Gholamreza Chaji

Driving schemes for a-Si and LTPS AMOLED displays

Design of stable active matrix organic light-emitting diode (AMOLED) displays comes with significant challenges that stem from the electrical property of the backplane materials, line parasitics in the matrix, and the opto-electronic property of the organic light-emitting diode (OLED). This paper reviews voltage and current programming schemes for AMOLEDs. Following a systematic review of pixel circuits, design considerations are examined for both current and voltage schemes with focus on stability and programming speed for both amorphous silicon (a-Si) and low temperature polysilicon (LTPS) pixel circuits. In particular, spatial parameter variations and stability, which hinder reliable operation of AMOLED display backplanes, are discussed. Analysis shows that while driving schemes reported hitherto maybe suitable for small and medium size displays, new schemes are critically needed for large-area high-resolution AMOLED displays.

P-25: A New Driving Method for a-Si AMOLED Displays Based on Voltage Feedback

We present a new driving technique for active-matrix organic light-emitting diode displays using amorphous silicon backplanes. The technique uses voltage feedback to compensate for threshold voltage shift of TFTs. Measurement results show less than 3.5% change in OLED current over 2700 hours of bias stress.

Merged phototransistor pixel with enhanced near infrared response and flicker noise reduction for biomolecular imaging

A top-illuminated, nonoffset amorphous silicon (a-Si) photo thin-film transistor structure is presented for biomolecular imaging applications. The device yields a high gate-modulated response to near infrared wavelengths, enhanced by trapped assisted absorption in the transistor. In addition, its flicker noise power is reduced by more than a factor of 3 by means of a switched biasing technique. Since the image sensor, readout, and amplification are the same elements, the pixel size can be made relatively small, enabling high resolution imaging capability over large area using standard, low-cost flat-panel technology.

AMOLED Pixel Circuit With Electronic Compensation of Luminance Degradation

A new voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diodes (AMOLEDs) is presented. In addition to compensating for the shift in threshold voltage of TFTs, the circuit is capable of compensating for OLED luminance degradation by employing the shift in OLED voltage as a feedback of OLED degradation

A novel current scaling active pixel sensor with correlated double sampling readout circuit for real time medical x-ray imaging

This paper presents a new current-programmed, current-output active pixel sensor (APS) based on a novel current scaling scheme and suitable for real time X-ray imaging (fluoroscopy) and a current mode CMOS readout circuit. The circuit is designed using hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) technology. The simulation results based on the measured characteristics of a-Si:H TFTs show that the proposed pixel circuit can successfully scale up the current data during readout and therefore provide higher gain compared with the conventional current mode APS circuits. The design can also compensate for characteristic variations (e.g. mobility and threshold voltage shift) in a-Si:H TFTs under prolonged gate voltage stress. The readout circuit exploits correlated double sampling (CDS) technique to reduce the offset current, low frequency noise and fixed-pattern noise (FPN) on the array operation.

Electrical Compensation of OLED Luminance Degradation

This letter presents a stable compensation scheme for active-matrix organic light-emitting-diode (AMOLED) displays based on the observed strong interdependence between the luminance degradation of organic light-emitting diodes (OLEDs) and its current drop under bias stress. This feedback-based compensation provides 30% improvement in luminance stability under 1600 h of accelerative stress. To employ this scheme in AMOLED displays, a new pixel circuit is presented that provides on-pixel electrical access to the OLED current without compromising the aperture ratio.

A Stable Voltage-Programmed Pixel Circuit for a-Si:H AMOLED Displays

Hydrogenated amorphous silicon (a-Si:H) active matrix organic light-emitting diode (AMOLED) displays are attractive given the potentially low manufacturing cost and ultimately low-temperature fabrication enabling using flexible substrates. Although the conventional two thin-film transistor (2-TFT) AMOLED voltage-programmed pixel circuit (VPPC) can provide high resolution and high yield, the 2-TFT VPPC is prone to image retention over time due to shift in the threshold voltage (VT-shift) of a-Si:H TFTs. This paper presents a new driving scheme that not only preserves the simplicity of the 2-TFT VPPC, but also demonstrates high uniformity. Experimental results indicate that the current drop in the new driving scheme is less than 11% after 15 days of operation whereas it is over 50% for the conventional driving scheme. Moreover, the new driving scheme is less sensitive to temperature variations due to an internal feedback mechanism. After a 70% change in the temperature, the current in the conventional driving scheme increases by as much as 300%. However, the current in the driving scheme presented here is approximately constant

Low-Cost Stable a-Si:H AMOLED Display for Portable Applications

A large sector of display market comprises portable devices including cell phones, personal organizers, PDAs, portable electronic games, etc. Important design considerations for displays employed in these applications are power consumption and cost. Hydrogenated Amorphous silicon (a-Si:H) active matrix organic light emitting diode (AMOLED) displays are promising technology for these applications. However, the a-Si:H AMOLED backplane suffers from the temporal instability. Although, several stable driving schemes have been proposed, they suffer from high implementation cost due to extra driving circuitry and high power consumption due to additional operating cycles. This paper presents a new driving scheme that provides a stable AMOLED display despite the aging effects in the a-Si:H thin film transistors and OLED, without increasing the driving complexity

Low-Power Low-Cost Voltage-Programmed a-Si:H AMOLED Display for Portable Devices

This paper presents a driving scheme to achieve highly stable low-power amorphous silicon (a-Si:H) active-matrix organic light-emitting diode (AMOLED) displays. Although the conventional 2-thin-film transistor (TFT) a-Si:H AMOLED display has demonstrated interesting features, including simplicity, it is prone to growing nonuniformity due to the temporal instability of the a-Si:H material. Several compensating techniques have been proposed to control the nonuniformity, but they tend to compromise the key attributes of the simple 2-TFT display such as low power consumption, high yield, high aperture ratio, low implementation cost, and fast programming. For mobile applications which have tight constrains on power consumption, cost, and escalating resolution requirements, we propose a new driving and addressing scheme that not only improves the backplane stability, but also compensates for the OLED luminance degradation while maintaining the attractive features of the simple 2-TFT pixel circuit. The overhead in power consumption and implementation cost is reduced by over 90% compared to existing compensation driving schemes.

A Current-Mode Comparator for Digital Calibration of Amorphous Silicon AMOLED Displays

We present a digital calibration driving scheme for stabilizing the uniformity of large area amorphous silicon active-matrix organic light emitting diode displays. A current-mode analog-to-digital converter (ADC) is used to extract the differential aging of the individual pixels. For the ADC, a configurable current comparator is designed that employs the output buffer of the source driver to reduce the die area. The comparator and pixel circuit were fabricated in 0.8-mum high-voltage CMOS and amorphous silicon technologies, respectively. Analysis and measurement results show a calibration refresh time of 2 s for a high-definition display (1920 times RGB times 1080). Moreover, the pixel current is highly stable despite a 5-V shift in the threshold voltage of the thin-film transistor driver.