Basic Led

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Application Note 1

Application Note Optoelectronics

INTRODUCTION

Light Emitting Diodes or LEDs are generally classi-fied as infrared emitting diodes or visible LEDs depend-

ing on the wavelength of the light they emit. Thisapplication note provides information on visible LEDs.

Refer to the ‘Optoelectronics Data Book’ for informa-tion on infrared emitting diodes.

PRINCIPLE OF EMISSION OF VISIBLEEMITTING DIODE (LED)

When a forward voltage is applied to the external

lead pins, a current flows through the P-N junction andlight is emitted from the LED chip.

When no voltage is applied to the P-N junction of an

LED, the junction is in thermal equilibrium as shown inFigure 2 (a). The Fermi level of the P layer is the same

as that of the N layer, so that the height of the potentialbarrier is set at VD.

When a forward voltage VF is applied to the junction,the height of the potential barrier is reduced to VD - VFas shown in Figure 2 (b). As a result, the holes in the Player and the electrons in the N layer are injected into

the other layer and diffused. This injection of carriers

causes excessively high carrier concentration com-pared with that in the thermal equilibrium state. The P-

N junction tends to return to its stable thermal equilib-rium state, thus recombination of carriers occurs.

Simultaneously, light is emitted with an energy equal tothe energy difference between those of the two carriers

before recombination.

The peak emission wavelength of the emitted light (λ)

differs according to the energy released during recombi-nation and the energy differs according to the material.

Figure 1. Basic Structure of LED

Figure 2. Band Model of PN Junction

Epoxy resin

N electrode

N layer

P layer

P electrodeLED chip

Au wire

(Enlarged view of LED chip)

Pin

PN junction

OP10-1

VD: Potential barrierElectrons

Fermi level

Conduction band

Forbidden band

Valence band

Fermi level

Forbidden band

Valence band

Conduction band

N layer

N layer

VD VF

Holes

P layer

P layer

R e c om-

b i n a t i on

R e c om-

b i n a t i on

(a) Thermal Equilibrium State

(b) When a Forward Voltage is Applied

Eg

Light Light

OP10-2

The Basics of Light Emitting Diodes

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Optoelectronics The Basics of Light Emitting Diodes

2 Application Note

GENERAL LED CHARACTERISTICS

Absolute Maximum Ratings

Conditions which should never be exceeded to pre-

vent destruction of the LED and which correlate withthe operating temperature.

Continuous Forward Current (IF),Peak Forward Current (IFM)

Current which causes the LED to emit light. Since

the LED generates a certain amount of heat as currentflows which affects the operating life, the current is lim-

ited by a forward current derating curve.

Reverse voltage (VR)

An LED is a diode designed for its light emittingcharacteristics. Unlike ordinary diodes, the reverse

voltage cannot be controlled by changing the concen-

tration of the PN junction. Therefore, if a 3 V or higherreverse bias is applied, the addition of a protective cir-cuit is recommended.

Power Dissipation (P)

The internal power dissipation of the LED. The life ofthe LED lengthens if it is used at a dissipation (junction

temperature) below a certain level (temperature).

Operating Temperature (TOPR)

Refers to the temperature range including the heatgenerated by the device during operation of the LED.

Operation under conditions where damage to the pack-age material does not occur is recommended.

Storage Temperature (TSTG)

Refers to the temperature range during non-opera-tion of the LED. Since it is important for the LED pack-

age to pass light, it is not possible to change thecontent of the filler material to improve the temperature

characteristics as in IC packages.

Forward Voltage (VF)

The voltage when forward current is applied to theLED. It differs according to the added impurities in thecrystal material.

Reverse Current (IR)

The current when reverse voltage is applied. It issufficiently small compared to the forward current. It is

recommended that a circuit which applies a reversebias should be avoided.

Luminous Intensity (IV)

Refers to the brightness measured at a distance of

one feet from the light source. Common units are theµcd and mcd. The magnitude of the numeric value and

the apparent brightness do not necessary correspond.In actuality, it is necessary to take into account the con-

trast, luminance and quantity of light.

Peak Emission Wavelength (λ), SpectrumRadiation Bandwidth (∆λ)

These characteristics differ according to the crystalmaterial and added impurities.

