Backplane suitable to form part of an emissive pixel array and system and methods of modulating same
A backplane operative to drive an array of emissive pixel elements is disclosed. Each pixel element comprises a pixel circuit drive element and an emissive element, wherein the pixel circuit drive element comprises a memory cell, a current source element, and a modulation element. The present invention improves on an emissive display by providing a backplane and modulation system that enables fabrication of multi-color or monochrome LED display systems that operate efficiently and without objectionable image artifacts. One aspect of the present invention is to realize the backplane in a single crystal silicon process with a minimum number of metal layers while providing each color with a suitable operating voltage and drive current.
1. A backplane operative to drive an array of emissive pixel elements, wherein each pixel element comprises a pixel circuit drive element and an emissive element, and wherein the pixel circuit drive element comprises a memory cell, a current providing element, and a modulation element, wherein the current providing element comprises a large L FET and a second FET transistor operative to provide a desired current at a desired voltage to a current mirror element, the desired voltage being set by a bias voltage applied to the gate of the large L FET and the FET transistor is operated in diode mode with its gate and drain connect to each other, to the large L FET and to the gate of the mirror current FET, wherein the current mirror FET is a large L FET, and wherein the modulation element comprises a FET configured as a pulse width modulation switch operative to impose pulse width modulation on a current applied to it wherein the pulse width data modulation switch is responsive to the data state of the memory cell, and wherein the voltage and current output of the modulation element is asserted onto a first electrode of the emissive element and the second electrode of the emissive element is connected to a second global voltage, and wherein the difference between the voltage applied to the first electrode of the emissive element and the voltage applied to the second electrode of the emissive element is within the required voltage range for the emissive element to emit light.
2. The backplane of claim 1 , wherein the modulation element of each pixel circuit drive element comprises a FET wherein the gate of the FET is modulated by a non-data driven pulse width modulation signal operative to control the intensity of the emissive element associated with that pixel circuit drive element without regard for the data state of the memory cell of the pixel circuit drive element and wherein the non-data driven FET and the data driven FET are connected in series.
3. The backplane of claim 1 , wherein the modulation element comprises p-channel FETs in an n-well.
4. The backplane of claim 1 , wherein the current providing element comprises p-channel FETs as the current mirror FET and the second FET in an n-well and the large L FET as an n-channel FET in a p-well or directly on a p-substrate.
5. The backplane of claim 3 , wherein the voltage applied to the second electrode of each emissive element of each pixel element is lower than the upper rail voltage of the array.
6. The backplane of claim 5 , wherein the voltage applied to the second electrode of each emissive element of each pixel element is lower than the lower rail voltage (ground) of the array.
7. The backplane of claim 1 , wherein the current providing element and the modulation element are placed in series.
8. The backplane of claim 1 , wherein the memory cell is an SRAM memory device.
9. The backplane of claim 1 , wherein the emissive element is a light emitting diode.
10. The backplane of claim 1 , wherein substantially all of the backplane is fabricated in a single piece of single crystal silicon.
11. The backplane of claim 1 , wherein the FETs of the modulation element are p-channel FETs placed in a first n-well of a p-substrate material and the p-channel FETs of the current providing element are placed in a second n-well of the same p-substrate material, and wherein the second n-well is biased to a voltage different than the bias voltage of the first n-well.
12. The backplane of claim 11 , wherein the first n-well is biased to the upper rail voltage of the device.
13. The backplane of claim 1 , wherein the emissive elements emit a plurality of colors of light.
14. The backplane of claim 1 , wherein the pixel drive circuit elements associated with a first color require a different bias voltage on its large L FET to the bias voltage required for the pixel drive circuit elements associated with a second color.