1. Field of the Invention
This disclosure relates to pixel cells for active matrix displays and more particularly to thin film transistor devices which employ field shields to improve performance.
2. Description of the Related Art
E-ink capsules react to voltages placed on them and are used for generating images. The capsule includes white and black ink (E-ink), which are reactive and move in accordance with the voltage applied to the capsule or cell. To change image content on an electrophoretic E-ink display, new image information is written for a certain amount of time (e.g., 500 ms-1000 ms). As the refresh rate of an active-matrix is usually higher, this results in addressing the same image content during a number of frames (e.g., at a frame rate of 50 Hz, 25 to 50 frames).
Referring to
Referring to
CDE is the capacitor 30 providing the display effect, Cst is the storage capacitor 28 and Cgd is the parasitic gate-drain capacitor (not shown) in the TFT 26. In the circuit 20, the previous row electrode forms the storage capacitor line.
In accordance with the invention, a pixel cell and method for making the same for an active matrix displays includes a pixel pad and a thin film field effect transistor which selectably couples a signal to activate/deactivate the pixel pad. A field shield is formed on an insulating layer and connected to the pixel pad/transistor through the insulating layer such that the field shield extends over at least a portion of pixel electrodes (e.g., the pixel pad) or the transistor. The field shield may extend over the thin film transistor and form a second gate used to enhance the performance of the thin film transistor and the pixel cell.
A method for forming a pixel for active matrix displays includes forming an insulating layer over a pixel stack, the pixel stack including a thin film transistor, a pixel pad and addressing lines, and patterning a conductive layer formed over the insulating layer to form a field shield which connects to the pixel pad. The field shield connects to the pixel pad through the insulating layer such that the field shield extends over at least a portion of the thin film transistor to form a second gate, which enhances the transistor performance.
Other steps may include activating the pixel pad and field shield to increase on-current for the thin film field effect transistor. The step of forming an insulating layer may include setting a thickness of the insulating layer in accordance with performance criteria for the thin film transistor. The step of patterning the conductive layer may include patterning the conductive layer such that the field shield of a first pixel cell extends over a thin film transistor of an adjacent cell or over a pixel pad of an adjacent cell.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
These and other aspects of the invention will be further explained with reference to the Figures, which are purely diagrammatical and not drawn to scale, and in which the same reference numbers refer to the same or equivalent parts, and in which:
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
The present disclosure provides an improved pixel circuit, which includes a field shield to enhance the performance of a thin film transistor associated with the pixel circuit. A field shielded backplane in accordance with embodiments of the present invention increases the optical performance of the display by adding an additional metal layer (e.g., a third metal layer) to a pixel stack. An additional function is further given to the field-shield layer, which includes improving the switching properties of the pixel TFTs by using the field-shield layer as a second gate of the TFTs. Embodiments herein enhance the on-current of the TFT, while retaining the high optical performance of the display.
Illustrative embodiments of the present invention will now be described in terms of organic TFT pixel circuits; however and TFT pixel circuit may benefit from the teachings of the present invention. In addition, the present invention may include any pixel circuits for devices such as liquid crystal displays, E ink displays or any other display or device.
Referring now to the drawings in which like numerals represent the same or similar elements and initially to
Table 1 shows illustrative materials which may be used for an active-matrix circuit with polymer electronics.
Stack 40 may be employed as a basis for providing features in accordance with the present invention. Stack 40 may be replaced with other pixel circuits and may still benefit from the teachings of the present invention.
Referring to
Referring to
Insulating layer 102 may include a photoresist material or other organic insulation material. Insulating layer 102 is formed over a semiconductor material 50 which forms a portion of a TFT 106. Semiconductor material 50 may include an inorganic or organic semiconductor material. In one embodiment, material 50 includes pentacene, although other materials may also be employed. After preparing insulating layer 102, conductive layer 104 is formed on insulating layer 102. Insulating layer 102 may be patterned to form openings to permit via connections 108 through insulating layer 102. Vias 108 may be formed concurrently with conductive layer (e.g., using a dual damascene technique) or formed separately from layer 104. Conductive layer 104 may include, e.g., Au, Al, Cu, Indium-tin oxide, Pd, Pt, ZnSnO3, SnO2:F, Ag or any other suitable conductor.
