Patent Publication Number: US-2023154390-A1

Title: Displays with current-controlled pixel clusters

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates to flat-panel display architectures having matrix-controlled pixel clusters. 
     BACKGROUND OF THE DISCLOSURE 
     Flat-panel displays are widely used in conjunction with computing devices, in portable electronic devices, and for entertainment devices such as televisions. Such displays typically employ an array of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the current. Displays using inorganic light-emitting diodes (LEDs) as pixel elements are also in widespread use for outdoor signage and have been demonstrated in a 55-inch television. 
     Displays are typically controlled with either a passive-matrix (PM) control scheme employing electronic control circuitry external to the pixel array or an active-matrix (AM) control scheme employing electronic control circuitry in each pixel on the display substrate associated with each light-emitting element. Both OLED displays and LCDs using passive-matrix control and active-matrix control are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066. 
     In a PM-controlled display, each pixel in a row is stimulated to emit light at the same time while the other rows do not emit light, and each row is sequentially activated at a high rate to provide the illusion that all of the rows simultaneously emit light. In contrast, in an AM-controlled display, data is concurrently provided to and stored in pixels in a row and the rows are sequentially activated to load the data in the activated row. Each pixel emits light corresponding to the stored data when pixels in other rows are activated to receive data so that all of the rows of pixels in the display emit light at the same time, except the row loading pixels. In such AM systems, the row activation rate can be much slower than in PM systems, for example divided by the number of rows. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control. 
     Passive-matrix row and column control circuits are typically provided on the sides of and external to a display area (e.g., including the display light-emitting pixels) on a display substrate of a display and comprise packaged integrated circuits (ICs). Active-matrix circuits are commonly constructed with thin-film transistors (TFTs) in a semiconductor layer formed over the display substrate and employ a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon or poly-crystalline silicon and is distributed over the entire flat-panel display substrate. The semiconductor layer is photolithographically processed to form electronic control elements, such as transistors and capacitors. Additional layers, for example insulating dielectric layers and conductive metal layers are provided, often by evaporation or sputtering, and photolithographically patterned to form electrical interconnections, or wires. In some implementations, small integrated circuits (ICs) with a separate IC substrate are disposed on a display substrate and control pixels in an AM display. The integrated circuits can be disposed on the display substrate using micro-transfer printing, for example as taught in U.S. Pat. No. 9,930,277. 
     Both active- and passive-matrix displays use electrical power to control the display and cause pixels to emit light. It is useful to reduce the power used by a display to reduce the operating costs of the display and, for portable displays powered by batteries, to increase the operating lifetime of the portable display between battery charges. There is an on-going need, therefore, for improved display efficiency. 
     SUMMARY 
     The present disclosure includes, among various embodiments, a current-selectable light-emitting-diode (LED) display comprising an array of pixels distributed in rows and columns. The pixels are grouped in mutually exclusive clusters. A cluster controller is connected to each pixel in a cluster of the mutually exclusive clusters to control the pixels in the cluster to emit light. Each of the cluster controllers comprises a selectable current source. Each of the selectable current sources comprises cluster current sources that are responsive to a current-select signal to enable one or more of the cluster current sources. 
     According to embodiments of the present disclosure, each of the cluster current sources in a cluster provides a different amount of current, each of the cluster current sources in the cluster provides a same amount of current, or some cluster current sources in the cluster provide the same amount of current and other cluster current sources in the cluster provide different amounts of current. 
     According to some embodiments, the cluster current sources are responsive to the current-select signal such that only one cluster current source is enabled by the current-select signal, such that no cluster current source is enabled by the current-select signal, or such that two or more cluster current sources whose current outputs are electrically connected in common are enabled by the current-select signal. 
     In some embodiments of the present disclosure, one or more of the cluster controllers are disposed between the pixels in the array. In some embodiments, each pixel comprises a pixel substrate comprising a fractured, broken, or separated pixel tether and each cluster controller comprises a cluster-controller substrate comprising a fractured, broken, or separated cluster-controller tether. A current-selectable LED display of the present disclosure can comprise a display substrate and the pixel substrate and the cluster-controller substrate can be each disposed directly on the display substrate. In some embodiments of the present disclosure, each of the clusters comprises a cluster substrate and the pixel substrates of the pixels and the cluster-controller substrate of the cluster controller in the cluster is disposed directly on the cluster substrate and the cluster substrate is disposed directly on the display substrate. 
     According to some embodiments, a current-selectable LED display of the present disclosure comprises a display substrate. For each of the clusters, each of the pixels in the cluster comprises a pixel substrate comprising a fractured, broken, or separated pixel tether, the cluster comprises a cluster substrate, the cluster controller is formed in or on and is native to the cluster substrate, the pixel substrates of the pixels in the cluster are disposed directly on the cluster substrate, and the cluster substrate is disposed directly on the display substrate. Each of the pixels can comprise a pixel substrate comprising a fractured, broken, or separated pixel tether disposed directly on the display substrate and the cluster controllers are formed in or on and are native to the display substrate. 
     According to some embodiments, for each of the clusters, each cluster controller in the cluster is operable to receive an image portion, receive a current-select signal corresponding to a luminance of the image portion, select a current of the selectable current source, and control the pixels in the cluster to emit light corresponding to the image portion. Each of the pixels can comprise LEDs and the cluster controller in each of the clusters can be operable to provide passive-matrix control to the LEDs in the cluster. 
     Each of the pixels can comprise one or more inorganic light-emitting diodes. Each of the light-emitting diodes can comprise a bare, unpackaged die comprising a separate, individual, and independent LED substrate. The LED substrate can have a (i) length no greater than 200 microns, (ii) a width no greater than 200 microns, (iii) a thickness no greater than 50 microns, or (iv) any combination of (i), (ii), and (iii). Each of the pixels can comprise a red LED operable to emit red light, a green LED operable to emit green light, and a blue LED operable to emit blue light. 
     According to some embodiments, the current-selectable LED display is a display for displaying images. According to some embodiments, the current-selectable LED display is a backlight and each pixel corresponds to a local-dimming zone of the backlight. The pixels and the cluster controllers can be comprised in a backlight and each of the pixels can correspond to a local-dimming zone of the backlight. 
     According to some embodiments of the present disclosure, a current-selectable LED display comprises a display row controller that provides row signals or a display column controller that provides column signals, or both. A first wire segment can be electrically connected to a first cluster in a row of clusters that conducts a signal between a cluster controller and the display row controller or a first wire segment can be electrically connected to a first cluster in a column of clusters that conducts a signal between a cluster controller and the display column controller, or both. A second wire segment can be electrically connected to a second cluster in the row of clusters or a second wire segment can be electrically connected to a second cluster in the column of clusters, or both. A signal regeneration circuit can be electrically connected to the first wire segment and electrically connected to the second wire segment that regenerates a signal conducted on the first wire segment and drives the regenerated signal onto the second wire segment. 
     According to some embodiments of the present disclosure, a current-selectable LED display comprises a display row controller that provides row signals. A first wire segment can be electrically connected to a first cluster in a row of clusters that conducts a signal between the display row controller and the first cluster. A second wire segment can be electrically connected to a second cluster in the row of clusters. A signal regeneration circuit can be electrically connected to the first wire segment and to the second wire segment that regenerates a signal conducted on the first wire segment and drives the regenerated signal onto the second wire segment. 
     According to some embodiments of the present disclosure, current-selectable LED display comprises a display column controller that provides column signals. A first wire segment can be electrically connected to a first cluster in a column of clusters that conducts a signal between the display column controller and the first cluster. A second wire segment can be electrically connected to a second cluster in the column of clusters. A signal regeneration circuit can be electrically connected to the first wire segment and to the second wire segment that regenerates a signal conducted on the first wire segment and drives the regenerated signal onto the second wire segment. The signal regeneration circuit can be micro-transfer printed onto a display substrate, the signal regeneration circuit can be micro-transfer printed onto a cluster substrate, the signal regeneration circuit can be native to a cluster substrate or a display substrate, or the signal regeneration circuit can be integrated into a common integrated circuit with the cluster controller. 
