Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
2002-06-26
2003-12-09
Baumeister, B. William (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S079000
Reexamination Certificate
active
06661032
ABSTRACT:
1. FIELD OF THE INVENTION
The present invention relates to a package structure of a light emitting diode (LED), and more particularly, to a LED package structure having an electro-static protective diode.
2. DESCRIPTION OF THE PRIOR ART
Because of the advantage of small size, low power consumption and long life span, LEDs are widely used in household appliances, computer peripherals and communication products. In 1994, NICHIA company successfully invented a blue Gallium-nitride (GaN) LED so that a full-colored LED can be achieved. Moreover, the usage of the LED expands to full-colored LED displays, traffic lights and car lamps, maybe further replaces the illuminative application of white lamps in the future illumination.
However, the blue GaN or bluish Indium Gallium-nitride (InGaN) LED easily suffers deterioration by static electricity since the P/N junction thereof is very close to the surface. Human body may carry static electricity with the magnitude of 1-2 kV during the dry environment. If pins of the LED happens to be touched, even small currents will destroy the susceptible LED of which typical range of operation voltage is about 0-5 V. And the unit price of the blue LED or the bluish LED is higher than a primary LED by 10-100 folds. Therefore, it is necessary for the blue LED and bluish LED to equip with electrostatic protective devices.
In order to solve the above-mentioned problem, a Zener diode parallel connects to the LED according to the prior art, so as to protect the LED dies from being damaged by static electricity.
FIG. 1
is a schematic diagram of electrical circuit connections of a LED
10
and its protective element, Zener diode
20
. In the circuit, the LED
10
is forward bias and the Zener diode
20
is reverse bias under normal operating voltage of 5V. Since typical Zener breakdown voltage is about 8V, the Zener diode
20
is not up to the Zener breakdown voltage during normal operation voltage resulting in an un-conductive situation without consuming electric energy. However, if high voltage such as 1-2 kV is provided to pins of light emitting elements, the LED
10
and Zener diode
20
are both conductive with large portions of currents through the Zener diode
20
due to its lower resistance. Therefore, a function of the Zener diode
20
to protect the LED
10
is achieved.
In order to form the circuit with Zener diode and LED in parallel connection as
FIG. 1
, several conventional techniques are available, but each one has its drawbacks. At first, please refer to
FIG. 2
of a schematic diagram of the first embodiment. A bottom surface
9
(n++; n-type heavy doped region) of a Zener diode
20
is mounted on a planar surface of a lampstand
15
(or say a reflector)by conductive silver pastes
14
. The lampstand
15
is a cone-shaped with a plane surface on its bottom, and under the plane thereof is a positive electrode of the leadframe
13
a
. The Zener diode
20
comprises a p region
25
in an n-type substrate
26
having a p-type electrode
27
and an n-type electrode
28
, respectively, formed thereon. Moreover, an n-type electrode
6
and a p-type electrode
5
of a LED
10
are, respectively, connects the p-type electrode
27
and the n-type electrode
28
of the Zener diode
20
by conductive solder balls
11
and
12
. The p region
25
of the Zener diode
20
electrically connects to a tip of a negative electrode of the leadframe
13
b
via a conductive wire
17
. Finally, a transparent substance, such as resin, encapsulates the aforesaid structures to form a LED package.
For a blue LED, the n-type electrode
6
and the p-type electrode
5
are positioned on the same side of the LED so that the aforesaid connections are workable. However, as to the red, yellow, or green LED, the n-type electrode
6
and the p-type electrode
5
are positioned on different sides of the LED structure so that the aforesaid connections are not workable. However, for the brightness of the LED is concerned, the aforesaid structure is better than others of the prior art because the light-emitting side of the aforesaid LED is upward and no wire-bonding pad or electrode formed thereon.
As for package techniques, there are some problems with the aforesaid structure. The areas of LED chip are several mil square (typical value is about 13×14 mil) and the height of the solder ball is about 4 mils. Therefore, it is very difficult to align the LED
10
which has the P-type electrode
5
and the n-type electrode
6
upside-down with the n-type electrode
28
and p-type electrode
27
of the Zener diode
20
. Therefore, the process efficiency and process yield are hardly improved. Please notices that larger solder balls are not suitable for the case or the risk of the short-circuit
10
between the p-type electrode
27
and n-type electrode
28
will increase. Moreover, the structure of the Zener diode
20
according to the present embodiment is more complicated than the conventional diode structure only stacked by the p and the n layers leading to the increase of the process cost. Therefore, the LED package technique of
FIG. 2
needs to be improved.
The second embodiment of the prior art, is disclosed by the U.S. Pat. No. 6,054,716 issued to Sonobe. Please refer to
FIG. 3
, The prime difference between the second and the first embodiment is the positions of a LED
53
and a Zener diode
55
. According to the second embodiment, the LED
53
and the Zener diode
55
are, respectively, mounted on the recess portion
61
and a flange
62
of the curve-surfaced lampstand provided at a tip of a positive electrode of the leadframe
61
a
. In the second embodiment, the Zener diode is a conventional one, stacked with a p-type layer and an n-type layer. The n-type electrode
55
a
of the Zener diode
55
connects the flange
62
of the lampstand via silver pastes
58
. The p-type electrode
55
b
of the Zener diode
55
connects a negative electrode of the leadframe
52
b
via a conductive wire
68
. The p-type electrode
65
of the LED
53
connects the positive electrode of the leadframe
52
a
via a conductive wire
66
. An n-type electrode
63
of the LED
53
connects present a p-type electrode
55
b
of the Zener diode
55
by a conductive wire
67
. Finally, the resin
73
is utilized to package and form a dome-shaped LED structure.
Because the wire bonding process is to the upward positions of the LED and the Zener diode, the process yield in accordance with the present embodiment is better than that of the first embodiment. However, three conductive bonding wires are needed in the present embodiment instead of one as the first embodiment so that less brightness of the present structure is anticipated. Moreover, the present embodiment described may have drawbacks as following: (1) The flange
62
and the recess portion
61
of the curve-surfaced lampstand, with different altitudes, but both need to be paste with the Silver paste and thus the prices of the producing facilities for that would be high. In addition, the area of the recess portion
61
of lampstand is small enough, not much to say the area of the flange
62
and thus bring about to highlight the difficulty of pasting the silver paste thereon. (2) if the LED such as a blue GaN LED or an InGaN LED has thinner substrate may increase the overflow risk resulting short circuit during silver pastes (or conductive paste) pasting process. (3) Only the LED having a p and n-type electrode positioned on one side is applicable for the present embodiment. Above all, the package technique of the second embodiment still needs to be improved.
Please refer to
FIG. 4
of a schematic diagram of the third embodiment according to the prior art. The present embodiment is in accordance with the U.S. Pat. No. 6,084,252 disclosed by Isokawa. A Zener diode
105
has an n-type electrode (not shown) formed at its bottom face mounted on a side of a positive electrode of the leadframe
107
a
of a lampstand with silver pastes. A p-type electrode formed at a top face of the Zener diode
105
electrically connects to a later
Lin Chi-Ming
Ma Li-Ching
Meng Hsiang-Chih
Baumeister B. William
Troxell Law Office PLLC
United Epitaxy Co., Ltd.
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