WO2005013381A1

WO2005013381A1 - Light emitting diode and light emitting device with the same

Info

Publication number: WO2005013381A1
Application number: PCT/KR2004/001926
Authority: WO
Inventors: Tae Kyung Yoo
Original assignee: Epivalley Co., Ltd.
Priority date: 2003-07-30
Filing date: 2004-07-30
Publication date: 2005-02-10
Also published as: KR20050014342A; KR100586676B1

Abstract

The present invention relates to a light emitting diode and a light emitting device including the light emitting diode having a first plurality of semiconductor layers including an active layer for emitting red light, and a second plurality of semiconductor layers which are grown on the first plurality of semiconductor layers and form a diode for raising an operating voltage of the first plurality of semiconductor layers; a blue LED connected in parallel to the red LED; and a green LED connected in parallel to the red LED and to the blue LED, thereby overcoms the problem of the conventional white LED using a fluorescent material and a blue LED.

Description

LIGHT EMITTING DIODE AND LIGHT EMITTING DEVICE WITH THE SAME

[Technical Field] The present invention relates to a light emitting diode (LED) structure for raising an operating voltage of entire LED elements by vertically integrating an additional diode onto an AIGalnP compound semiconductor LED.

[Background Art] A conventional LED includes, as indicated in FIG. 1 , an n-type clad layer 11 and a p-type clad layer 13 formed respectively at the upper and lower sides of an active layer 12 on a substrate 10, for trapping electrons and holes to emit light, a contact layer 14 connected to an external electrode, and p-type electrode 18 and an n-type electrode 19 for forming metal electrodes by a semiconductor process. A current-voltage characteristic of the conventional LED is illustrated in