     According to some embodiments, integrated circuits (e.g., bare, unpackaged die) each comprise one of the cluster controllers. Each of at least a portion of the integrated circuits can comprise the one of the cluster controllers and a signal regeneration circuit. 
     According to some embodiments, the selectable current source comprises a programmable current reference that determines the current range of a cluster current source. 
     According to some embodiments, a current-selectable light-emitting-diode (LED) backlight for a display comprises pixels distributed in an array of rows and columns, wherein the pixels are grouped in mutually exclusive clusters; and cluster controllers. Each cluster controller is connected to each pixel in a cluster of the mutually exclusive clusters to control the pixels in the cluster to emit light. Each of the cluster controllers can include a selectable current source. 
     According to some embodiments, a method of forming a current-selectable light-emitting-diode (LED) display, the method comprising: providing (i) pixels each comprising light emitters (e.g., non-native light emitters) on a pixel source wafer, (ii) a cluster source wafer comprising cluster substrates, and (iii) a display substrate. Mutually exclusive clusters can be formed to include a cluster controller and ones of the pixels. The cluster controller can include a selectable current source and can be operable to control the ones of the pixels to emit light with the selectable current source. Forming the mutually exclusive clusters can include printing the pixels from the pixel source wafer to the cluster substrates of the cluster source wafer. Subsequently, the mutually exclusive clusters can be printed from the cluster source wafer to the display substrate. In some embodiments, the mutually exclusive clusters are comprised in a backlight. 
     Embodiments of the present disclosure provide display control methods, designs, structures, and devices that reduce the power used by a display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, aspects, features, and advantages of the present disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a schematic plan view of a display comprising pixel clusters according to illustrative embodiments of the present disclosure; 
         FIGS.  2 A- 2 C  are schematic plan views of a pixel cluster according to illustrative embodiments of the present disclosure; 
         FIGS.  3 A and  3 B  are schematics of pixels in a cluster according to illustrative embodiments of the present disclosure; 
         FIGS.  4 A- 4 C  are schematics of selectable current sources and a timing switch according to illustrative embodiments of the present disclosure; 
         FIG.  5    is a schematic of a current source and enable circuit according to illustrative embodiments of the present disclosure; 
         FIG.  6    is a diagram of a display with clusters displaying an image with clusters having different luminances according to illustrative embodiments of the present disclosure; 
         FIGS.  7 A- 7 D  are perspectives of substrates according to illustrative embodiments of the present disclosure; 
         FIGS.  8 A- 8 B  are flow diagrams according to illustrative embodiments of the present disclosure; 
         FIGS.  9 A- 9 B  are schematic plan views of a display system according to illustrative embodiments of the present disclosure; and 
         FIG.  10 A  is a schematic of a regeneration circuit,  FIG.  10 B  is a perspective of a regeneration circuit on a cluster substrate, and  FIG.  10 C  is a schematic diagram of a regeneration circuit disposed in or as part of a cluster controller, according to illustrative embodiments of the present disclosure. 
     
    
    
     Features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The figures are not drawn to scale since the variation in size of various elements in the Figures is too great to permit depiction to scale. 
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     Embodiments of the present disclosure provide light-emitting information displays and backlights that require less power. As used herein, the generic term ‘display’ refers to both an information display that shows information, such as an image, text, or video, to a viewer, such as a micro-LED display, and to a local-area-dimming backlight that provides structured illumination to a light-valve display, such as a liquid crystal display (LCD). Each pixel of a backlight can variably illuminate multiple pixels in an LCD thereby providing local-area dimming. For conciseness, the word ‘display’ is used in the following. Unless otherwise clear from context, where a ‘display’ is described, analogous embodiments of a backlight, with or without corresponding light control feature(s), such as an LCD layer, present, are also contemplated. 
     According to some embodiments of the present disclosure and as illustrated in  FIGS.  1 ,  2 A,  2 B, and  2 C , a current-selectable light-emitting diode (LED) display system  90  comprises display pixels  24  distributed in an array of rows and columns. Pixels  24  are grouped in mutually exclusive pixel clusters  20  so that no pixel  24  is in more than one cluster  20  and every pixel  24  is in a cluster  20 . A cluster controller  22  controls pixels  24  in each cluster  20  and each cluster controller  22  is connected to each pixel  24  in cluster  20  of pixels  24  so that pixels  24  emit light responsive to cluster row signals  26  and cluster column signals  28 . Each cluster controller  22  comprises a selectable current source  30 . A selectable current source  30  is responsive to a current-select signal  40  (discussed below with respect to  FIGS.  4 A- 4 C ) to select a current range provided to pixels  24  to emit light. The current range limits the maximum amount of current that can be supplied to pixels  24  and therefore limits the maximum brightness (luminance) of pixels  24 . Thus, selecting a different current range using current-select signal  40  can alter brightness characteristics of pixels  24 . 
     Cluster controllers  22  can receive control signals, for example display row signals  17  (e.g., row-select or timing signals) from a display row controller  16  and display column signals  19  (e.g., column-data, current-select signals  40 , or timing signals) from a display column controller  18 . Display row and column controllers  16 ,  18  can receive display signals (e.g., display control signals) from a display controller  14  or can themselves constitute a display controller  14 . Display controller  14  can receive image data (image pixels) from an external source. Display row signals  17  and display column signals  19  can include data signals, row or column select signals, and timing signals, for example providing active-matrix control to pixel clusters  20  by providing image pixel data for each display pixel  24  from display column controller  18  through display column wires  19  to each cluster  20  in a row of clusters  20  selected by display row controller  16  through display row wires  17 . For illustrative clarity, display row signals  17  and display row wires  17  are designated with the same identifier since display row signals  17  are carried on display row wires  17  and are not easily distinguished in the drawings. Similarly, display column signals  19  and display column wires  19  are designated with the same identifier since display column signals  19  are carried on display column wires  19  and are not easily distinguished in the drawings. 
     Clusters  20  and pixels  24  can be disposed on a display substrate  10 , for example a glass or polymer substrate, within a display area  12  comprising all of pixels  24  and at least some of cluster controllers  22 . Display area  12  can be, for example, a convex hull of pixels  24 . Thus, in some embodiments, at least a portion of cluster controllers  22  are disposed between pixels  24  on display substrate  10 . In contrast, display row controller  16 , display column controller  18 , and display controller  14  can be disposed on display substrate  10  external to display area  12 , for example adjacent to the edges or sides of display area  12 . Display row controller  16 , display column controller  18 , and display controller  14  can be packaged integrated circuits mounted on display substrate  10 . According to some embodiments, display row controller  16 , display column controller  18 , and display controller  14  can each be one or more unpackaged bare die, for example disposed on display substrate  10  by micro-transfer printing, or a thin-film transistor circuit disposed on display substrate  10 . 
     As shown in  FIG.  2 A , a cluster controller  22  of a cluster  20  can receive display row signals  17  and display column signals  19  from display row controller  16  and display column controller  18 , respectively. Cluster controller  22  can be directly connected to each pixel  24  in cluster  20  and can provide both cluster row signals  26  and cluster column signals  28  to provide either active- or passive-matrix control of pixels  24 . As shown in  FIG.  2 B , pixels  24  in a cluster  20  can receive display row signals  17  and display column signals  19  from display row controller  16  and cluster controller  22  can receive display column signals  19  from display column controller  18 . According to the illustrations herein, a wire (e.g., display row wires  17  and display column wires  19 ) incorporating dashes indicates that additional clusters not shown in the Figure can be connected to the wire e.g., as shown in  FIGS.  1 ,  2 B, and  3   , and  FIGS.  9 A- 9 B  discussed below. As shown in more detail in  FIGS.  3 A,  3 B , according to some embodiments, display row signals  17  from display row controller  16  can also serve as anode control lines for LEDs  60 . 
