Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2001-03-01
2002-10-29
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S099000, C257S084000, C257S603000, C257S173000
Reexamination Certificate
active
06472688
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light emitting device having a LED (light-emitting diode), and a display device using the same.
A conventional semiconductor light emitting device is shown in FIG.
10
. In this semiconductor light emitting device
100
, a LED chip
101
is placed on the top of a first lead
102
, and one electrode of the LED chip
101
is electrically connected to the first lead
102
. The other electrode of the LED chip
101
is electrically connected to a second lead
104
via a gold wire
103
. The LED chip
101
and top portions of the first and second leads
102
,
104
are sealed with resin
105
which is transparent to a light from the LED chip
101
. The LED chip
101
is structured, though not illustrated, to incorporate p-n junction formed by direct joining of a p-type layer and a n-type layer, or double hetero-junction formed by insertion of an active layer between the p-type layer and the n-type layer. The p-n junction portion or the active layer emits light when a forward direction voltage is applied between the p-type layer and the n-type layer.
When an AC voltage is applied to the semiconductor light emitting device
100
, a current passes through the LED chip
101
only when the voltage is in forward direction because of rectification of the LED chip.
However, the semiconductor light emitting device
100
has a weakness for a reverse direction voltage, so that the LED chip
101
breaks if reverse direction voltage is excessively generated in AC driving. In the process of handling a substrate of the LED chip, the LED chip
101
also breaks if applied a static electric voltage in reverse direction.
In order to overcome this weakness, a semiconductor light emitting device as shown in
FIG. 11
has been proposed. The semiconductor light emitting device
110
has a first lead
111
, a second lead
112
, a Zener diode chip
115
and a LED chip
116
. The Zener diode chip
115
is die-bonded to a concave portion provided on the top of the first lead
111
. The LED chip
116
has a p-side electrode and a n-side electrode formed on one surface thereof, and the LED chip
116
is facedown-bonded to the surface of the Zener diode chip
115
. The Zener diode chip
115
and the LED chip
116
are parallel-connected in opposite directions to each other. These parallel-connected Zener diode chip
115
and LED chip
116
are connected to the first lead
111
and also to the second lead
112
via a gold wire
114
. The Zener diode chip
115
, the LED chip
116
, and top portions of the first and the second leads
111
,
112
are sealed with resin
117
, by which the semiconductor light emitting device
110
is constituted.
When an AC voltage is applied to the semiconductor light emitting device
110
and when the voltage is in forward direction, a current passes through the LED chip
116
and the LED chip
116
emits light while the Zener diode chip
115
is in OFF state till the voltage reaches a certain level. On the other hand, when the voltage is in reverse direction, the Zener diode chip
115
is turned on so that high voltage is not applied to the LED chip
116
. Accordingly, the Zener diode chip
115
prevents the LED chip
116
from breaking due to reverse voltage.
A display device
120
using the conventional semiconductor light emitting device
110
is shown in FIG.
12
. The display device
120
is a dynamic driving display device, in which a plurality of leads are provided in a vertical direction and a horizontal direction. On junctions of these leads, semiconductor light emitting devices are disposed in the form of matrix and connected. The semiconductor light emitting devices are made up of LEDij and Zener diode Zij (i=1 to 3, j=1 to 3). A transistor TRk (k=1 to 6) is provided to each one end of the leads. Those transistors TRk are each controlled so as to light a desired semiconductor light emitting device LEDij, by which images are displayed on the display device
120
.
It is noted that the display device
120
is a full color semiconductor light emitting device having red LEDil (i=1 to 3), green LEDi
2
(i=1 to 3), and blue LEDi
3
(i=1 to 3), where the forward direction threshold voltage of the red LEDil (i=1 to 3) is 2.1V, and the forward direction threshold voltage of the green LEDi
2
(i=1 to 3) and the blue LEDi
3
(i=1 to 3) is 3.5V.
However, the display device
120
has a drawback that when transistors TR
2
and TR
5
are turned on to light LED
22
, a leakage current is passed through Zener diode Z
11
, as a result of which not only LED
22
but also LED
21
and LED
12
are slightly lit up, as shown with an arrow A.
Further, because the forward direction threshold voltage of the red LEDil is 2.1V, when the display device
120
is driven at 3.5V or more corresponding to the forward direction threshold voltage of green LEDi
2
(i=1 to 3) and blue LEDi
3
(i=1 to 3), red LEDil (i=1 to 3) suffers from high remaining voltage in its driver IC (integrated circuit). This imposes a load on the constant current driver IC of the red LEDi
1
(i=1 to 3) and raises temperature thereof, causing lower reliability of the driver IC and lower reliability of the display device
120
as well.
SUMMARY OF THE INVENTION
Accordingly, a first object of the present invention is to provide a semiconductor light emitting device which is not damaged by a reverse direction voltage at the time of AC driving and free from malfunction due to a leakage current caused by matrix connection.
A second object of the present invention is to provide a highly-reliable semiconductor light emitting device in which when LEDs with different forward direction threshold voltages are connected in parallel so that a large load is not imposed on a constant current driver IC driving a specified LED and therefore the specified constant current driver IC is free from temperature rise.
In order to accomplish the above objects, the present invention provides a semiconductor light emitting device comprising a LED and a protection circuit made up of Zener diodes connected in series in opposite directions to each other, wherein the LED and the protection circuit are connected in parallel.
According to the semiconductor light emitting device of this invention, when a forward direction voltage below the breakdown voltage of the protection circuit is applied to the LED, the reverse-direction Zener diode in the protection circuit is turned off and so the current is not passed through the protection circuit, while on the contrary, the current is passed through the LED and the LED emits light. On the other hand, when a reverse direction voltage below the breakdown voltage of the protection circuit is applied to the LED, the forward direction Zener diode in the protection circuit is turned off and so the current is not passed through the protection circuit, while at the same time, the LED is turned off and therefore the current is not passed through the LED either.
When a forward direction voltage above the breakdown voltage of the protection circuit is applied to the LED, the reverse direction Zener diode in the protection circuit breaks down and so the current is passed through the protection circuit. When a reverse direction voltage above the breakdown voltage of the protection circuit is applied to the LED, the forward direction Zener diode in the protection circuit breaks down and so the current is also passed through the protection circuit. In other words, it can be said that when a voltage above the breakdown voltage of the protection circuit is applied in either forward or reverse direction to the semiconductor light emitting device, the current is allowed to pass through the protection circuit in the direction of applied voltage, which prevents a high voltage applied to the LED, and as a result the LED is protected from damage or breakage. It is noted that in the protection circuit, a total voltage obtained by adding a Zener voltage of the reverse direction Zene
Lee Eddie
Nguyen Joseph
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