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NASA CASE NO. LAR 15_1B_4-3--SB

PRINT FIG. _5__

The invention disclosed in this document resulted from research in aeronautical

and space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the patent licensing regulations applicable to U.S.

Government-owned inventions (37 CFR 404.1 et seq.).

To encourage commercial utilization of NASA-owned inventions, it is NASA

policy to grant licenses to commercial concerns. Although NASA encourages

nonexclusive licensing to promote competition and achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve early

practical application of the invention.

Address informational requests for this invention to the Technology

Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia

23681-0001.

Address applications for license to the Patent Counsel Office, Mail Stop 212,

3 Langley Boulevard, Hampton, Virginia 23681-0001.

Serial No.: 08/644,654

Filed: 04/23/96 LaRC
NASA Case No. LAR 1 5 1 84-1 -SB 

INCREASED EFFICIENCY LED 

In a semiconductor light emitting diode (LED) a large proportion ofthe light produced 

will be internally reflected when it strikes the end face ofthe LED at an 

semiconductor-air interface. This is due to the high index of refraction of most LED 

semiconductor materials. This problem may be partially overcome by modification of 

the shape ofthe LED by a reverse taper ofthe sides ofthe LED. Light is redirected by 

the taper to strike the interface at an angle closer to normal. This allows light to exit 

the LED that would be totally internally reflected in an untapered LED. 

The novelty ofthe present invention lies in the tapering ofthe sides ofthe LED for 

increased through transmission of light. Prior art devices have made use surface 

modifications such as roughening, addition of guiding layers, use of index matching 

materials and hemispherical shaping ofthe emitting end ofthe LED. Current technology 

LEDs have a transmission efficiency of only a few percent. An increase in this 

efficiency would be of great value to any technology that makes use of LEDs for light 

generation. The present invention is related to LAR 15050-SB, Collection of Light 

from Optical Fibers of NA > 1. 

Inventors: Claudio Oliviera Egalon Robert S. Rogowski 

Address: 2658 N. Armistead Ave. B-2 109 Wicomico Turn 

Hampton, VA 23666 

SSN: 

Employer: AS&M 

Initial Evaluator: Joesph S. Heyman 

Serial No.: 081644,654 

Filed: 4/23/96 
LAR 15184-1-SB -1- PATENT APPLICATION

INCREASEDEFFICIENCYLED

Origin of the Invention

5 The invention described herein was jointly made by an employee of

the United States Government and a contract employee during the

performance of work under NASA Contract No. NAS-1-19236. In

accordance with 35 U.S.C. 202, the contractor elected not to retain title.

10 Background of the Invention

1. TechnicatEield of the Invention

15

The present invention relates to increasing the efficiency of light

generation in light emitting diodes (LEDs).

2. Discussion o_atedJ_t

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In an LED, light is produced at the p-n junction of the diode. The

LED is designed to allow the light produced to escape through an end

face. However not all of the light is able to escape a diode in air because

a portion is totally internally reflected at the interface between the diode

and surrounding air. Due to the very high refractive index of most

semiconductor materials, the portion of the light totally internally

reflected is very large.

The portion of light transmitted by the LED will depend on the

index of refraction of the LED as well as that of the medium in which the

LED is placed. Often the medium will be air. Herein the transmitting

portion of the LED will be known as the air semiconductor interface,
LAR 15184-1 -SB -2- PATENT APPLICATION

however this should be understood to include any other medium in which

an LED may operate. A critical angle, e c, measured away from

perpendicular, may be defined. Light impinging the air semiconductor

interface at an angle less than the critical angle will refract through the

5 interface. Light impinging at an angle greater than the critical angle will

be totally internally reflected, remaining in the semiconductor.

One method used to overcome the problem of total internal

reflection is to encapsulate the LED in plastic to better match refractive

indices and reduce reflection at the interface. This allows a small

10 increase in efficiency.

A second method is to shape the air-semiconductor interface into a

hemisphere, thereby altering the angle at which light strikes the interface.

This allows a greater portion of the light to be transmitted at the

interface. The hemispherical shape, while allowing more light to be

1 5 transmitted also contributes to diffusion of the light, thereby decreasing

intensity.