     According to some embodiments of the present disclosure and as illustrated in  FIG.  2 C , cluster controller  22  can comprise multiple integrated circuits, for example unpackaged, micro-transfer printed, bare die disposed at least partly or completely between pixels  24  providing a cluster row controller  22 R and a cluster column controller  22 C to enable passive- or active-matrix control of pixels  24 . 
     According to embodiments of the present disclosure and as illustrated in  FIGS.  3 A and  3 B , pixels  24  of clusters  20  can comprise one or more light emitters  60 , for example micro-light-emitting diodes  60  that each emit different colors of light, for example red LEDs that emit red light, green LEDs that emit green light, and blue LEDs that emit blue light when provided with enough current at a suitable voltage. Display row signals  17  (e.g., display row-select signals) or cluster row signals  26  (e.g., cluster row-select signals) and cluster column signals  28  (e.g., cluster column-data signals) can provide enough current at suitable voltages to drive each of LEDs  60  in each pixel  24 . Display or cluster row signals  16 ,  26  and display column or cluster column signals  18 ,  28  can comprise one or more of row-select, timing, column-data signals, or current-select signals  40  but are not limited to such and can implement any suitable control and data function desired. 
     As shown in  FIG.  3 A , a separate selectable current source  30  is provided for each color of LEDs  60  and a common voltage provided either by cluster controller  20  or externally, for example by display row controller  16 . As shown in  FIG.  3 B , a common selectable current source  30  is provided for all colors of LEDs  60  and different voltages provided for each color of LEDs can be provided either by cluster controller  20  or externally, for example by display row controller  16 . In some embodiments, both a common voltage and selectable current source  30  are provided to all of the different colors of LEDs  60 . In some embodiments, the colors of LEDs  60  are controlled in a color sequential fashion and a single selectable current source  30  is provided to all of the different colors of LEDs  60  in cluster  20 . By providing different voltages or selectable current sources to different colors of LEDs  60 , the realized efficiency of LEDs  60  can be improved, since different colors of LEDs  60  can have different efficiencies at different voltages and currents. Furthermore, a voltage provided to LEDs  60  (for example from display row controller  16  or cluster controller  20 ) can be different from an operating voltage provided to cluster controller  20 . Since LEDs  60  and cluster controller  20  can comprise different semiconductor material (e.g., a compound semiconductor and silicon, respectively) that operate efficiently at different voltages, for example cluster controller  22  can operate at a lower voltage than LEDs  60 , providing different voltages can improve overall realized efficiency. 
     Pixels  24  can comprise light emitters  60 , for example light-emitting diodes  60 , for example inorganic light-emitting diodes  60 , for example micro-light emitting diodes  60  having a length or width no greater than one hundred microns, for example no greater than fifty microns, no greater than twenty microns, no greater than fifteen microns, no greater than twelve microns, or no greater than ten microns, and, optionally, a thickness no greater than fifty microns, for example no greater than twenty microns, no greater than ten microns, or no greater than five microns. As discussed further below, micro-light-emitting diodes  60  can be bare, unpackaged die, for example integrated circuit die, and can be micro-transfer printed from a micro-light-emitting diode source wafer to display substrate  10  and can comprise a broken (e.g., fractured) or separated LED tether  61  as a consequence of micro-transfer printing. Cluster controllers  22  can likewise be unpackaged bare die, for example integrated circuit die, and can be micro-transfer printed from a cluster controller source wafer to display substrate  10  and comprise a broken (e.g., fractured) or separated controller tether  23  as a consequence of micro-transfer printing. Cluster controllers  22  can have a length or width no greater than two hundred microns, for example no greater than one hundred microns, no greater than fifty microns or no greater than twenty microns, and, optionally, a thickness no greater than fifty microns, for example no greater than twenty microns, no greater than ten microns, or no greater than five microns. Micro-transfer printed integrated circuits, for example micro-LEDs  60 , are relatively small and can therefore be provided at a high density and resolution on display substrate  10 . Likewise, cluster controllers  22  can be very small and can therefore be provided between pixels  24  in display area  12  on or over display substrate  10 . 
     Each cluster controller  22  can comprise a single selectable current source  30  so that all of pixels  24  and LEDs  60  in each cluster  20  are driven with a single selected cluster current source  36 . In some embodiments, each cluster controller  22  can comprise a selectable current source  30  for each color of LED  60  (e.g., three selectable current sources  30 , one for each of the red-light emitting, green-light emitting, and blue-light emitting LEDs in a cluster  20 . In some embodiments a selectable current source  30  can be provided for each row or column of pixels  60  or for each color of LED  60  in each row or column of pixels in cluster  20 . In some embodiments, separate selectable current sources  30  can share some components but are nonetheless capable of providing different current ranges. For example, cluster current sources  36  can comprise a current reference and different current references can be provided for and shared by each color of LEDs  60 . Furthermore, the range of a cluster current source  36  can be specified by the input current reference. Different cluster current source  36  ranges can be provided by a programmable current source. Thus, current-select signal  40  can program a programmable current source, thereby selecting a cluster current source  36  range. As used herein, selecting a range of a cluster current source  36  is the same as selecting a cluster current source  36 . 
     A selectable current source  30  is a circuit that provides electrical current in two or more ranges that are selected by a current-select signal  40 . Current-select signal  40  can be a digital value presented on one or more wires to the selectable current source  30  circuit or current-select signal  40  can be an analog value. For example,  FIGS.  4 A- 4 C  illustrate selectable current sources  30  according to embodiments of the present disclosure and Table 1 is a table illustrating example current ranges associated with each of four different current-select signals  40  presented as a two-bit binary value to selectable current source  30 . The ranges and circuits illustrated in  FIGS.  4 A- 4 C  and Table 1 are exemplary and not limiting. Those knowledgeable in the digital and analog electronic arts will appreciate that there are many ways to implement selectable current source  30  and many possible current ranges that are useful in a display system  90 , such as a backlight. 
     As shown in Table 1, four different luminance values corresponding to the four different possible two-bit binary values selected by current-select signal  40  are each associated with one of four different current ranges: 0 to 1 μA, 0 to 4 μA, 0 to 16 μA, and 0 to 64 μA. These ranges are selected as suitable for micro-LEDs, but other ranges are possible and are included in the present disclosure. Moreover, the logarithmic progression of the different selectable current ranges is exemplary; some embodiments can comprise other progressions, for example linear or a power series. According to some embodiments of the present disclosure, one of current-select signals  40  can indicate no cluster current source  36  is selected so that all of the cluster current sources  36  are disabled or effectively turned off. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 00 
                 Luminance level 0 
                  0 to 1 μA 
               
               
                 01 
                 Luminance level 1 
                  0 to 4 μA 
               
               
                 10 
                 Luminance level 2 
                 0 to 16 μA 
               
               
                 11 
                 Luminance level 3 
                 0 to 64 μA 
               
               
                   
               
            
           
         
       
     
     In some embodiments and as shown in  FIG.  4 A , selectable current source  30  comprises four different cluster current sources  36  of different ranges with outputs connected in parallel and with a high-impedance output so that any one of cluster current sources  36  can be active at time, for example each providing a current range as illustrated in Table 1 and represented by current-source symbols of different sizes. A larger current-source symbol represents a cluster current source  36  that can provide current over a relatively larger range (not necessarily to scale). A demultiplexer  32  converts the binary current-select signal  40  into enable circuit control signals  35  that each enable a single different cluster current source  36  with respective enable circuit  34 . 
