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 source element, and a modulation element, wherein the modulation element receives a global static voltage originating outside the pixel circuit, imposes pulse width modulation on that voltage and delivers the pulse width modulated voltage onto the current source element, which in turn delivers a pulse width modulated current at a required voltage to a first electrode of an emissive device, and wherein a second electrode of the emissive element is connected to a global voltage wherein the difference between the voltage of the modulated current and the second global voltage satisfies the voltage drive requirement of the emissive elements, and wherein the current source comprises a large L FET and a second FET transistor operative to provide a desired current at a desired voltage to an emissive element, the desired voltage being set by a bias voltage applied to the gate of the large L FET, and wherein the gate of the second FET transistor is biased to ground, and wherein the modulation element comprises a FET configured as a pulse width modulation switch operative to impose pulse width modulation responsive to the data state of the memory cell as asserted on its gate, and wherein the output of the modulation element is asserted onto the current source element.
2. The backplane of claim 1 , wherein the modulation element of each pixel circuit drive element comprises a second FET connected in series with the FET connected to the memory cell wherein the gate of the second 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.
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 FETs of the current source comprise p-channel FETs in an n-well.
5. The backplane of claim 3 , wherein the second 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 source 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 10 , wherein the FETs of the modulation element and the large L FET of the current source element are p-channel FETs placed in a first n-well of a p-substrate material and the second FET of the current source element is 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 second 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 13 , 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.