Kapil Sakariya

Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic

This paper presents design considerations along with measurement results pertinent to hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) drive circuits for active matrix organic light emitting diode (AMOLED) displays. We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the OLED layer is integrated directly above the TFT circuit layer, to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Various drive circuits on glass and plastic were fabricated and tested. Integration of on-panel gate drivers is also discussed where we present the architecture of an a-Si:H based gate de-multiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.

Threshold voltage instability of amorphous silicon thin-film transistors under constant current stress

We investigate the time-dependent shift in the threshold voltage of amorphous silicon thin-film transistor stressed with constant drain current. We observe a nonsaturating power-law time dependence, which is in contrast to the conventional stretched exponential that saturates at prolonged stress time. The result is consistent with the carrier-induced defect creation model and corroborates the nonlinear dependence of the rate of defect creation on the band-tail carrier density.

Amorphous silicon back-plane electronics for OLED displays

This paper reviews design considerations along with measurement results pertinent to amorphous silicon (a-Si:H) thin-film transistor (TFT) drive circuits for active matrix organic light-emitting diode displays, and follows from work presented earlier (A. Nathan et al., 2002), (A. Nathan et al., 2003). We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the organic light-emitting diode layer is integrated directly above the TFT circuit layer to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Integration of on-panel gate drivers is also discussed, where we present the architecture of an a-Si:H-based gate demultiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.

Accelerated stress testing of a-Si:H pixel circuits for AMOLED displays

Electronics reliability testing is traditionally carried out by accelerating the failure mechanisms using high temperature and high stress, and then predicting the real-life performance with the Arrhenius model. Such methods have also been applied to organic light-emitting diode (OLED) testing to predict lifetimes of tens of thousands of hours. However, testing the active matrix OLED thin-film transistor (TFT) backplane is a unique and complex case where standard accelerated testing cannot be directly applied. This is because the failure mechanism of pixel circuits is governed by multiple material and device effects, which are compounded by the self-compensating nature of the circuits. In this paper, we define and characterize the factors affecting the primary failure mechanism and develop a general method for accelerated stress testing of TFT pixel circuits in a-Si AMOLED displays. The acceleration factors derived are based on high electrical and temperature stress, and can be used to significantly reduce the testing time required to guarantee a 30 000-h display backplane lifespan.

Stability analysis of current programmed a-Si:H AMOLED pixel circuits

In this paper, we present self-compensating current mirror-based pixel circuits, and analyze basic stability issues to provide a deeper understanding of circuit operation, and the impact of thin film transistor bias nonidealities, which can lead to the long-term (and gradual) instabilities in pixel drive current. The analysis also provides the circuit designer a means to tailor the pixel drive current stability to the long-term brightness degradation characteristics of the organic light-emitting diode.

A-Si Amoled Display Backplanes on Flexible Substrates

In view of its maturity and low-cost, the amorphous silicon (a-Si) technology is an attractive candidate for active matrix organic light emitting diode (AMOLED) display backplanes on flexible substrates. However, the a-Si material comes with significant intrinsic shortcomings related to speed (mobility) and stability of operation, requiring novel threshold-voltage-shift (δVT) compensated thin-film transistor (TFT) pixel circuits and architectures to enable stable OLED operation. But given the dramatic progress in efficiency of OLED materials over recent years, the drive current requirement has been significantly lowered, thus relaxing the constraints on a-Si TFTs. For compatibility to plastic substrates, the a-Si TFT process temperature must be reduced from the conventional 300°C to ∼150°C or below, which tends to compromise the integrity of thin-film materials and device performance. Hence, optimizing the TFT process for high device performance with limited thermal budget is a necessary step towards flexible AMOLEDs with a-Si backplanes. This paper reviews the design and process challenges, and specifically examines the performance of TFTs and δVT- compensated integrated pixel driver circuits on plastic substrates with respect to current driving ability and long term stability. More importantly, lifetime tests of circuit degradation behaviour over extended time periods demonstrate highly stable drive currents and its ability to meet commercial standards.

57.2: Extreme AMOLED Backplanes in a‐Si with Proven Stability

Instability has long been a barrier to the use of a-Si AMOLED backplanes. We present here the first demonstration of proven stability of a-Si AMOLED pixels. Over 7000h of stability data is shown for pixel circuits that compensate for threshold-voltage shift, temperature, and OLED degradation (extreme compensation). This demonstrates that stable AMOLED backplanes are achievable using well-established and proven a-Si TFT technology in mainstream use by the flat panel display industry.

Analysis and characterization of self-compensating current programmed a-Si:H active matrix organic light-emitting diode pixel circuits

The organic light-emitting diode (OLED) drive current provided by a simple 2 thin film transistor (TFT) hydrogenated amorphous silicon (a-Si:H) active matrix OLED (AMOLED) voltage programmed pixel circuit drops over time due to threshold voltage (VT) increase in the drive TFT. VT shift compensating current programmed 4-TFT circuits have been developed to overcome this problem and to keep the drive current constant, thereby prolonging the life of the display. In this article, we present a thorough analysis of the static and dynamic operation of such circuits and derive theoretical limits for the extent of compensation. Measurement results of circuit characteristics and stability are presented which demonstrate that it is feasible to use a-Si:H for AMOLED back planes. The advantages and drawbacks of current programmed circuits are also outlined from the perspective of active matrix back-plane design.

Amorphous silicon TFT circuit integration for OLED displays on glass and plastic

This paper reviews design considerations along with measurement results pertinent to amorphous silicon (a-Si:H) thin film transistor (TFT) drive circuits for active matrix organic light emitting diode (AMOLED) displays. We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the OLED layer is integrated directly above the TFT circuit layer, to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Integration of on-panel gate drivers is also discussed where we present the architecture of an a-Si:H based gate de-multiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.

Pixel circuits and drive schemes for glass and elastic AMOLED displays

— Flexible AMOLED displays pose unique opportunities and challenges for a-Si. Leveraging the existing a-Si process infrastructure is the fastest and lowest-cost route to flexible AMOLEDs. However, the displays must maintain high performance, long lifetimes, and high uniformity despite low-temperature processes and mechanical stress. New pixel circuits and drive schemes shown here demonstrate that high-performance flexible AMOLED displays are possible using well-established a-Si technology.