     In some embodiments and as shown in  FIG.  4 B , selectable current source  30  comprises four cluster current sources  36  each having the same range (as illustrated with current-source symbols of the same size) connected in parallel. Enable circuits  34  enable one, two, three, or four of cluster current sources  36  in response to current-select signal  40 , thus providing 0 to 1 μA, 0 to 2 μA, 0 to 3 μA, or 0 to 4 μA (if each cluster current source  36  provides 0 to 1 μA while other ranges can be achieved with other cluster current sources  36 ). In some embodiments, the same-range cluster current sources  36  of  FIG.  4 B  could be replaced by the different-range cluster current sources  36  of  FIG.  4 A , providing different combinations of different current ranges, e.g., 0 to 5 μA (ranges 1 and 2 combined) or 0 to 21 μA (ranges 1, 2, and 3 combined). 
     In some embodiments and as shown in  FIG.  4 C , in some embodiments selectable current source  30  can comprise multiple cluster current sources  36  and any one or combination of cluster current sources  36  can be active at the same time and can be connected in parallel so that the total cluster current sources  36  by selectable current source  30  is the sum of all of the activated cluster current sources  36 . The cluster current sources  36  can have the same range (e.g., as in  FIG.  4 B ) or have different ranges (e.g., as in  FIGS.  4 A and  4 C ). 
     Embodiments of the present disclosure can operate with any of a variety of cluster current sources  36 .  FIG.  5    illustrates a generic cluster current source  36  that is enabled with enable circuit  34 , for example comprising two control transistors  52 A,  52 B responsive to enable circuit control signals  35 A and  35 B (collectively enable circuit control signal  35 ), respectively and a transistor  52 C with a connected source and drain driving a capacitor C to form a sample and hold circuit that controls the gate of current source  36  (a transistor  52 ). When the gate voltage control signal on current source  36  transistor  52  is low, leakage through capacitor C and the cluster current source  36  transistor  52  is reduced, saving power. In some embodiments, an optional control transistor  52 D responsive to enable circuit control signal  35 C can short capacitor C and ensure that the gate of current source  36  transistor  52  is grounded to further reduce leakage in capacitor C and current sources  36 . When the gate voltage is high current can flow through cluster current source  36 . The range of currents provided by cluster current source  36  can depend on the size of transistor  52  in cluster current source  36  (a larger transistor  52  can provide a greater current range) or current reference  38 . As shown in  FIG.  5   , the gate control signal is connected to multiple cluster current sources  36  in parallel so that the multiple cluster current sources  36  are enabled in common. In some embodiments, enable circuit  34  drives only a single cluster current source  36 . According to some embodiments, current reference  38  can be part of enable circuit  34  or can be shared among multiple enable circuits  34  (as shown with the dotted line connection to the output of current reference  38 ) in order to save circuitry. In some embodiments, one or more current reference  38  can be disposed in a display row controller  16  and connected to one or more cluster controller  22 , saving circuitry in cluster controller  22 . 
     Once cluster current source  36  is enabled, the provided current can be turned on or off with a switch  50  (for example comprising one or more transistors  52 ) in response to a timing signal  42  and the current provided to a cluster row signal  26  or cluster column signal  28  to turn LEDs  60  on or off. According to some embodiments of the present disclosure, cluster controller  22  is a passive-matrix controller for pixels  24  in cluster  20  and timing signal  42  is a pulse-width modulation or pulse-density modulation signal that uses temporal modulation to control the luminance of pixels  24  at a constant current. 
     According to embodiments of the present disclosure, LEDs  60  emit light most efficiently at a particular current. This efficient current can be different for different LEDs, for example LEDs made with different materials or that emit different colors of light (e.g., due to having different compositions of a binary or ternary compound semiconductor). It is useful, therefore, to operate LEDs  60  at their most efficient current to provide a power-efficient display and to select different efficient currents for different corresponding types of LEDs  60 . Passive-matrix control can provide higher currents for shorter periods of time that, in some embodiments, match currents needed for efficient LED  60  operation. 
     LED  60  in pixel  24  can emit different amounts of light in response to a control signal (e.g., timing signal  42 ) and the number of light levels (the luminance) is determined by the range of the control signal. However, if pixel  24  only operates within a subset of the range, the number of realized luminance levels is decreased. For example, if pixel  24  only operates at relatively low luminance levels, the higher luminance levels are never activated, and the reduced number of different luminance levels can lead to perceptible contouring (pixelization) in an image pixel. Thus, contouring is reduced if the actual luminance range of a display pixel  24  is matched to the desired luminance of a desired image pixel. Furthermore, transistors  52  (and some other components, such as capacitors) in cluster current sources  36  can leak current and the larger the transistor  52  (or other components) the more current can leak. Leakage can be reduced by reducing the voltage provided to a gate of a transistor or across a capacitor, for example by reducing the voltage output by enable circuit  34 . Although the leakage of a single transistor  52  can be relatively small, if the leakage occurs for every pixel  24  in a high-resolution display, the power wasted can be considerable, especially for portable display applications in which power efficiency is an important consideration. Thus, leakage is reduced if cluster current source  36  for an LED  60  provides only the current required for a desired LED luminance range. If additional current is provided but not used in a cluster current source  36 , additional current leakage also occurs, reducing efficiency. 
     Therefore, according to embodiments of the present disclosure, a current-selectable light-emitting-diode display comprises pixels  24  arranged and controlled in clusters  20 . Each cluster  20  has a selected range of electrical current necessary to operate pixels  24  in cluster  20 . The desired range can be determined by analyzing image pixel values input to cluster  20 , for example a portion of an image corresponding to cluster  20 , to determine the brightest image pixel in cluster  20  and selecting the smallest luminance range of selectable current source  30  that can provide the desired luminance in cluster  20  according to the brightest image pixel. By selecting the smallest luminance range, power leakage is reduced in selectable current source  30  and the number of luminance levels in each cluster  20  is maintained, improving power efficiency, and reducing image contouring. Use of a larger number of clusters  20  within display  90  of a given size can also enable further reductions in image contouring and improvements in efficiency (e.g., more clusters  20  decreases cluster size for a given resolution, thereby allowing for improved matching of luminance ranges to current sources  36 ). 
     For example, and with reference to a simplified small example illustrated in  FIG.  6   , an image can be divided into a four-by-four array of sixteen clusters  20 , labeled  20 A- 20 P. (In practice, for example, a 2k display might have 8192 clusters  20  each having 256 pixels  24 .) Clusters  20 A,  20 D,  20 E,  20 H,  20 I,  20 L,  20 M, and  20 P (dark clusters  20 ) include only pixels  24  that are relatively dark and clusters  20 B,  20 C,  20 F,  20 G,  20 J,  20 K,  20  N, and  20 O (bright clusters  20 ) include a range of pixels  24  that are both dark and light. Current for dark clusters  20  can be provided with a relatively small current range (e.g., 0 to 1 μA) and bright cluster  20  can be provided with a relatively large current range (e.g., 0 to 64 μA). Dark clusters  20  will therefore have reduced current leakage and current-selectable light-emitting-diode display system  90  will have increased power efficiency. Furthermore, pixels  24  in dark clusters  20  can have reduced contouring because the reduced luminance range (because of the reduced current range of dark clusters  20 ) has the same number of luminance levels as clusters  20  with a greater luminance range. Since the human visual system has increased sensitivity to different luminance levels primarily in darker areas, embodiments of the present disclosure can provide displays with reduced visible contouring in darker areas without reducing luminance for a given image bit depth, and with reduced power usage and increased power efficiency. In effect, current-selectable light-emitting-diode display system  90  having clusters  20  provided with different current ranges can be a high-dynamic range (HDR) display. 