Bergh, et. al. (US Pat No 3,739,217, herein incorporated by

reference) discusses the problem of total internal reflection in an LED. To

overcome this problem the semiconductor surface is roughened through

20 chemical or mechanical means. According to Bergh, et. al., light striking

the rough surface will pass through at approximately the same rate as it

would in an LED having a flat surface. In a rectangular shaped LED any

light that impinges on an air-semiconductor interface at an angle that

allows total internal reflection will never escape as each interface will be

25 struck at the same angle. However, in a roughened LED that light which

is internally reflected has a greater chance of exiting the LED upon

striking the rough surface a second or succeeding time. This effect is a

result of the random reflection at the rough surface, the light will strike at

a random angle and this angle will change with each reflection from the
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LAR 15184-1 -SB -3- PATENT APPLICATION

roughened surface. Thus, an overall increase in efficiency is achieved by

allowing light that has totally internally refracted to have a chance to

escape on subsequent interactions with the air-semiconductor interface.

Yamanaka, et. al. (US Pat No 4,080,245, herein incorporated by

reference) also make use of geometric modifications of the emitting

surface of an LED for the purpose of increasing light transmission

efficiency across the air-semiconductor interface.

Figueroa, et. al. (US Pat No 4,856,014, herein incorporated by

reference) makes use of internal channels to guide light to the airsemiconductor interface at an angle less than the critical angle.

Haitz (US Pat No 5,087,949, herein incorporated by reference)

geometrically alters the air-semiconductor interface through the use of

diagonally cut facets. These facets allow a greater portion of the light

produced to strike an interface at an angle less than the critical angle.

Additionally, as with the Bergh invention, totally internally reflected light

will strike the air-semiconductor interface at a varying angle since the

LEDis not rectangular, thus allowing light to escape upon striking an

interface multiple times.

Fletcher, et. al, (US Pat No 5,233,204, herein incorporated by

reference) present an LED having a thick transparent layer overlying the

light generation region of the LED. This improvement is directed towards

LEDs that make use of an electrically conductive transparent window

layer between the light generation region and a top metal contact. The

thick transparent layer allows more light to escape from the sides of the

LED than would be able to with a thin electrically conductive transparent

window. One embodiment of the invention makes use of non-parallel

sides for the thick electrically conductive transparent window layer so

that more light may refract through the interface between the window

and the surrounding medium.
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LAR 15184-1-SB -4- PATENT APPLICATION

Egalon and Rogowski (US app. ser. no. 08/ _ _, filed

herein incorporated by reference) concerns untrapping light in an optical

fiber having high numerical aperture. It also makes use of geometric

modifications of a reflecting surface to modify the angle of incidence of

light so that it may refract through an air interface instead of being totally

internally reflected.

Sugary of the In_ention

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15

It is an object of the present invention to provide a p-n type

semiconductor LED capable of efficiently transmitting light produced at

the p-n junction through the air-semiconductor interface.

It is a further object of the present invention to do so without the

use of plastic encapsulation or difficult machining of the semiconductor

material into a hemispherical shape.

To achieve the forgoing objects a LED is presented that makes use

of reverse tapering to alter the path of light produced such that more is

able to be transmitted through the air-semiconductor interface.

20 Brief OescdptJo_of the Drawings

25

Fig. 1 is a drawing showing the generation of light at the p-n

junction of a semiconductor LED.

Fig. 2 is a drawing showing the total internal reflection of a light

ray at a air-semiconductor interface in a semiconductor LED.

Fig. 3 is a drawing of light being refracted through a hemispherical

interface.

Fig. 4 is a drawing of a semiconductor having a reverse taper,

showing a sample light path for a beam being refracted through the
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LAR 15184-1-SB -5- PATENT APPLICATION

interface, dashed lines show how the same beam would be totally

internally reflected in an untapered LED.

Fig. 5 is a drawing of a semiconductor LED with tapered sides and

the addition of an end face with a refractive index chosen to reduce total

internal reflection.

Description of_ Preferred Embodiments

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Making reference to Fig. 1, an n-p semiconductor is shown. At

the p-n junction 14 between n type semiconductor region 10 and p type

semiconductor region 12 photons 16 are produced when electricity is

applied to electrical contacts 18. Oxide layer 20 serves as an electrical

insulator.

Making reference to Fig. 2, total internal reflection is shown in a

semiconductor LED similar to that in Fig. 1. A light ray 22 is produced at

the p-n junction 14 as in Fig. 1. The light ray travels to the airsemiconductor interface :)4 where it is totally internally reflected.

Fig. 3 shows a semiconductor LED in which the air-semiconductor

interface 24 is shaped in a hemisphere. Light rays 22 produced at the pn junction 14 are refracted through the air-semiconductor interface 24.

Note that these light rays are dispersed as a result of refracting through

the curved surface.

