Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-12-28
2001-10-23
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169300, C345S076000
Reexamination Certificate
active
06307322
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns electronic circuitry which uses thin-film transistors (TFT's) and, in particular, circuitry which uses polysilicon TFT's in designs that are sensitive to the gate to source threshold voltage of the TFT's.
Display technology pervades all aspects of present day life, from televisions to automobile dashboards to laptop computers to wristwatches. At the present time, cathode-ray tubes (CRTs) dominate display applications in the 10-40 inch (diagonal) display size. CRTs, however, have many disadvantages including weight, lack of ruggedness, cost, and the need for very high driving voltages.
Recently, passive-matrix liquid-crystal displays (PMLCDs) and active-matrix liquid crystal displays (AMLCDs) have become dominant in midrange display applications because of their use in lap-top computers. For smaller pixel sizes and also for large projection displays, the AMLCD is becoming increasingly important. A major drawback of AMLCDs, however, is the requirement of a backlight, usually a fluorescent light source, that substantially increases the size and weight of the display. It also leads to reduced efficiency since the back illumination is applied continuously even for pixels in the off state.
Another type of display, an active matrix electroluminescent (AMEL) display, emits light by passing a current through a light emitting material. In the case of an AMEL display, an alternating current (AC) is passed through an inorganic light emitting material. The inorganic light emitting material is arranged such that dielectrics are present on either side of the emitting material. Relatively high voltages are required to generate sufficient light from the electroluminescent (EL) material. The relatively high voltages are typically between 100-200 volts.
Yet another type of active matrix display uses light emitting diodes (LED's) or organic light emitting diodes (OLED's). Only the devices that correspond to illuminated pixels are drawing power. Accordingly, displays which use LED's or OLED's may provide better power efficiency for equivalent illumination. Both LED's and OLED's are advantageously implemented in an active matrix configuration to form AMLED and AMOLED displays.
Each pixel in an active matrix display includes at least one and typically two thin film transistors (TFT's). The thin film transistor receives video data from a column driver on the display when the display row containing the pixel is selected. The TFT stores the received video data onto the capacitance of the pixel.
One material which may be used to form active matrix LCD displays is polysilicon. It is difficult to design peripheral circuitry, such as column drivers, with TFT's, however, because field effect transistors made from polysilicon exhibit different turn-on thresholds. The turn-on threshold of a metal oxide semiconductor (MOS) transistor is the gate to source voltage, VGS, needed to establish a conductive path between the source and the drain. Variation in the turn-on threshold for the polysilicon thin-film transistors is a problem which affects both the pixel structure and peripheral circuitry of AMEL, AMLED and AMOLED technologies. It also affects AMLCD displays in which a portion of the peripheral circuitry is implemented in thin-film transistors.
Displays which use OLED devices for the pixels, moreover, have another problem. OLED devices are current-mode devices, that is to say, the amount of light emitted by an OLED varies with the current that is conducted by the OLED. To form a gray-scale OLED display, it is desirable to carefully control the current that flows through each pixel. Polysilicon threshold variation complicates this task causing OLED pixel circuits to be designed with four or more transistors to minimize the effect of threshold variation and, so, improve the uniformity of the displayed images. Some of these pixel structures are described in copending patent application Ser. No. 09/064,696 entitled ACTIVE MATRIX LIGHT EMITTING DIODE PIXEL STRUCTURE AND CONCOMITANT METHOD.
The addition of transistors to the pixel structure, however, has a negative impact on device yield and also reduces the portion of the pixel area that emits light, reducing the brightness of the display.
SUMMARY OF THE INVENTION
The present invention is embodied in a circuit design technique polysilicon thin-film transistor (TFT) circuitry which is sensitive to threshold variations among the TFT's used to implement the circuitry. The circuit is designed such that thin-film transistors that are sensitive to threshold variations are made larger than other thin-film transistors in the circuitry to minimize threshold variations among similar transistors implemented in the circuit.
According to one aspect of the invention, the circuitry is the pixel structure of an active matrix display. The pixel structure includes two transistors, a select transistor which is active when data is being written to the pixel and a drive transistor that is sensitive to threshold variations.
According to another aspect of the invention, the drive transistor in the pixel structure includes a capacitance sufficient to hold an electrical potential that keeps the transistor in a conducting state for an image field interval.
According to yet another aspect of the invention, the pixel structure includes only the select transistor and the drive transistor. The pixel storage capacitance is entirely realized by the gate to channel capacitance of the drive transistor.
According to another aspect of the invention, the pixel structure includes a capacitor which is smaller than the capacitor of a conventional active matrix pixel structure. This capacitance holds the pixel in a non-illuminated state when the drive transistor is turned off.
According to yet another aspect of the invention, the pixel structure is used for an AMOLED display and the charge value stored on the gate to source capacitance of the drive transistor controls the level of current that is applied to the OLED device.
According to yet another aspect of the invention, the larger drive transistor has the same width to length ratio as the drive transistor of a conventional active matrix display.
REFERENCES:
patent: 4042854 (1977-08-01), Luo et al.
patent: 4393380 (1983-07-01), Hosokawa et al.
patent: 4471347 (1984-09-01), Nakazawa et al.
patent: 4602192 (1986-07-01), Nomura et al.
patent: 4928095 (1990-05-01), Kawahara
patent: 5079483 (1992-01-01), Sato
patent: 5408109 (1995-04-01), Heeger et al.
patent: 5652600 (1997-07-01), Khormaei et al.
Dawson Robin Mark Adrian
Ipri Alfred Charles
Kane Michael Gillis
Shen Zilan
Stewart Roger Green
Burke William J.
Sarnoff Corporation
Vu David
LandOfFree
Thin-film transistor circuitry with reduced sensitivity to... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Thin-film transistor circuitry with reduced sensitivity to..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thin-film transistor circuitry with reduced sensitivity to... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2614597