     For example, given an image with an eight-bit image pixel depth (256 luminance levels) and a two-bit current range corresponding to Table 1, the number of luminance levels at luminance level 0 is 256 and the number of additional luminance levels at each of luminance levels 1, 2, and 3 is 192 (because the lower luminance values in the larger current ranges are redundant with those of the lower current ranges) for a total of 832 luminance levels available (but only 256 are available in any one cluster  20 ). Thus, in this example, an approximately four-fold increase in available luminance levels across display  90  is realized as compared to an equivalent display without selectable current sources  30  or clusters  20 . This example specifies eight bits, but as will be appreciated by those knowledgeable in the display arts, any number of bits greater than one can be used in a design according to embodiments of the present disclosure, for example ten bits or twelve bits. 
     Display systems  90  according to embodiments of the present disclosure can comprise light-emitting diodes (LEDs)  60  made with compound semiconductor materials and LED substrates separate, distinct, and individual from display substrate  10 . As shown in  FIG.  7 A , each LED  60  can comprise a broken (e.g., fractured) or separated LED tether  61  broken (e.g., fractured) or separated as a consequence of micro-transfer printing LEDs  60  from an LED source wafer (e.g., a compound semiconductor substrate such as GaN or GaAs) to display substrate  10 . Similarly, cluster controller  22  can comprise a broken (e.g., fractured) or separated controller tether  23  broken (e.g., fractured) or separated as a consequence of micro-transfer printing cluster controller  22  from a cluster-controller source wafer (e.g., a semiconductor substrate such as silicon) to display substrate  10 . Thus, in some embodiments LEDs  60  and cluster controller  22  are disposed directly on display substrate  10  or directly on layers disposed on display substrate  10 .  FIG.  7 A  illustrates one cluster  20  disposed on display substrate  10  but display systems  90  of the present disclosure can comprise multiple clusters  20  disposed on display substrate  10 , for example an array of clusters  20  defining a display area  12 , such as is shown in  FIG.  1   . 
     In some embodiments, and as illustrated in  FIG.  7 B , LEDs  60  and cluster controller  22  are micro-transfer printed onto a cluster substrate  62  that is separate, individual, and distinct from display substrate  10  and separate, individual, and distinct from LEDs  60  and any LED substrates and cluster controller  22 . LEDs  60  and a cluster controller  22  of a cluster  20  can be disposed on cluster substrate  62 . A single cluster  20  can be disposed on a single cluster substrate  62  or multiple clusters  20  can be disposed on a single cluster substrate  62 . Cluster substrates  62  can be disposed on display substrate  10 , for example by micro-transfer printing or other assembly processes, such as surface-mount technology. Clusters  20  on cluster substrates  62  can be surface-mount devices or can be micro-assembled, for example by micro-transfer printing cluster substrates  62  from a cluster source wafer to display substrate  10  so that cluster substrates  62  can comprise a broken (e.g., fractured) or separated cluster tether  63  as a consequence of micro-transfer printing. Clusters  20  on cluster substrates  62  can be packaged in order to be appropriately disposed by surface-mount technology. Cluster substrates  62  can comprise a same material as display substrate  10  or can be a different material. 
     As illustrated in  FIG.  7 C , cluster controller  22  in each cluster  20  can be formed in or on and native to cluster substrate  62  rather than micro-assembled on cluster substrate  62 , for example where cluster substrate  62  is a semiconductor substrate such as a silicon substrate and by using photolithographic processes found in the integrated circuit industry. Cluster controller  22  can be an integrated circuit. As also illustrated in  FIG.  7 C , pixels  24  with LEDs  60  can be micro-assembled on a pixel substrate  64  and pixel substrate  64  can be micro-assembled on cluster substrate  62  so that pixel substrate  64  can comprise a fractured or separated pixel tether  65  as a consequence of micro-assembling pixel substrate  64  from a pixel source wafer to cluster substrate  62 . Pixel substrates  64  can comprise material similar to or the same as cluster substrate  62  or display substrate  10 . One or more pixels  24  with pixel substrates  64  can be disposed directly on cluster controller  22 , so that cluster controller  22  can occupy a substantial amount of space on cluster substrate  62  or cluster controller  22  can be disposed between pixels  24  (e.g., as shown in  FIG.  7 C ). Cluster substrate  62  can be assembled on display substrate  10  or layers on display substrate  10 . 
     According to some embodiments and as shown in  FIG.  7 D , cluster controller  22  can be formed in or on and native to display substrate  10 , for example where display substrate  10  is a semiconductor substrate and, e.g., with photolithographic processing and materials, for example a silicon substrate in a micro-display. LEDs  60  in pixels  24  can be assembled, for example by micro-transfer printing, directly on display substrate  10  or layers on display substrate  10 , as shown in  FIG.  7 A , or can be disposed on pixel substrates  64  and pixel substrates  64  can be assembled, for example by micro-transfer printing, onto display substrate  10  or layers disposed on display substrate  10 , as shown in  FIG.  7 D . 
     Embodiments of the present disclosure illustrate in  FIGS.  7 B- 7 D  use cluster substrates  62  or pixel substrates  64 , or both, to provide a compound micro-assembled structure. Such structures can be tested before assembly on display substrate  10 . For example, clusters  20  on cluster substrates  62  as shown in  FIGS.  7 B and  7 C  can be tested before assembly on display substrate  10 . Similarly, pixels  24  disposed on pixel substrates  64  can be tested before micro-assembly on cluster substrates  62  or display substrate  10 . By testing clusters  20  or pixels  24  before assembly, any defective cluster controllers  22  or pixels  24  can be discarded and not assembled on display substrate  10  or cluster substrate  62 , thereby improving display system  90  yields and reducing costs. For example, either or both cluster substrate  62  or pixel substrate  64  can comprise probe pads for automated testing and micro-assembly systems can be programmed to discard or not assemble any defective clusters  20  or defective pixels  24 . 
     According to embodiments of the present disclosure and as illustrated in  FIG.  8 A , display system  90  can operate by first providing a display system  90  in step  100 . Display system  90  then receives an image, for example display controller  14  receives an image comprising image pixel values arranged in rows and columns corresponding to display pixel  24  rows and columns, in step  105 . The image is then analyzed to determine the appropriate cluster current source  36  for each cluster  20 , for example by display controller  14 , in step  110 , and the corresponding current-select signal  40  chosen for each cluster  20 . The determination can be based on the current required to provide the greatest desired luminance of any display pixel  24  in each cluster  20 . The image data and current-select signal  40  are then sent to each cluster  20 , for example through display row and display column controllers  16 ,  18  and display row wires  17  and display column wires  19  to cluster controllers  22  of each cluster  20  in step  115 . In response to received current-select signal  40 , cluster controller  22  enables circuit  34  to enable circuit control signal  35  to select cluster current source  36 . Timing signal  42  (for example provided by display row and display column controllers  16 ,  18  or generated internally by cluster controller  22 ) then controls switch  50  to display the received cluster image data with LEDs  60  in each cluster  20  in step  125 . Timing signal  42  can be a pulse-width modulation, pulse density modulation, or delta sigma signal that provides a constant current to LEDs  60 , thereby improving the efficiency of display system  90 . Cluster controller  22  can provide passive-matrix control to LEDs  60 , reducing the needed control circuits in cluster  20 . 
     Embodiments illustrated in  FIG.  8 A  can use a display controller  14  to analyze the image data associated with each cluster  20  and determine the appropriate cluster current source  36  for each cluster  20 . According to some embodiments of the present disclosure and as illustrated in  FIG.  8 B , the image data analysis to determine the appropriate cluster current source  36  for a cluster  20  is performed in cluster  20 . Thus, the analysis for each cluster  20  can be performed simultaneously and the communication bandwidth for cluster  20  is reduced, thereby increasing display system  90  frame rate. In some such embodiments, additional circuits must be provided in each cluster controller  22  to enable the analysis and determination, but since all that is necessary is to determine the greatest image pixel value of the cluster image data for each color or the colors together, e.g., find a greatest value, the circuitry can be simple and can be implemented directly in logic rather than requiring a stored-program machine (e.g., a computer or CPU and memory). 