Fig. 4 shows an LED according to the present invention. As in the

LED of Fig. 1 an n-type semiconductor region 10 is surrounded by a ptype semiconductor region 12. Light rays are produced at the p-n

junction 14. A light ray 22 traveling at angle _ reflects off of the reverse

tapered side of the LED 26 and continues to the air-semiconductor

interface 24. It strikes the interface 24 at an angle c and refracts

through. Dashed lines 26' show the sides of an untapered LED. Dashed
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LAR 15184-1 -SB -6- PATENT APPLICATION

arrows 22' represent the path of the same light ray in an untapered LED.

Light ray 22' strikes the interface 24 at an angle e' (not shown) and is

totally internally reflected. Angle (_ is the angle at which the sides of the

LED are reverse tapered. Making reference now to Fig. 5, another

embodiment of the present invention is shown. As in the LED of Fig. 1

an n-type semiconductor region 10 is surrounded by a p-type

semiconductor region 12. In addition to the tapered side of the LED 26

as in Fig. 4 another layer 28 is added between the surface of the

semiconductor and the air 30. This layer 28 is chosen to have an index

of refraction lower than that of the semiconductor materials. The use of

a lower index of refraction material in the interface region helps to reduce

total internal reflection.

In the examples are shown an n-p type LED, that is an n-type

semiconductor is surrounded by a p-type semiconductor. This

arrangement may be preferable because the refractive index of a p type

semiconductor may be made slightly lower than that of an n type

semiconductor, providing a waveguide configuration. However a p-n

type LED may also be used with the reverse taper surface to some

advantage. As the difference in refractive indices is generally small

between n and p type semiconductors the waveguiding effect would tend

to be small.

The index of refraction of region 10 is given to be n 1, the index of

refraction of region 12 is given as n 2, and the index of refraction of the

medium surrounding the LED is given as n o. Light impinging the airsemiconductor interface will refract through the interface if the critical

condition is met. For light in region 10 this is that sin c < no / nl, that

is, light striking the interface at an angle closer to the normal angle than

e will refract through, and light striking the interface at an angle closer to

parallel than e will be totally internally reflected. Light in region 12 must
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LAR 15184-1 -SB -7- PATENT APPLICATION

meet the condition sin e < no/n 2. In the case where the LED is in air,

n o = 1, generally n 1 and n 2 are both much higher than 1. In addition, as

noted above, n I is approximately equal to n 2 SO in a first approximation

the conditions for the two regions may be considered to be the same.

The above conditions may be used to determine an advantageous

choice for a, the angle of taper of the sides of the semiconductor, for

purposes of this discussion only nl will be used, n 2 may be trivially

substituted when necessary, however here we take them to be

equivalent:

0< sin e < (no/n 1) (1)

where:

and,

and,

e = (n/2-8) when (n/2-e) _<a

c = 2_- (n/2-8) when (n/2-8) > a

(2)

(3)

0 < 8 < n/2 (4)

Equation one gives the critical conditions for light to refract through the

air-semiconductor interface as discussed above. Equation two merely

states that light traveling at an angle that does not cause it to intersect

with the tapered sides will not have any dependance on (x. Note that the

angle e is measured from the normal angle to the side of an untapered

semiconductor. Thus, (n/2-e) gives e which is measured from the normal

angle to the air-semiconductor interface. Equation three shows the

dependance on cx of light traveling at an angle e such that it intersects

with the tapered sides of the semiconductor. Note here that since the

distance from the production of light at the p-n junction to the tapered

sides of the semiconductor is non-zero, the given conditions for

intersection do not strictly hold over small distances. That is, in an LED

having a short optical path from the p-n junction to the air-semiconductor
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LAR 15184-1-SB -8- PATENT APPLICATION

interface compared with the width of the LED, the conditions given in

Eqns. 2 and 3 are only approximations. Some light having (n/2-0) < a

will obey Eqn. 2 rather than Eqn. 3. The angle a may therefore be

chosen by optimizing Eqns. 1 and 3. This optimization will depend on

the ratio of width to height of the LED as well as factors such as the rate

of absorption of light by the LED material, and the values of n 1 and n o.

In optimizing equations 1 and 3 for a it may become apparent that

a single value of a is not the best solution. Instead, a varying a,

providing a curved taper rather than a straight reverse taper may enable

a larger amount of light to be untrapped.

Other applications and modifications of the present invention will

be apparent to those skilled in the art. The above embodiments are not

exhaustive but rather given by way of example. It is understood that the

present invention is capable of numerous modifications within the scope

of the following claims.
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LAR 15184-1-SB -11- PATENT APPLICATION

INCREASEDEFFICIENCYLED

Abstract of the Disclosure

5 In an LED a large portion of the light produced is lost due to total

internal reflection at the air-semiconductor interface. A reverse taper of

the semiconductor is used to change the angle at which light strikes the

interface so that a greater portion of the light is transmitted.
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