Agilent Technologies, INC. - US 2006/0097269 A1
05/11/2006
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.

LED structure

Background of the invention

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.

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

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.

Claims

1. A light emitting diode structure comprising: a substrate; and a plurality of layer structures formed on said substrate, each one of said plurality of layer structures comprising a tunnel junction and a quantum well active region surrounded by a pair of cladding layers, said plurality of layer structures arranged in a vertical stack with respect to said substrate such that a first one of said plurality of layer structures is closest to said substrate and a second one of said plurality of layer structures is furthest from said substrate.

2. The structure of claim 1 wherein said cladding layer is comprised of AlGaInN.

3. The structure of claim 1 wherein said substrate is comprised of a material selected from the group containing Al2O3, SiC, AlN, and GaN.

4. The structure of claim 1 wherein said tunnel junction is comprised of an n doped AlGaInN layer.

5. The structure of claim 4 wherein said n doped AlGaInN layer is doped to a concentration in the range from about 2·1020/cm3 to about 3·1020/cm3 with an n dopant.

6. The structure of claim 5 wherein said n dopant is silicon.

7. The structure of claim 4 wherein said n doped AlGInN layer has a thickness in the range from about 100 to about 500 angstroms.

8. The structure of claim 1 wherein said tunnel junction is comprised of a p doped AlGaInN layer.

9. The structure of claim 1 wherein said quantum well active region is comprised of a plurality of InGaN quantum wells separated from each other by GaN barrier layers.

10. The structure of claim 1 wherein said tunnel junction is operable to be reverse biased.

11. A method for forming a light emitting diode structure comprising: a substrate; and forming a plurality of layer structures on said substrate, each one of said plurality of layer structures comprising a tunnel junction and a quantum well active region surrounded by a pair of cladding layers, said plurality of layer structures arranged in a vertical stack with respect to said substrate such that a first one of said plurality of layer structures is closest to said substrate and a second one of said plurality of layer structures is furthest from said substrate.

12. The method of claim 11 wherein said cladding layer is comprised of AlGaInN.

13. The method of claim 11 wherein said substrate is comprised of a material selected from the group containing Al2O3, SiC and GaN.

14. The method of claim 11 wherein said tunnel junction is comprised of an n doped AlGaInN layer.

15. The method of claim 14 wherein said n doped AlGaInN layer is doped to a concentration in the range from about 2·1020/cm3 to about 3·1020/cm3 with an n dopant.

16. The method of claim 15 wherein said n dopant is silicon.

17. The method of claim 14 wherein said n doped AlGaInN layer has a thickness in the range from about 100 to about 500 angstroms.

18. The method of claim 11 wherein said tunnel junction is comprised of a p doped AlGaInN layer.

19. The method of claim 11 wherein said quantum well active region is comprised of a plurality of InGaN quantum wells separated from each other by GaN barrier layers.

20. The method of claim 11 wherein said tunnel junction is operable to be reverse biased.

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