     Therefore, according to embodiments of the present disclosure and as illustrated in  FIG.  8 B , display system  90  can operate by first providing a display system  90  in step  100 . Display system  90  then receives an image, for example display controller  14  receives an image comprising image pixel values arranged in rows and columns corresponding to display pixels  24 , in step  105 . Image data for each cluster  20  is then communicated to each cluster  20 , for example through display row and display column controllers  16 ,  18  and display row wires  17  and display column wires  19  to cluster controllers  22  of each cluster  20  in step  114 . The image is then analyzed in each cluster  20  to determine the appropriate cluster current source  36  for cluster  20 , for example by cluster controller  22 , in step  112 , and the corresponding current-select signal  40  chosen for each cluster  20 . The determination can be based on the current required to provide the greatest desired luminance of any display pixel  24  in each cluster  20 . In response to current-select signal  40 , each cluster controller  22  enables circuit  34  to enable circuit control signal  35  to select cluster current source  36  in step  120 . Timing signal  42  (for example provided by display row and display column controllers  16 ,  18  or generated internally by cluster controller  22 ) then controls switch  50  to display the received cluster image data with LEDs  60  in each cluster  20  in step  125 . Timing signal  42  can be a pulse-width modulation, pulse density modulation, or delta sigma signal that provides a constant current to LEDs  60 , thereby improving the efficiency of display system  90 . Cluster controller  22  can provide passive-matrix control to LEDs  60 , reducing the number and size needed in control circuits in cluster  20 . Thus, clusters  20  can be externally controlled, e.g., by display row and display column controllers  16 ,  18 , using active-matrix circuits, each cluster  20  can control display pixels  24  in the cluster using passive-matrix circuits. 
     Display substrates  10  of large-format displays can have signal-carrying wires (e.g., display row wires  17  and display column wires  19 ) that are lengthy (e.g., greater than one meter). Such long wires have a finite resistance and can experience parasitic capacitance and therefore signals carried on the wires can degrade significantly over the extent of display substrate  10 .  FIG.  9 A  illustrates display row wires  17  and display column wires  19  directly connected to each cluster  20  and cluster controller  22  in an array of clusters  20  disposed over display substrate  10 . According to some embodiments and as illustrated in  FIG.  9 B , display system  90  can comprise signal regeneration circuits  70  that regenerate signals (e.g., display row signals  17  and display column signals  19 ) In some such embodiments, display row wires  17  and display column wires  19  each comprise separate wire segments that are indirectly electrically connected through signal regeneration circuits  70 . Thus, according to embodiments of the present disclosure and as shown in  FIG.  9 B , a display system  90  can comprise an array of display pixels  24  distributed in rows and columns. A first wire segment (e.g., first display row wire segment  17 A or first display column wire segment  19 A) is electrically connected to a first cluster  20  or first cluster controller  22  and a second wire segment (e.g., second display row wire segment  17 B or second display column wire segment  19 B) is electrically connected to a second cluster  20  or second cluster controller  22 . Signal regeneration circuit  70  is operable to regenerate a signal conducted on the first wire segment and drive the regenerated signal onto the second wire segment. 
       FIG.  10 A  illustrates a simple signal regeneration circuit  70 . A gate of a transistor  52  is connected to first wire segments (e.g., first display row wire segment  17 A or first display column wire segment  19 A), transistor  52  source is connected to power P, the transistor  52  drain is connected through a resistor R to ground G and second wire segments (e.g., second display row wire segment  17 B or second display column wire segment  19 B). When a signal is received on the transistor  52  gate, transistor  52  is turned on and transistor  52  drain is pulled high to regenerate the signal connected to transistor  52  gate. As will be appreciated by those knowledgeable in electronic circuit design, many other signal regeneration circuits  70  are possible and are contemplated in various embodiments of the present disclosure. One or multiple clusters  20  or cluster controllers  22  can be connected to each first and to each second wire segment and embodiments of the present disclosure can comprise more than two wire segments (e.g., more than two display row wire segments  17 B or more than two display column wire segments  19 B) for each wire (e.g., display row wire  17  or display column wire  19 ) and one or multiple clusters  20  or cluster controllers  22  can be connected to each of the more than two wire segments. Signal regeneration circuits  70  can be disposed on display substrate  10  separately from other circuits (for example signal regeneration circuits  70  can be unpackaged, bare integrated-circuit dies micro-transfer printed to display substrate  10  and can have broken (e.g., fractured) or separated tethers), as shown in  FIG.  9 B . In some embodiments, signal regeneration circuits  70  can be disposed on cluster substrate  62 , either as a separate unpackaged, bare integrated circuit die or native to cluster substrate  62 , for example as shown in  FIG.  10 B , or as a part of cluster controller  22 , for example as shown in  FIG.  10 C . Signal regeneration circuits  70  can enable good signal propagation over large display substrate  10  and enable larger display systems  90  with faster frame rates and fewer display pixel errors. 
     Display substrate  10  can be any useful substrate on which cluster controllers  22  and an array of pixels  24  can be suitably disposed, for example glass, plastic, resin, fiberglass, semiconductor, ceramic, quartz, sapphire, or other substrates found in the display or integrated circuit industries. Display substrate  10  can be flexible or rigid and can be substantially flat. Display row wires  17  and display column wires  19  can be wires (e.g., photolithographically defined electrical conductors such as metal lines) disposed on display substrate  10  that conduct electrical current from display row controllers  16  and display column controllers  18 , respectively, to cluster controllers  22 . Similarly, cluster row wires  26  and cluster column wires  28  can be wires (e.g., photolithographically defined electrical conductors such as metal lines) disposed on display substrate  10  that conduct electrical current from cluster controllers  22  to pixels  24  and LEDs  60 . 
     Generally, display substrate  10  has two opposing smooth sides suitable for material deposition, photolithographic processing, or micro-transfer printing of micro-LEDs  60  or cluster controllers  22 . Display substrate  10  can have a size of a conventional display, for example a rectangle with a diagonal of a few centimeters to one or more meters. Display substrate  10  can include polymer, plastic, resin, polyimide, PEN, PET, metal, metal foil, glass, a semiconductor, or sapphire and have a transparency greater than or equal to 50%, 80%, 90%, or 95% for visible light. In some embodiments of the present disclosure, LEDs  60  emit light through display substrate  10 . In some embodiments, LEDs  60  emit light in a direction opposite display substrate  10 . Display substrate  10  can have a thickness from 5 microns to 20 mm (e.g., 5 to 10 microns, 10 to 50 microns, 50 to 100 microns, 100 to 200 microns, 200 to 500 microns, 500 microns to 0.5 mm, 0.5 to 1 mm, 1 mm to 5 mm, 5 mm to 10 mm, or 10 mm to 20 mm). According to some embodiments of the present disclosure, display substrate  10  can include layers formed on an underlying structure or substrate, for example a rigid or flexible glass or plastic substrate. 
     In some embodiments, display substrate  10  can have a single, connected, contiguous display area  12  (e.g., a convex hull including pixels  24  that each have a pixel functional area such as the light-emitting area of LEDs  60  in pixels  24 ). The combined functional area of light emitters  60  can be less than or equal to one-quarter of display area  12 . In some embodiments, the combined functional areas of light emitters  60  is less than or equal to one eighth, one tenth, one twentieth, one fiftieth, one hundredth, one five-hundredth, one thousandth, one two-thousandth, or one ten-thousandth of the contiguous system substrate area. Thus, remaining area over display substrate  10  is available for additional functional elements such as cluster controllers  22 . 