Conductive layer 104 connects to a pixel pad 110 formed from layer 48 (
Referring to
An ideal pixel TFT delivers a high on-current, while it occupies a small area in the pixel. One problem includes that this ideal is never reached, especially not with organic electronics. The higher the on-current, the larger the display that can be made. The smaller the TFT area, the higher the yield of the active-matrix backplane.
In accordance with the present invention, conductive layer 104 provides a field-shield for a display which offers the opportunity to increase the on-current of the TFT 106, while its area is not increased, by using the layer 104 as a “second gate” of the TFT 106 (see
Referring to
During the remaining frame time (i.e. the hold time), the other rows are addressed. The TFTs 226 are then in their non-conducting state and the charge on the pixel capacitors 228 and 230 is retained. Between image updates the active-matrix is at rest at 0 V for the row and column electrodes, the pixel pads and the common electrode.
CDE is the capacitor 230 providing the display effect, Cst is the storage capacitor 228, and Cgd is the parasitic gate-drain capacitor in the TFT 226. In the circuit 200, the previous row electrode forms the storage capacitor line. The pixel pad 110 forms a second gate 215 of the TFT 226.
Referring to
Without a field shield, the on-current of the TFT is lower when the pixel is charged to a negative voltage compared to charging to a positive voltage. This is due to the lower source-gate voltage when charging to a negative voltage. Pixel charging to a negative voltage is therefore one of the determining factors for the size of the pixel TFT.
With the field shielding, the on-current during charging to a negative voltage is increased due to the negative voltage on the second gate 215 (
A number of alternate embodiments may be employed using different configurations of second gate 215 in a display.
Referring to
Referring to
This stack can be made in a number of ways. One possibility is to omit stripping of the resist (layer 150) that is used for patterning the semiconductor 50, followed by patterning the second insulating layer 152 in the region of the TFT 106. The resist 150 may be about 1 micron thick, while the insulating layer 152 can be much thicker (e.g., 5 microns). Other thicknesses are also contemplated.
Referring to
The second gate 215 is connected to the next pixel pad 110′ in an adjacent pixel cell. An equivalent circuit is illustratively shown in
Referring to
To illustrate some of the advantages of the structure shown in
The field shield may extend over a portion of the thin film field effect transistor of an adjacent pixel cell to form a capacitive relationship with a thin film field effect transistor of the adjacent pixel cell to enhance performance of that transistor Likewise, the field shield may extend over a portion of the pixel pad of an adjacent pixel cell to form a capacitive relationship with a pixel pad of the adjacent pixel cell to enhance performance of that pixel pad.
The embodiments described herein may be combined with other embodiments, for example, the gap between the field shield and the TFT (e.g., second gate) may be adjusted as described when the second gate is connected to the pixel pad of a next row. Other combinations are also contemplated.
Advantages aspects of the present invention include the TFT area can be made smaller while keeping the optical display performance optimal. This increases the yield of the displays and enables larger displays to be made with the current performance of the organic TFTs. Application areas include all active-matrix displays. These displays may include a field shielded design. The present invention can also be applied to other technologies other than organic electronics, for example, amorphous silicon or poly-crystalline silicon may also be employed.
While the invention has been described by way of examples and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
This application is a Divisional of application Ser. No. 11/993,779, filed Mar. 20, 2008, which is a national entry of PCT/IB2006/052136, filed Jun. 27, 2006, which claims the priority of U.S. provisional application 60/695,665, filed Jun. 30, 2005, the entirety of which are incorporated herein for reference.
Number | Date | Country | |
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60695665 | Jun 2005 | US |
Number | Date | Country | |
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Parent | 11993779 | Mar 2008 | US |
Child | 13167673 | US |