Radiation Diagram, Half Valueof Viewing Angle

Represents the directivity distribution of the LED lumi-

nous intensity as a relative luminous intensity value.Generally, the luminous intensity is highest along the

normal optical axis and decreases as the angle with

respect to the optical axis increases. The angle at whichthe luminous intensity drops to 50% of the peak value is

called the halfpower angle. It can be used as a guideshowing the sharpness of the directivity.

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The Basics of Light Emitting Diodes Optoelectronics

Application Note 3

VISIBLE LED MATERIALSPresently, most LEDs use semiconducting materials

of the III-V group of chemical compounds. Gap, GaAsPand GaAlAs shown in Table 1 are three common mate-

rials for visible LEDs.

GaP (Gallium Phosphide) LEDs

(Red, Yellow-green, Green)

GaP, with a large forbidden band width (energy dif-ference between conduction band bottom and

valence band top) of 2.26 eV, can emit light from redto green. Because of the short 550 nm basic absorp-tion end (550 nm), this crystal is transparent to visible

light. Despite the indirect transition band structure, it

provides a high luminous efficiency because its emis-

sion is affected by the carrier recombination via exci-tons. GaP LEDs have a number of advantages. First,

they have been manufactured in mass productionquantities for a long time and the production technol-

ogy is mature. In addition, they are free from emissionwavelength deviation so that the characteristics areeasy to control.

Zn and O are dopants for red LEDs and N is adopant in yellow-green LEDs to increase the luminous

efficiency.

The external quantum efficiency of green and yel-low-green LEDs is about 0.15% and about 0.3%,

respectively. This is an order of magnitude smaller thanthat of GaP red LEDs (4% or less for the commercially

available one). However, due to their high luminousefficiency, these LEDs provide sufficiently high lumi-nance for practical use. Since yellow-green LEDs vary

in color tone, depending upon the amount of N dopant,it is important to control the amount of the N dopant.

GaAs1-xPx (Gallium-Arsenide-Phosphide)LEDs (Red, Sunset Orange, Yellow)

Mixed crystals of GaP and GaAs are used. By vary-

ing the mixing ratio ‘x’, different luminous colors fromred to yellow are obtained. Since the same vapor

growth and the same dopant diffusion technology as forSi can be used in manufacturing this material, it wasthe first put to practical use as an LED material.

The relation between the ‘x’ value and emission

wavelength is as shown in Table 1. With x ≥ 0.4 thecrystal structure changes from a direct transition type toan indirect transition type. Nitrogen is doped in orange

(x = 0.65) and yellow (x = 0.85) LEDs to improve theluminous efficiency as in the GaP LEDs. Commercially

available orange and yellow LEDs generally provideluminous efficiency of about 0.3% and about 0.12%,respectively.

Table 1. Crystal Material and Radiation Color (Emission Wavelength)

CRYSTALMATERIAL

RADIATIONCOLOR

FORBIDDENBAND (eV)

PEAK EMISSIONWAVELENGTH (nm)

SERIES

(AlxGa1-x)yIn1-yP Red 1.91 647 ZR

(AlxGa1-x)yIn1-yP Orange 1.98 627 ZJ

(AlxGa1-x)yIn1-yP Sunset Orange 2.03 609 ZS

(AlxGa1-x)yIn1-yP Amber 2.09 591 ZV

(AlxGa1-x)yIn1-yP Yellow-green 2.18 570 ZE

(AlxGa1-x)yIn1-yP Green 2.21 560 ZG

GaP: ZnO Red 2.26 695 PR

GAP: N Yellow-green 2.26 565 EG

GaP Green 2.26 555 KG

GaAS0.35P0.65 Red 1.95 635 HDGaAS0.25P0.75 Sunset Orange 2.03 610 HS

GaAS0.15P0.85 Yellow 2.1 585 HY

Ga0.63Al0.37As Red 1.9 660 TR, UR

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Application Note 5

Red LEDs with low Al mixing ratios have a quantumefficiency of approximately 3.4 percent, which dimin-

ishes as the Al mixing ratio is increased. Green LEDs,which have the highest Al mixing ratio, have a quantum