     Cluster controller  22  can be, for example, a bare, unpackaged integrated circuit disposed between rows and columns of pixels  24  micro-transfer printing or formed in cluster substrate  62  or display substrate  10  that provides control, timing (e.g., clocks) or data signals (e.g., column-data signals) through cluster row wires  26  and cluster control wires  28  to pixels  24  to enable pixels  24  to emit light in display system  90 . Cluster controller  22  can comprise a single integrated circuit or can comprise multiple integrated circuits, e.g., electrically connected integrated circuits. The integrated circuit(s) can be micro-transfer printed as unpackaged dies and can comprise broken (e.g., fractured) or separated controller tether(s)  23 . 
     The array of pixels  24  can be a completely regular array (e.g., as shown in  FIG.  1   ) or can have pixel rows or pixel columns of pixels  24  that are offset from each other, so that pixel rows or pixel columns of pixels  24  are not disposed in a straight line and can, for example, form a zigzag line (not shown in the Figures) or, as another example, have non-uniform spacing(s). Cluster controllers  22  can be disposed between rows or columns of pixels  24  even when pixels  24  are arranged in a regular array, at least in part because cluster controllers  22  can be micro-integrated-circuits comprising bare, unpackaged die of a size that can be disposed between rows or columns, or both, of pixels  24  by micro-transfer printing. 
     Pixels  24  can be passive-matrix pixels  24 , can be analog or digital (e.g., including one or more analog or digital controllers), and can comprise one or more light-controlling or light-responsive elements, e.g., inorganic micro-light-emitting diodes  60 . Pixels  24  can comprise micro-light-emitting diodes  60 . Inorganic light-emitting diodes  60  can have a small area, for example having a length and a width each no greater than 20 microns, no greater than 50 microns, no greater than 100 microns, or no greater than 200 microns. Such small, light emitters  60  leave additional area on display substrate  10  for more or larger wires or additional functional elements such as cluster controllers  22 . When active, pixels  24  can be controlled at a constant current with timing signals  42  such as temporal pulse-width modulation signals provided by cluster controller  22 . Pixels  24  can comprise a red-light-emitting diode  60  that emits red light, a green-light-emitting diode  60  that emits green light, and a blue-light-emitting diode  60  that emits blue light (collectively light-emitting diodes  60  or LEDs  60 ) under the control of cluster controller  22 . In certain embodiments, light emitters  60  that emit light of other color(s) are included in pixel  24 , such as a yellow light-emitting diode  60 . Light-emitting diodes  60  can be mini-LEDs  60  (e.g., having a largest dimension no greater than 500 microns) or micro-LEDs  60  (e.g., having a largest dimension of no greater than 100 microns). Pixels  24  can emit one color of light or white light (e.g., as in a black-and-white display) or multiple colors of light (e.g., red, green, and blue light as in a color display). 
     According to some embodiments of the present disclosure, pixels  24  comprise inorganic micro-light-emitting diodes  60  that have a length, a width, or both over array substrate  10  or pixel substrate  64  that is no greater than 100 microns (e.g., no greater than 50 microns, no greater than 20 microns, no greater than 15 microns, no greater than 12 microns, no greater than 10 microns, no greater than 8 microns, no greater than 5 microns, or no greater than 3 microns). Such relatively small, light emitters  60  disposed on a relatively large display substrate  10  (for example a laptop display, a monitor display, or a television display) take up relatively little area on display substrate  10  so that the fill factor of LEDs  60  on display substrate  10  (e.g., the aperture ratio or the ratio of the sum of the areas of LEDs  60  over display substrate  10  to the convex hull area of display substrate  10  that includes LEDs  60  or minimum rectangular area of the array of pixels  24  such as display area  12 ) is no greater than 30% (e.g., no greater than 20%, no greater than 10%, no greater than 5%, no greater than 1%, no greater than 0.5%, no greater than 0.1%, no greater than 0.05%, or no greater than 0.01%). For example, an 8K display (having a display array  12  bounding 8192 by 4096 display pixels  24 ) over a 2-meter diagonal 9:16 display with micro-LEDs  60  having a 15-micron length and 8-micron width has a fill factor of much less than 1%. An 8K display having 40-micron by 40-micron pixels  24  can have a fill factor of about 3%. According to some embodiments of the present disclosure, the remaining area not occupied by light emitters  60  is used at least partly to dispose cluster controllers  22  between light emitters  60 . 
     In contrast to embodiments of the present disclosure, existing prior-art flat-panel displays have a desirably large fill factor. For example, the lifetime of OLED displays is increased with a larger fill factor because such a larger fill factor reduces current density and improves organic material lifetimes. Similarly, liquid-crystal displays (LCDs) have a desirably large fill factor to reduce the necessary brightness of the backlight (because larger pixels transmit more light), improving the backlight lifetime and display power efficiency. Thus, prior displays cannot provide integrated cluster control because there is no space on their display substrates for additional or larger functional elements, such as cluster controllers  22 , in contrast to embodiments of the present disclosure. 
     In some embodiments, integrated circuits such as LEDs  60  or cluster controllers  22  are made in or on a native semiconductor wafer and have a semiconductor substrate and are micro-transfer printed to a non-native substrate, such as pixel substrate  64 , cluster substrate  62 , or display substrate  10 . Any of pixel substrate  64 , cluster substrate  62 , and display substrate  10  can include glass, resin, polymer, plastic, ceramic, or metal and can be non-elastomeric. Cluster substrate  62  can be a semiconductor substrate and cluster controller  22  can be formed in or on and native to cluster substrate  62 . Semiconductor materials (for example doped or undoped silicon, GaAs, or GaN) and processes for making small integrated circuits are well known in the integrated circuit arts. Likewise, backplanes such as display substrates  10  and means for interconnecting integrated circuit elements on the backplane are well known in the display and printed circuit board arts. 
     In a method according to some embodiments of the present disclosure, integrated circuits are disposed on the display substrate  10  by micro transfer printing. In some methods, integrated circuits (or portions thereof) or LEDs  60  are disposed on pixel substrate  64  to form a heterogeneous pixel  24  and pixel  24  is disposed on cluster substrate  62  or display substrate  10  using compound micro-assembly structures and methods, for example as described in U.S. patent application Ser. No. 14/822,868 filed Aug. 10, 2015, entitled Compound Micro-Assembly Strategies and Devices. However, since pixels  24  or clusters  20  can be larger than the integrated circuits included therein, in some methods of the present disclosure, pixels  24  or clusters  20  are disposed on display substrate  10  using pick-and-place methods found in the printed-circuit board industry, for example using vacuum grippers. Pixels  24  or clusters  20  can be interconnected on display substrate  10  using photolithographic methods and materials or printed circuit board methods and materials. 
     In certain embodiments, display substrate  10  includes material, for example glass or plastic, different from a material in an integrated-circuit substrate, for example a semiconductor material such as silicon or GaN. LEDs  60  can be formed separately on separate semiconductor substrates, assembled onto cluster substrates  62  or pixel substrates  64  to form pixels  24  and then the assembled units are located on the surface of cluster substrate  62  or display substrate  10 . This arrangement has an advantage that the integrated circuits, clusters  20 , or pixels  24  can be separately tested on cluster substrate  62  or pixel substrate  64  and the cluster  20  or pixel  24  modules accepted, repaired, or discarded before clusters  22  or pixels  24  are located on display substrate  10 , thus improving yields and reducing costs. 
