Active matrix electroluminescent display and method of operation

Abstract

An active matrix electroluminescent display (AMELD) having an improved light emitting efficiency and methods of operating the AMELD to produce gray scale operation comprises a plurality of pixels, each pixel including a first transistor having its gate connected to a select line, its source connected to a data line and its drain connected to the gate of a second transistor, the second transistor having its source connected to the data line and its drain connected to a first electrode of an electroluminescent (EL) cell. The EL cell's second electrode is connected to alternating high voltage means. A method for producing gray scale performance including the step of varying the length of time the second transistor is on while the alternating voltage is applied to the EL cell is also disclosed.

1. An electroluminescent display comprising an array of pixels, each pixel including
 a first transistor having its gate connected to a select line, its source connected to a data line and its drain connected to the gate of a second transistor;  
 the second transistor having it source connected to the data line and its drain connected to a first electrode of an electroluminescent cell; and  
 said electroluminescent cell having a second electrode which is connected to means for providing an alternating voltage power source with the voltage power source means being connected between the second electrode and a source of reference potential.  
 
     
     
       2. The display of  claim 1  wherein the means for providing an alternating voltage power source comprises a resonant alternating current high voltage power source. 
     
     
       3. The display of  claim 2  wherein the power source includes:
 first means for receiving an input voltage;  
 a resistor connected at one end and in series through a first switch to the first means and at another end to the second electrode of the electroluminescent cell;  
 an inductor connected to the first means and in series through a second switch to a source of reference potential;  
 a third switch connected across the first means, the inductor, the first switch and the resistor;  
 a comparator having an input connected to the second electrode of the electroluminescent cell and its output connected to an input of a set/reset latch, the latch having a second input, and first and second outputs;  
 wherein the first output of the latch, when activated, closes the first and second switches, the second output of the latch, when activated opens the first and second switches and closes the third switch;  
 wherein the values of the resistor and the inductor are chosen to provide a multiplication of the voltage applied to the first means.  
 
     
     
       4. The display of  claim 1  wherein the second transistor is a drift type MOS transistor. 
     
     
       5. The display of  claim 4  further comprising a capacitor connected between the gate of the second transistor and a source of reference potential. 
     
     
       6. The display of  claim 4  further comprising a capacitor connected between said data line and the gate of the second transistor. 
     
     
       7. A method of operating an active matrix electroluminescent display, said display comprising a plurality of pixels, each pixel including a first transistor having its gate connected to a select line, its source connected to a data line and its drain connected to the gate of a second transistor; the second transistor having its source connected to the date line and its drain connected to a first electrode of an electroluminescence cell, the electroluminescent cell having a second electrode, the method comprising the steps of
 applying voltages to the select and data lines to enable the second transistor of a given pixel;  
 applying a power source to the second electrode of the electroluminescent cell of the given pixel for a period of time; and  
 disabling the second transistor of the given pixel prior to the conpletion of said period of time.  
 
     
     
       8. In an electroluminescent display comprising an array of pixels, where each pixel contains a circuit for controlling application of energy to an electroluminescent cell associated with each pixel in said array of pixels, a method of providing gray scale illumination during a frame time comprising the steps of:
   subdividing said frame time into a plurality of LOAD periods and a plurality of ILLUMINATE periods;        loading, during each LOAD period, data from a data line into said circuit; and        varying, during each of said ILLUMINATE periods, a voltage on the data line, to selectively illuminate said electroluminescent cell in response to said voltage and said data.      
     
     
       9. The method of  claim 8  wherein said voltage on said data line is a linear ramp.  
     
     
       10. The method of  claim 8  wherein said voltage on said data line is a step function.  
     
     
       11. The method of  claim 8  wherein, during each ILLUMINATE period, a high voltage power supply applies at least one pulse to said circuit and, in response to said voltage, said at least one pulse is applied to said electroluminescent cell.  
     
     
       12. An electroluminescent display comprising an array of pixels for providing gray scale illumination during a frame time, where said frame time is divided into a number of LOAD and ILLUMINATE periods, each pixel comprising:
   a first transistor and a second transistor;        said first transistor having a first transistor gate, a first transistor source and a first transistor drain, where said first transistor gate is connected to a select line, said first transistor source is connected to a data line and said first transistor drain is connected to a second transistor gate of said second transistor;        said second transistor having said second transistor gate, a second transistor source and a second transistor drain, where said second transistor source is connected to said data line and second transistor drain is connected to an electroluminescent cell;        during each of said LOAD periods and when a select line signal on the select line activates the first transistor, said data line supplies, through said first transistor, a data signal to the second transistor gate where said data signal is stored; and        during each of said ILLUMINATE periods, said data line supplies a voltage to said second transistor to control illumination of said electroluminescent cell.      
     
     
       13. The display of  claim 12  wherein said voltage is a linear ramp.  
     
     
       14. The display of  claim 12  wherein said voltage is a step function.  
     
     
       15. An electroluminescent display comprising an array of pixels for providing gray scale illumination during a frame time, where said frame time is divided into a number of LOAD and ILLUMINATE periods, each pixel comprising:
   a control circuit, connected to a select line, a data line and a first electrode of an electroluminescent cell, for selectively applying energy to said electroluminescent cell in response to signals carried by said select line and said data line;        during each of said LOAD periods and when a select line signal on the select line activates the control circuit, said data line supplies a data signal to the control circuit where said data signal is stored; and        during each of said ILLUMINATE periods, in response to a state of said stored data signal, said control circuit applies pulsed energy from a power supply means to a second electrode of said electroluminescent cell for a particular period of time.      
     
     
       16. The display of  claim 15  wherein a number of ILLUMINATE periods and LOAD periods that are used to illuminate said electroluminescent cell during a frame time is equivalent to a number of bits used to define a number of levels of gray.  
     
     
       17. The display of  claim 15  wherein said control circuit further comprises:
   a first transistor and a second transistor;        said first transistor having a first transistor gate, a first transistor source and a first transistor drain, where said first transistor gate is connected to a select line, said first transistor source is connected to a data line and said first transistor drain is connected to a second transistor gate of said second transistor; and        said second transistor having said second transistor gate, a second transistor source and a second transistor drain, where said second transistor source is connected to said data line and second transistor drain is connected to a first electrode of an electroluminescent cell.      
     
     
       18. The display of  claim 15  wherein a number of ILLUMINATE periods and LOAD periods that are used to illuminate said electroluminescent cell during a frame time is equivalent to a number of bits used to define a number of levels of gray.  
     
     
       19. An electroluminescent display comprising an array of pixels, each pixel comprising:
   a first transistor, a second transistor and an electroluminescent cell;        said first transistor having a first transistor gate connected to a select line, a first transistor source connected to a data line, and a first transistor drain connected to a second transistor gate of said second transistor;        said second transistor having a second transistor source connected to said select line and a second transistor drain coupled to a first electrode of said electroluminescent cell; and        said electroluminescent cell having a second electrode coupled to means for providing an alternating current to the electroluminescent cell.      
     
     
       20. The display of  claim 19  further comprising:
   a first capacitor, connected between said second transistor drain and said first electrode of said electroluminescent cell, for coupling said second transistor to said electroluminescent cell.     
     
     
       21. The display of  claim 19 , further comprising:
   a second capacitor, connected between said second electrode of said electroluminescent cell and said means for providing an alternating current, for coupling said electroluminescent cell to said means for providing alternating current.