efficiency as low as around 0.06 percent. Given thevisual sensitivity of the human eye, the highest lumi-nous intensity is obtained at emission wavelengths

between orange (630 nm) and yellow (590 nm).Figure 5 (a) illustrates the structure of the general-

purpose, low-current (AlxGa1-x)yIn1-yP LEDs. Ann-(AlxGa1-x)0.5In0.5P cladding layer, (AlxGa1-x)0.5In0.5P

active layer, and p-(AlxGa1-x)0.5In0.5P cladding layerare sequentially deposited on an n-GaAs substrate to

form a double heterojunction structure. Luminous effi-ciency can be enhanced by making the Al mixing ratio(x) in the cladding layer higher than that in the active

layer as injected carriers can be easily confined withinthe active layer.

(AlxGa1-x)yIn1-yP LEDs are fitted with an optical

reflection layer known as DBR (Distributed BraggReflector, which combines mixed crystal layers having

different refractive indexes) between the substrate andthe n-(AlxGa1-x)0.5In0.5P cladding layer to further

enhance luminous efficiency.

Figure 5 (b) illustrates the current blocking structure,

in which a current blocking layer is inserted immedi-ately under the P electrode. The current blocking layer

blocks the current that would otherwise pass under the

electrode, thereby enabling the luminosity to be effi-ciently extracted from the active layer and thus provid-

ing even higher luminous intensity.

Because quad-crystal LEDs are capable of a

broader emission range from red to green, they areexpected to replace the conventional high-luminance

AlGaAs LEDs used in outdoor display boards and thelow-current LEDs used in cellular phones.

PRECAUTIONS FOR USE

Table 2 shows precautions that must be taken toprotect the quality and reliability of LED products.

NOTE: ‘x’ in part number indicates alphabet which shows emission color.

Figure 5. Structures of (AlxGa1-x)yIn1-yP LED

Table 2. Precautions for Use

CLASSIFICATIONS TYPE LED PRODUCTS

A Lead pin typeLED lamps (except for mini-mold LED lamps, chip LED devices)LED panel displays (LT9200x, LT9230x series and thin case mold type)LEDs for bar graphic displays (all resin mold type)

B Mini-mold type Mini-mold LED lamps

C Substrate typeNumeric LEDs (substrate type)Dot matrix LEDs (LT5007xx and LT5008xx and LT5013T and LT50414S series)

D Case mold type

LED panel displays (LT9002x, LT9010x, LT9210x, and LT9220x, LT9400x series)Numeric LEDs (mold type)Dot matrix LEDsLEDs for bar graphic display (case mold type)

E Chip device type Chip LED devices

P electrode

Current diffusion layer

p-(AlXGa1-X)0.5 In 0.5 P cladding layer

(AlXGa1-X)0.5 In 0.5 active layer

n-(AIXGa1-X)0.5 In 0.5 Pcladding layer

n-DBR layer

n-GaAs layer

N electrode

(a)Low current LED

P electrode

Current diffusion layer

Current blocking layer

p-(AlXGa1-X)0.5 In 0.5 P cladding layer

(AlXGa1-X)0.5 In 0.5 P active layer

n-(AlXGa1-X)0.5 In 0.5 P cladding layer

n-DBR layer

n-GaAs layer

N electrode

(b)Current blocking type LED OP10-5

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6 Application Note

Lead Pin Type

LEAD FORMING METHOD

Avoid forming a lead pin with the lead pin base as afulcrum: be sure to hold a lead pin firmly when forming.

Lead pins should be formed before soldering.

INSTALLATION

Installation on a PWBWhen mounting an LED lamp on a PWB, do not

apply physical stress to the lead pins.

When an LED Lamp is Mounted Directly on a PWBIf the bottom face of an LED lamp is mounted

directly on single-sided PWB, the base of the lead pins

may be subjected to physical stress caused by PWBwarp, cutting or clinching of lead pins. Prior to use, be

sure to check that no disconnection inside of the resinor damage to resin etc., is found.

When an LED lamp is mounted on a double-sidedPWB, the heat during soldering affects the resin; there-

fore, keep the LED lamp more than 1.6 mm afloatabove the PWB.