     In some embodiments of the present disclosure, providing display system  90 , display substrate  10 , clusters  20 , or pixels  24  can include forming conductive wires (e.g., display row wire  17 , display column wire  19 , cluster row wire  26 , and cluster column wire  28 ) on display substrate  10 , cluster substrate  62 , or pixel substrate  64  by using photolithographic and display-substrate processing techniques, for example photolithographic processes employing metal or metal oxide deposition using evaporation or sputtering, curable resin coatings (e.g. SU8), positive or negative photo-resist coating, radiation (e.g. ultraviolet radiation) exposure through a patterned mask, and etching methods to form patterned metal structures, vias, insulating layers, and electrical interconnections. Inkjet and screen-printing deposition processes and materials can be used to form patterned conductors or other electrical elements. The electrical interconnections, or wires, can be fine interconnections, for example having a width of less than fifty microns, less than twenty microns, less than ten microns, less than five microns, less than two microns, or less than one micron. Such fine interconnections are useful for interconnecting micro-integrated circuits, for example as bare dies with contact pads and used with cluster substrate  62  and pixel substrate  64 . Alternatively or additionally, wires can include one or more crude lithography interconnections having a width from 2 μm to 2 mm, wherein each crude lithography interconnection electrically interconnects circuits, device, or modules on display substrate  10 . For example, electrical interconnections cluster row wire  26 , and cluster column wire  28  can be formed with fine interconnections (e.g., relatively small high-resolution interconnections) while display row wire  17  and display column wire  19  are formed with crude interconnections (e.g., relatively large low-resolution interconnections). 
     In some embodiments, red, green, and blue LEDs (e.g., micro-LEDs  50 ) are micro transfer printed to pixel substrates  64 , cluster substrate  62 , or display substrate  10  in one or more transfers and can comprise fractured or separated LED tethers  61  as a consequence of micro-transfer printing. For a discussion of micro-transfer printing techniques that can be used or adapted for use in methods disclosed herein, see U.S. Pat. Nos. 8,722,458, 7,622,367 and 8,506,867, each of which is hereby incorporated by reference in its entirety. The transferred light emitters  60  are then interconnected, for example with conductive wires and optionally including connection pads and other electrical connection structures. 
     In some embodiments of the present disclosure, an array of display pixels  24  (e.g., as in  FIG.  1   ) can include at least 40,000, 62,500, 100,000, 500,000, one million, two million, three million, six million, eight million, or thirty-two million display pixels  24 , for example for a quarter VGA, VGA, HD, 4K, 5K, 6K, or 8K display having various pixel densities (e.g., having at least 50, at least 75, at least 100, at least 150, at least 200, at least 300, or at least 400 pixels per inch (ppi)). In some embodiments of the present disclosure, light emitters  60  in pixels  24  can be considered integrated circuits, since they are formed in a substrate, for example a wafer substrate, or layer using integrated-circuit processes. The substrate or layer need not necessarily be silicon, for example III-V semiconductor wafers or layers can be used to form light emitters  60  using integrated-circuit processes. Light emitters  60  are considered integrated circuits (or portions thereof) in the context of this disclosure. 
     In some embodiments of the present disclosure, light emitters  60  are inorganic micro-light-emitting diodes  60  (micro-LEDs  60 ), for example having light-emissive areas of less than 10, 20, 50, or 100 square microns. In some embodiments, light emitters  60  have physical dimensions that are less than 100 μm, for example having at least one of a width from 2 to 50 μm (e.g., 2 to 5 μm, 5 to 10 μm, 10 to 20 μm, or 20 to 50 μm), a length from 2 to 50 μm (e.g., 2 to 5 μm, 5 to 10 μm, 10 to 20 μm, or 20 to 50 μm), and a height from 2 to 50 μm (e.g., 2 to 5 μm, 5 to 10 μm, 10 to 20 μm, or 20 to 50 μm). Light emitters  60  can have a size of, for example, one square micron to 500 square microns. Such micro-LEDs  60  have the advantage of a small light-emissive area compared to their brightness as well as color purity providing highly saturated display colors and a substantially Lambertian emission providing a wide viewing angle. Such small light emitters  60  also provide additional space on display substrate  10  for additional functional elements or larger wires. 
     In some embodiments, LEDs  60  are formed in substrates or on supports separate from display substrate  10 . For example, LEDs  60  can be made in a native compound semiconductor wafer. Similarly, cluster controllers  22  can be separately formed in a semiconductor wafer such as a silicon wafer e.g., in CMOS. LEDs  60 , or cluster controllers  22  are then removed from their respective source wafers and transferred, for example using micro-transfer printing, to display substrate  10 , cluster substrate  62 , or pixel substrate  64 . Such arrangements have the advantage of using a crystalline semiconductor substrate that provides higher-performance integrated circuit components than can be made in the amorphous or polysilicon semiconductor available in thin-film circuits on a large substrate such as display substrate  10 . Such micro-transferred LEDs  60  or cluster controllers  22  can comprise a broken (e.g., fractured) or separated LED tether  61  or controller tether  23  as a consequence of a micro-transfer printing process. 
     According to various embodiments, display system  90  can include a variety of designs having a variety of resolutions, light emitter  60  sizes, and display substrate  10  areas. 
     By employing a multi-step transfer or assembly process, increased yields are achieved and thus reduced costs for display systems  90  of the present disclosure. Additional details useful in understanding and performing aspects of the present disclosure are described in U.S. patent application Ser. No. 14/743,981, filed Jun. 18, 2015, entitled Micro Assembled Micro LED Displays and Lighting Elements, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     As is understood by those skilled in the art, the terms “over”, “under”, “above”, “below”, “beneath”, and “on” are relative terms and can be interchanged in reference to different orientations of the layers, elements, and substrates included in the present disclosure. For example, a first layer on a second layer, in some embodiments means a first layer directly on and in contact with a second layer. In other embodiments, a first layer on a second layer can include another layer or layers there between. 
     As is also understood by those skilled in the art, the terms “column” and “row”, “horizontal” and “vertical”, and “x” and “y”, “top” and “bottom”, and “left” and “right” are arbitrary designations that can be interchanged (unless otherwise clear from context). 
     Throughout the description, where apparatus and systems are described as having, including, or comprising specific components, or where processes and methods are described as having, including, or comprising specific steps, it is contemplated that, additionally, there are apparatus, and systems of the disclosed technology that consist essentially of, or consist of, the recited components, and that there are processes and methods according to the disclosed technology that consist essentially of, or consist of, the recited processing steps. 
     It should be understood that the order of steps or order for performing certain action is immaterial so long as operability is maintained. Moreover, two or more steps or actions in some circumstances can be conducted simultaneously. The disclosure has been described in detail with particular express reference to certain embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the following claims. 
     PARTS LIST 
     
         
         G ground 
         P power 
         C capacitor 
         R resistor 
           10  display substrate 
           12  display area 
           14  display controller 
           16  display row controller 
           17  display row signals/display row wires 
           17 A first display row wire segment 
           17 B second display row wire segment 
           18  display column controller 
           19  display column wire/display column signals 
           19 A first display column wire segment 
           19 B second display column wire segment 
           20  pixel cluster/cluster 
           22  cluster controller 
           22 C cluster column controller 
           22 R cluster row controller 
           23  controller tether 
           24  pixel/display pixel 
           26  cluster row wire/cluster row signal 
           28  cluster column wire/cluster column signal 
           30  selectable current source 
           32  demultiplexer 
           34  enable circuit 
           35  enable circuit control signal 
           35 A enable circuit control signal 
           35 B enable circuit control signal 
           35 B enable circuit control signal 
           35 C enable circuit control signal 
           36  cluster current source 
           38  current reference 
           40  current-select signal 
           42  timing signal 
           50  switch 
           52  transistor 
           52 A transistor 
           52 B transistor 
           52 C transistor 
           52 D transistor 
           60  light-emitting diode/LED/light emitter 
           61  LED tether 
           62  cluster substrate 
           63  cluster tether 
           64  pixel substrate 
           65  pixel tether 
           70  signal regeneration circuit 
           90  display or backlight system 
           100  provide display system step 
           105  receive image step 
           110  analyze image to determine current source for each cluster step 
           112  analyze cluster image data to determine current source for each cluster step 
           114  send image data to each cluster step 
           115  send image data and current-select signal to clusters step 
           120  each cluster selects current source step 
           125  each cluster displays image step