Abstract:
The efficiency of LEDs is increased by incorporating multiple active in series separated by tunnel junction diodes. This also allows the LEDs to operate at longer wavelengths.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The operating efficiency of light emitting diodes (LEDs) may be improved in a number of ways. These include improvements in the quality of the semiconductor layers and the design of the structure to maximize coupling of light out of the LED.  
         [0002]     The operating efficiency of LEDs based on AlGaInN or InGaN decreases as the net drive current is increased as is shown in graph  110  of  FIG. 1  for a green GaInN LED. This effect exists in addition to the well-known effect in LEDs where efficiency decreases due to heating brought on by increases in the drive current. The effect limits the performance of AlGaInN or InGaN at high drive currents. Additionally, for AlGaInN or InGaN LEDs, a wavelength shift to shorter wavelengths occurs as the current increases.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003]     In accordance with the invention multiple active regions in series separated by tunnel junctions are incorporated into AlGaInN or InGaN LEDs. For a fixed input power, LEDs in accordance with the invention require higher drive voltages but the current and current densities are reduced by a factor of n, where n is the number of active regions. The ability to operate at a lower drive current improves the efficiency of the AlGaInN or InGaN LEDs and reduces the wavelength shift due to drive currents. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  shows efficiency roll-off versus current.  
         [0005]      FIG. 2  shows an embodiment in accordance with the invention  
         [0006]      FIG. 3  shows a schematic of an LED structure in accordance with the invention.  
         [0007]      FIG. 4  shows the shows the shift of dominant wavelength with the forward drive current.  
         [0008]      FIG. 5  shows the relative efficiency as a function of wavelength for a constant drive current. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]      FIG. 2  shows LED structure  200  in accordance with the invention. N-type (Al x In (1-x) ) y  Ga (1-y)  N cladding layer  215  with x=0, y=0 or x=0 is grown to a typical thickness in the range from about 2 μm to about 5 μm on substrate  210  which is typically Al 2 O 3 , SiC GaN, or AlN. Quantum well active region  220 , typically including one to ten InGaN quantum wells and typically separated from one another by GaN barrier layers, is grown over AlGaInN cladding layer  215 . Growth of quantum well active region  220  is followed by growing p-type AlGaInN cladding layer  224  to a thickness typically in the range from about 0.03 μm to about 0.5 μM thick. Next tunnel junction  225  is formed by growing heavily doped p++ AlGaInN layer  226  to a thickness typically in the range from about 100 to 500 angstroms, followed by growing heavily doped n++ AlGaInN layer  227  to a thickness typically in the range from about 100 to 500 angstroms. P++ AlGaInN layer  226  is heavily p doped, typically with magnesium to a concentration typically in the range from about 6·10 19 /cm 3  to about 1·10 20 /cm 3 . N++ AlGaInN layer  227  is heavily n doped, typically with silicon to a concentration much greater than 1·10 20 /cm 3 , for example, in the range from about 2·10 20 /cm 3  to about 3·10 20 /cm 3 . Layer structure  297  comprises n-type AlGaInN cladding layer  215 , quantum well active region  220 , p-type AlGaInN cladding layer  224  and tunnel junction  225 .  
         [0010]     Following tunnel junction  225 , n-type AlGaInN cladding layer  230  is grown. Then second quantum well active region  235  is grown over n-type AlGaInN cladding layer  230 . Second quantum well active region  235  is similar to quantum well active region  220 . P-type AlGaInN cladding layer  240  is susbsequently grown over quantum well active region  235 . Next tunnel junction  245  is formed by growing p++ AlGaInN layer  246  to a thickness typically in the range from about 100 to 500 angstroms, followed by n++ AlGaInN layer  247  to a thickness typically in the range from about 100 to 500 angstroms. P++ AlGaInN layer  246  is heavily p doped, typically with magnesium to a concentration in the range from about 6·10 19 /cm 3  to about 1·10 20 /cm 3 . N++ AlGaInN layer  247  is heavily n doped, typically with silicon to a concentration much greater than 1·10 20 /cm 3 , for example, in the range from about 2·10 20 /cm 3  to about 3·10 20 /cm 3 . Layer structure  299  functions as an LED and is the basic building block for LED structure  200 . Layer structure  299  comprises n-type AlGaInN cladding layer  230 , quantum well active region  235 , p-type AlGaInN cladding layer  240  and tunnel junction  245 . Layer structure  299  may be repeated an arbitrary number of times in the vertical stack for LED structure  200 , as desired.  
         [0011]     Finally, an n-type AlGaInN layer is grown over the last tunnel junction in the vertical stack, for example, n-type AlGaInN cladding layer  250  is grown over tunnel junction  245  for LED structure  200 . Quantum well active region  255 , similar to quantum well active regions  220  and  235 , is then grown over n-type AlGaInN cladding layer  250  and p-type AlGaInN cladding layer  270  is grown over quantum well active region  250 . Layer structure  298  comprises n-type AlGaInN cladding layer  250 , quantum well active region  255  and p-type AlGaInN cladding layer  270 . Layer structure  298  functions as an LED.  
         [0012]     Tunnel junctions  245  and  225  in  FIG. 2  are reverse biased in the operation of LED structure  200 . Reverse biasing tunnel junctions  245  and  255  allows the current to flow through active regions  255 ,  235  and  220  in series. If there are a total of n quantuum well active regions in LED structure  200 , an applied voltage V to LED structure  200  will be divided approximately (because of possible parasitic voltage drops across contacts) equally across the n quantum well active regions so that there is a voltage drop of V/n across each layer structure  299  where each layer structure  299  is associated with a quantum well active region. This reduces the current and also the current density by a factor n in each quantum well active region  220 ,  235  and  255  while increasing the efficiency of LED structure  200 . For example, with respect to  FIG. 2 , three quantum well active regions  220 ,  235  and  255  are explicitly shown so the voltage drop across each quantum well region  220 ,  235  and  255  is about one third of the applied voltage, V, with the drive current, I, reduced by a factor of three from the single quantum well region case.  FIG. 3  shows a schematic of LED structure  200  indicating that layer structures  297 ,  299  and  298  function as LEDs. Typically, in an embodiment in accordance with the invention, a total of about two to ten layer structures is used.  
         [0013]     Graph  410  in  FIG. 4  shows the shift of dominant wavelength with the forward drive current. For AlGaInN or InGaN LEDs the wavelength typically shifts towards shorter wavelengths as the drive current increases as seen in graph  410 . Graph  510  in  FIG. 5  shows the relative efficiency as a function of wavelength for a constant drive current of about 20 mA. The relative efficiency improves as the In amount is decreased. To obtain the desired dominant wavelength at the highest forward drive current, quantum well active regions are grown with the appropriate composition of InGa(Al)N. If operation of the LED occurs at less than the highest forward drive current in accordance with the invention, the dominant wavelength will be longer. Hence, a composition with less In is used to obtain the same desired dominant wavelength while improving the efficiency of the LED.  
         [0014]     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.