Installation Using a Holder

When a holder is used during an LED lamp posi-

tioning, holder A should be designed to be smallerthan the inside diameter of the lead pins. Holder Bshould be designed to be larger than the outside

diameter of the lead pins.

NOTE: Pay attention to the thermal expansion coefficient of the

material used for the holder. Since the holder expands and contractsas a result of preheat and soldering heat, mechanical stress may be

applied to the lead pins, resulting in disconnection.

Installation to the Case

When the LED is fixed to a case as showin in Figure10, do not fix part A with adhesives. A hole of the case

should be designed to be smaller than the outsidediameter of LED lamp resin.

Figure 6. Lead Forming Method

Figure 7. Installation on a PWB

Hold a lead pin firmly when forming

OP10-6

PWB

No

Yes

OP10-7

NOTES:

1. The lead pin pitch should match the PWB pin-hole pitch:absolutely avoid widening or narrowing the lead pins.

2. When positioning an LED lamp, basicallyemploy an LED with tie-bar cut or use a spacer.

Figure 8. LED Lamp Mounted Directly on a PWB

Figure 9. Installation Using a Holder

Figure 10. Installation to the Case

1.6 mm

OP10-8

Holder A

B

OP10-9

Case

A

OP10-10

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Application Note 7

SOLDERING CONDITIONS

Solder the lead pins under the conditions shown in

Table 3.

NOTES:

1. Avoid dipping resin into soldering bath.

2. Avoid applying stress to lead pins while they are heated. For

example, when the LED lamp is moved with the heat applied to

the lead pins during manual soldering or solder repair, disconnec-

tion may occur.

CLEANING

The package resin may be penetrated by solvents

used in cleaning. Refer to Table 4 for usable solvents.and to Table 5 for Cleaning Methods.

NOTE: There is a world-wide movement to restrict the use of chlorof-

luorocarbon (CFC) based solvents and we recommend that you

avoid their use. However, before using a CFC substitute solvent,

carefully check that it will not penetrate the package resin.

NOTES:

1. The effect on the device from ultrasonic cleaning differs depend-

ing on the size of the cleaning bath, ultrasonic output, duration,

board size and device mounting method.

2. Cleaning with water is not allowed with the lead pins resin-tubu-lated: water may remain, thus causing the lead pins to rust.

3. Please contact our representative before using a cleaning solvent

or method not given in Tables 4 and 5.

Table 3. Soldering Conditions

TYPE OFSOLDERING

CONDITIONS

Manual soldering 295°C ±5°C, within 3 seconds.

Wave soldering 260°C ±5°C, within 5 seconds.

Reflow solderingPreheating 70°C to 80°C, within30 seconds. Soldering 245°C ±5°C,within 5 seconds

Solderingiron

YesNo

OP10-11

Table 4. Solvents

SOLVENT USABLE

Ethyl alcohol Yes

Isopropyl alcohol Yes

Chloresen No

Acetone No

Trichlorethylene No

Table 5. Cleaning Methods

CLEANINGMETHOD

USABLE REMARKS

Solventcleaning

YesImmersing up to one minute atroom temperature

Ultrasoniccleaning

Yes/NoTest the cleaning under actualconditions and check for abnor-malties before actual use

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8 Application Note

Mini-mold Type

LEAD FORMING METHOD



INSTALLATION ON A PWB

When mounting on a PWB, we recommend you to:

• Form lead pins.

• Design the product so that lamps will not be mountedin the same direction as the warp of the PWB.

• Utilize a jig to fix location of lamps.

SOLDERING CONDITION

Table 6 shows the lead pin soldering conditions.

NOTES:

1. Avoid dipping resin into soldering bath.

2. Avoid applying stress to lead pins while they are heated.

3. Do not warp the PWB after soldering.

CLEANING

The package resin may be penetrated by solventsused in cleaning. Refer to Table 7 for usable solvents

and Table 8 for Cleaning Methods.





NOTES:





or method not given in Tables 7 and 8.

Figure 11. LED Forming

Figure 12. Mounting an LED on a PWB

Figure 13. Lamp Direction

Figure 14. Utilizing a Jig to Fix Location of Lamps

Hold a lead pin firmlywhen forming

OP10-12

PWB

Yes

No

OP10-13

YesNoOP10-14

Soldering iron

PWB

Jig

OP10-15


TYPE OFSOLDERING

CONDITIONS


Wave soldering 260°C ±5°C, within 5 seconds.

