Active solid-state devices (e.g. – transistors – solid-state diode – Encapsulated
Reexamination Certificate
2000-06-20
2002-10-29
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Encapsulated
C257S099000, C257S100000, C257S098000
Reexamination Certificate
active
06472765
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light emitting device having a plastic encapsulant, and more specifically, relates to a plastic encapsulated semiconductor light emitting device having a light emitting element and a light-permeating cover which can convert the wavelength of the light emitted from the light emitting element into a light of different wavelength to irradiate it outside of the cover.
2. Description of the Prior Art
A prior art semiconductor light emitting device as shown in
FIG. 2
has a plurality of leads
1
,
2
and a semiconductor light emitting element
3
secured with an adhesive agent such as solder (not shown) on a bottom surface
6
of a dished or cup-like portion (header)
5
formed at the end of the lead
1
. A lead wire
4
is connected between an electrode formed on a top surface of the semiconductor light emitting element
3
and a metal post of the lead
2
for electrical connection. A plastic encapsulant
9
is formed by the well known transfer molding or casting method to seal each one end of the leads
1
,
2
, the semiconductor light emitting element
3
and the lead wire
4
. The plastic encapsulant
9
comprises a sealing portion
10
formed into a substantially cylindrical shape, and a lens portion
11
formed into a substantially hemispherical shape integral with the sealing portion
10
. The plastic encapsulant
9
mainly contains an epoxy resin or the like having a light permeability, and as required, some additives may be mixed in the epoxy resin such as a scattering agent such as silica or the like and a pigment of a certain non-light emitting substance, however, no fluorescent particle is added to the plastic encapsulant
9
.
Attached to the plastic encapsulant
9
is a light-permeating cover
12
which comprises a lower portion
13
formed into a substantially cylindrical shape, and an upper portion
14
formed into a substantially hemispherical shape integral with the lower portion
13
. The lower and upper portions
13
,
14
have respectively their shapes matched to the sealing and lens portions
10
,
11
of the plastic encapsulant
9
. When the cover
12
is attached to the plastic encapsulant
9
, the lens portion
11
is inserted into the opening
15
provided at one end of the lower portion
13
to fit the plastic encapsulant
9
in the cover
12
so that an inner surface of the cover
12
is in the close and tight contact with an outer surface of the plastic encapsulant
9
. Therefore, the cover
12
attached to the encapsulant
9
does not easily come off out of the plastic encapsulant
9
although an external force such as vibration is applied to the cover
12
.
When the semiconductor light emitting element
3
is electrically operated, it produces a light, a part of which is reflected on side surfaces of the dished portion
5
formed in the upper portion of the lead
1
. However, the other part of the light is emitted from the top of the semiconductor light emitting element
3
without being reflected on the side surfaces of the dished portion
5
. Both of these two lights are condensed or converged by the lens portion
11
of the plastic encapsulant
9
to irradiate it outside of the plastic encapsulant
9
.
When the light emitting element
3
comprises for example a GaAlP, GaP or GaN semiconductor, it generates a light of red, green or blue color respectively. When the light emitting element
3
comprises a GaAs semiconductor, the semiconductor light emitting device emits infrared radiation. If no cover
12
is attached to the semiconductor light emitting device, the light projected from the plastic encapsulant
9
offers an extremely sharp directivity of emission light intensity around the tip of the lens portion
11
of the plastic encapsulant
9
with respect to angular distance from the central axis of the plastic encapsulant
9
as shown in
FIG. 3
which exemplifies that the light is practically not radiated over the angular range of ±30 degrees. The color of the light emitted from this semiconductor light emitting device is determined by the inherent emission light wavelength of the semiconductor light emitting element
3
.
On the other hand, when the cover
12
is attached to the plastic encapsulant
9
, the light of the semiconductor light emitting element
3
passes through the plastic encapsulant
9
, and then is radiated outside of the cover
12
with scattering of the light by the fluorescent particles
16
dispersed in the cover
12
.
The fluorescent particles
16
refer to substances which absorb a light irradiated thereto, while radiate a light of the wavelength different from that of the absorbed light. Therefore, the light emitted from the cover
12
offers an extremely wide directivity of emission light intensity as shown in
FIG. 4
, and the scattered light is spread out to the entire angle range of ±90 degrees. Because the fluorescent particles
16
are excited by the light irradiated from the semiconductor light emitting element
3
through the plastic encapsulant
9
, the fluorescent particles
16
radiate lights of the wavelength different from that of the light from the semiconductor light emitting element
3
. A part of the light emitted from the semiconductor light emitting element
3
passes through an area of the light-permeating cover
12
without fluorescent particles
16
so that it is then directly irradiated outside of the cover
12
in the absence of excited fluorescent particles. Consequently, the unchanged light emitted from the semiconductor light emitting element
3
is mixed with the wavelength-converted light emitted from the fluorescent particles
16
to emit the light of the mixed color wavelengths outside of the cover
12
.
In the prior art semiconductor light emitting device, the cover
12
has approximately uniform thickness in the whole area with neglect of the production errors and the uneven shrinkage of the plastic materials. The light from the semiconductor light emitting element
3
is intense at the tip of the lens portion
11
so that there is a large likelihood of the light component which does not impinge on the fluorescent particles
16
and passes through and outside of the cover
12
without the wavelength-conversion of the major component of emission light from the semiconductor light emitting element
3
. On the other hand, only weak light from the semiconductor light emitting element
3
passes through the sealing portion
10
of the plastic encapsulant
9
, and the major portion of the weak light impinges on the fluorescent particles
16
for wavelength-conversion. Therefore, when the cover
12
is applied to the plastic encapsulant
9
for the surface light emission indication with scattered light, wavelength of the light irradiated outwardly from the semiconductor light emitting device varies with the light emission angle with respect to a central axis of the plastic encapsulant
9
in the prior art semiconductor light emitting devices. Thus, for example, an undesirable problem arises that the emission light color changes depending upon the area of the light emitting surface so that the visible emission light does not have a specific, uniform and exact color. Obviously, this defect would give rise to bad impression to people that the semiconductor light emitting device is of poor quality.
Therefore, an object of the present invention is to provide a plastic encapsulated semiconductor light emitting device capable of emitting light with nearly uniform light wavelength at any angular distance from the central axis of the plastic encapsulant with a light-permeating cover attached thereon.
SUMMARY OF THE INVENTION
The plastic encapsulated semiconductor light emitting device according to the present invention comprises a plurality of leads
1
,
2
; at least one of which is formed with a dished portion
5
; a semiconductor light emitting element
3
attached on the bottom surface of the dished portion
5
for electrical connection of the light emitting element
3
across the lea
Kawae Hiroyuki
Sano Takeshi
Bachman & LaPointe P.C.
Chaudhuri Olik
Sanken Electric Co. Ltd.
Trinh Hoa B.
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