TFT is stable under short-term bias stress for over 50000h. Also, we present a pixel circuit based on this technique for active matrix organic light emitting diode displays.

52.2: A Low‐Cost Stable Amorphous Silicon AMOLED Display with Full VT‐ and VOLED Shift Compensation

We present a new simple and low cost pixel circuit and driving scheme for the a-Si AMOLED displays using conventional AMLCD drivers. Measurement results show significant stability and high immunity to temperature and mobility variations which makes the pixel circuit and driving scheme attractive for implementation in different technologies. A 9-inch a-Si AMOLED display fabricated based on this driving scheme shows high uniformity and long lifetime.

Stable RGBW AMOLED display with OLED degradation compensation using electrical feedback

The active matrix organic light emitting diode (AMOLED) display is a strong candidate for the next generation display because of attributes such as low power consumption, wide viewing angle, highly saturated colors, fast response, and high contrast ratio. More importantly, its structural simplicity (compared to the active matrix liquid crystal display) gives it the potential for low fabrication costs. Since the OLED is a current driven device, an active matrix of thin film transistors (TFTs) is needed to provide a programmable current source at each pixel [1]. Thus, OLED luminance becomes extremely sensitive to the temporal instability and spatial non-uniformity of the TFTs which can result in Mura, image sticking, and reduced lifetime [1–4].

Effect of threshold voltage instability on field effect mobility in thin film transistors deduced from constant current measurements

The field effect (FE) mobility of thin film transistors is normally extracted using static measurement methods, which inherently rely on the assumption that the device remains stable during the measurement duration. However, these devices, particularly those based on emerging materials, can show large instability during the measurement, typically exhibiting hysteresis in the static characteristics. This letter looks at the effect of threshold voltage shift in FE mobility extracted using the conventional method, and introduces an alternative and more accurate technique of measuring device characteristics. The technique decouples the effect of transient phenomena, thus permitting extraction of the true device FE mobility, which turns out to be either over or underestimated depending on the magnitude and direction of threshold voltage shift.

Transparent Semiconducting Oxide Technology for Touch Free Interactive Flexible Displays

Amorphous oxide semiconductor thin film transistors and sensors constitute fundamental building blocks for a new generation of applications ranging from interactive displays and imaging to future electronic systems that are unconstrained by form factor. This makes the quest for high mobility materials processed at low temperatures even more compelling, to enable the layering of circuits and systems on plastic and possibly even paper substrates. Transparency is also an attractive feature that enables seamless embedding of electronics for the immersive ambient. This paper reviews the current status of the ubiquitous oxide semiconductor technology for flexible and transparent interactive displays, along with demonstrated examples of continuous thin film and nanowire systems for the transistor and sensor. Issues related to photo-sensing and active matrix operation are discussed along with solutions addressing the problem of threshold voltage instability and its compensation for fast recovery, particularly after light stress. Physics-based compact models for expedient design and simulation of analog and digital circuits are reviewed along with examples of key system building blocks. Finally we attempt to conceptualize a thin film transistor (TFT)-based fully heterogeneously integrated and autonomous system that can be realized using a combination of oxide and other technological routes.

Ambipolar thin-film transistors fabricated by PECVD nanocrystalline silicon

We report on directly deposited plasma-enhanced chemical vapor deposition (PECVD) nanocrystalline silicon (nc-Si:H) ambipolar thin-film transistors (TFTs) fabricated at 260 °C. The ambipolar operation is achieved adopting Cr metal contacts with high-quality nc-Si:H channel layer, which creates highly conductive Cr silicided drain/source contacts, reducing both electron and hole injection barriers. The n-channel nc-Si:H TFTs show a field-effect electron mobility (meFE) of 150 cm2/Vs, threshold voltage (VT) ~ 2 V, subthreshold slope (S) ~0.3 V/dec, and ON/OFF current ratio of more than 107, while the p-channel nc-Si:H TFTs show a field-effect hole mobility (mhFE) of 26 cm2/Vs, VT ~ -3.8 V, S ~0.25 V/dec, and ON/OFF current ratio of more than 106. Complementary metal-oxide-semiconductor (CMOS) logic integrated with two ambipolar nc-Si:H TFTs shows reasonable transfer characteristics. The results presented here demonstrate that low-temperature nc-Si:H TFT technology is feasible for total integration of active-matrix TFT backplanes.

P-14: Low-Cost Amoled Televeision with IGNIS Compensating Technology

MaxLife™ is a breakthrough backplane solution for AMOLED television. We demonstrate a compensating technology with a prototype that monitors and corrects for the OLED and TFT aging at each pixel for expected lifetimes of 50,000h+. This provides a tremendous boost in realizing low-cost AMOLED TVs using amorphous or poly-Silicon backplanes.

Device-Circuit Interactions and Impact on TFT Circuit-System Design

This paper reviews the importance of device-circuit interactions (DCI) and its consideration when designing thin film transistor circuits and systems. We examine temperature- and process-induced variations and propose a way to evaluate the maximum achievable intrinsic performance of the TFT. This is aimed at determining when DCI becomes crucial for a specific application. Compensation methods are then reviewed to show examples of how DCI is considered in the design of AMOLED displays. Other designs such as analog front-end and image sensors are also discussed, where alternate circuits should be designed to overcome the limitations of the intrinsic device properties.

High-precision, fast current source for large-area current-programmed a-Si flat panels

Although current-programmed pixel circuits lead to a highly stable amorphous silicon (a-Si) active matrix backplane, they are prone to a long settling time due to the large parasitic capacitance coupled with the low mobility of a-Si thin film transistors (TFTs). This paper presents a fast-current source that can significantly improve the settling time of the a-Si active matrix. Also, to reduce the effect of mismatches a differential offset cancellation technique is introduced

P-38: Unique Electrical Measurement Technology for Compensation, Inspection, and Process Diagnostics of AMOLED HDTV

A unique measurement and compensation technology for AMOLED high definition television is presented. The technique electrically measures every TFT and OLED in the backplane, useful for inspection and diagnostics in the factory, and subsequent compensation of mura, TFT- and OLED-degradation.