Reflow solderingPreheating 70°C to 80°C, within30 seconds. Soldering 245°C ±5°C,within 5 seconds

Table 7. Solvents

SOLVENT USABLE

Ethyl alcohol Yes


Chloresen No

Acetone No

Trichlorethylene No


CLEANINGMETHOD

USABLE REMARKS

Solventcleaning


Ultrasoniccleaning

Yes/NoTest the cleaning under actualconditions and check for abnor-malties before actual use

No NoOP10-16

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Application Note 9

SubstrateType

LEAD FORMING METHOD



SOLDERING CONDITION


CLEANING

In principle, no cleaning is required. If cleaning isrequired, take care to prevent the solvent from entering

the package. Otherwise disconnections may occur inthe LED.

NOTES:

1. There is a world-wide movement to restrict the use of chlorofluo-

rocarbon (CFC) based solvents and we recommend that you

avoid their use. However, before using a CFC substitute solvent,carefully check that it will not penetrate the package resin.



board size and device mounting method. Test the cleaning

method under actual conditions and check for abnormalities

before actual use.


or method not given in Table 10.

Case-mold Type

LEAD FORMING METHOD



SOLDERING CONDITION


CLEANING

The package resin may be penetrated by solventsused in cleaning. Refer to Table 12 for usable solvents

and Table 13 for Cleaning Methods.


luorocarbon (CFC) based solvents and we recommend that youavoid their use. However, before using a CFC substitute solvent,




TYPE OFSOLDERING

CONDITIONS


Wave soldering 260°C ±5°Cm within 5 seconds.

Table 10. Solvents

SOLVENT USABLE

Ethyl alcohol Yes


Chloresen No

Acetone No

Trichlorethylene No

Yes NoOP10-17



TYPE OFSOLDERING

CONDITIONS


Wave soldering 260°C ±5°Cm within 5 seconds.

Reflow solderingPreheating 70°C to 80°C, within30 seconds. Soldering 245°C ±5°C,within 5 seconds.

Table 12. Solvents

SOLVENT USABLE

Ethyl alcohol Yes


Chloresen No

Acetone No

Trichlorethylene No

Yes NoOP10-18

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10 Application Note

NOTES:






Chip LED Device Type

MOUNTING TO A PWB

Design the product so that the devices will not be

mounted in the same direction as the warp of the PWB.

SOLDERING CONDITION

Solder the lead pins under the following conditions:

• In manual soldering, do not move the lead pins withthe soldering edge.

• Avoid applying excessive solder reinforcement.

• Do not try to correct the position of the devices aftersoldering.

• Do not warp PWB after soldering.

CLEANING

The package resin may be penetrated by solvents

used in cleaning. Refer to Table 15 for usable solventsand Table 16 for Cleaning Methods.





NOTES:






3. Since the device is very small, it may be damaged by excessive

stress. Pay special attention to the transport method and handling.


CLEANINGMETHOD

USABLE REMARKS

Solventcleaning


Ultrasonic

cleaningYes/No

Test the cleaning under actualconditions and check for abnor-malties before actual use

Figure 17. Mounting to a PWB

Figure 18. Solder Reinforcement

Figure 19. PWB Warping

YesNoOP10-19

YesNoOP10-21

No YesOP10-22


TYPE OFSOLDERING

CONDITIONS

Manual soldering 300°C ±5°C, within 5 seconds

Reflow soldering

Preheating 100°C to 150°C, within2 minutes. Soldering 245°C ±5°C, with-in 5 seconds. Gradual cooling (avoidingquenching).

Table 15. Solvents

SOLVENT USABLE

Ethyl alcohol Yes


Chloresen No

Acetone No

Trichlorethylene No


CLEANINGMETHOD

USABLE REMARKS

Solventcleaning


Ultrasoniccleaning

Yes/NoTest the cleaning under actualconditions and check for abnor-

malties before actual use

Within 5 s

Within 2 min. 30 s

245

200

150

100

0

T e m p e r a t u r e ( C )

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Basic Led