FIG. 2. If a forward voltage is applied to a p-n diode, no current flows until it reaches a threshold voltage. However, if the forward voltage exceeds the threshold voltage, current flows into the p-n diode. For example, when current of 20 mA is used, the voltage at that current is defined as an operating voltage. The threshold voltage is determined by the properties of semiconductor elements constituting the LED. If a reverse voltage is applied, no current flows until the LED reaches its endurable voltage and then excessive current flows at any voltage. This voltage is called a breakdown voltage. The breakdown voltage is also determined by the properties of semiconductor elements and artificially adjustable impurity levels. In the conventional LED structure, a forward threshold voltage is mainly determined by the properties of semiconductor materials and the impurity concentrations of p-n junction materials. In case of a GalnP/AIGalnP-based element constituting a red LED, the threshold voltage of a composition corresponding to light of a wavelength of 630 nm is about 1.2 - 1.5 volts, and the operating voltage at a current of 20 mA is about 1.8 - 2.2 volts. Meanwhile, if a blue LED achieving a different wavelength has a semiconductor construction of AIGalnN/lnGaN/GaN, it is different from the red LED in a semiconductor composition, and the threshold voltage and operating voltage determined by the composition achieving a corresponding wavelength are about 2.0 - 2.7 volts and 2.8 - 4.0 volts, respectively. If the red LED is connected in parallel to a blue LED within a package so as to be used at a single voltage, only the red LED operates before the blue LED operates. Therefore, in obtaining three primary colors of light, it is difficult to drive red, green, blue LEDs assembled into one package by one external operating voltage. Usually, since the blue and green LEDs are similar to each other in a threshold voltage and an operating voltage, they can be connected in parallel to each other by combining respective brightness characteristics. However, since the red LED is different from the blue or green LED in the operating voltage, an additional voltage should be supplied. [Disclosure] [Technical Problem] Accordingly, the present invention relates to a red LED diode of a new structure which is capable of adjusting an operating voltage to adjust chromaticity and obtain various colors by connecting LEDs having different operating voltages in parallel to one another, and it is an object of the present invention to provide a red LED which is capable of being driven at an external single voltage by approximating a threshold voltage and an operating voltage of the red LED to those of a blue LED and a green LED and connecting the three diodes in parallel to one anther. It is another object of the present invention to provide an LED that is capable of being driven at a single voltage by constructing tri-color LEDs within one package without changing a driving circuit. [Technical Solution] According to an aspect of the present invention, a light emitting diode includes a substrate, a first plurality of semiconductor layers which are formed on the substrate and have an active layer for emitting red light, and a second plurality of semiconductor layers which are formed on the first plurality of semiconductor layers and form a diode for raising an operating voltage of the first plurality of semiconductor layers. Preferably, the first plurality of semiconductor layers includes an n-type AIGalnP clad layer 21 , a GalnP/AIGalnP quantum well active layer 22, a p-type AIGalnP clad layer 23 and a p-type semiconductor layer 24, which are sequentially formed on the substrate 20, the second plurality of semiconductor layers includes a p-type AIGalnP layer 25, an n-type AIGalnP layer 26 and an n-type contact layer 27, an ohmic contact electrode 29 is connected to the substrate 20, and an ohmic contact electrode 28 is connected to the n-type contact layer 27. Preferably, the diode formed by the second plurality of semiconductor layers is a Zener diode. Preferably, the diode formed by the second plurality of semiconductor layers has a reverse breakdown voltage of 3 volts or less. Preferably, the band gaps of the second plurality of semiconductor layers are larger than the band gap of the active layer so as not to absorb light generated from the active layer. Preferably, an operating voltage of the light emitting diode is 2.5 - 4.5 volts. According to another a spect of the present invention, a light emitting device includes a red light emitting diode (LED) having a first plurality of semiconductor layers including an active layer for emitting red light, and a second plurality of semiconductor layers which are grown on the first plurality of semiconductor layers and form a diode for raising an operating voltage of the first plurality of semiconductor layers; a blue LED connected in parallel to the red LED; and a green LED connected in parallel to the red LED and to the blue LED. Preferably, the red, blue and green LEDs are assembled into one package to produce white light. According to a further aspect of the present invention, a light emitting device includes a first light emitting diode (LED) having a first plurality of semiconductor layers including an active layer for emitting a first color, and a second plurality of semiconductor layers which are grown on the first plurality of semiconductor layers and form a diode for raising an operating voltage of the first plurality of semiconductor layers; a second LED connected in parallel to the first LED, for emitting a second color; and a third LED connected in parallel to the first LED and to the second LED, for emitting a third color, wherein the first, second and t hird L EDs a re a ssembled i nto one p ackage a nd e mit I ight combined by the first, second and third colors by one operating voltage. [Advantageous Effects] As describe above, since a red LED of a new structure according to the present invention can adjust an operating voltage determined in a conventional LED p-n junction, it is applicable to a device requiring a high operating voltage. For example, as illustrated in FIG. 6, the red LED is connected in parallel to blue and green LEDs each having a higher operating voltage than the red LED by about 1.0 volt. Then multiwavelength light can be emitted within one package by a single external voltage of an external circuit construction. Generally, cellular phones or electronic products achieve white light by coating fluorescent materials on the blue LED, but in these applications, circuit conditions use voltage conditions determined only by the properties of the blue LED. Therefore, if a red L ED is added to a n interior circuit of an application product in order to obtain a white LED, there is needed a circuit alteration suitable for the red L ED. T he present i nvention provides a n ew red L ED for constructing an LED having the same operating conditions to obtain three primary colors without changing an external circuit. This structure is effective to overcome problems of a white LED in which a blue LED is combined with a fluorescent material having short life time and low conversion efficiency.

[Description of Drawings] Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: FIG. 1 is a cross-sectional view of a conventional AIGalnP-based LED structure; FIG. 2 is a graph illustrating a current-voltage characteristic of a conventional LED; FIG. 3 is a cross-sectional view of an AIGalnP-based LED structure according to the present invention; FIG. 4 is a graph illustrating a current-voltage characteristic of a diode integrated onto an LED according to the present invention; FIG. 5 is a diagram illustrating a current-voltage characteristic of an element in which an additional diode is vertically integrated onto an LED according to the present invention; and FIG. 6 is a circuit diagram illustrating an embodiment in which an element according to the present invention is connected in parallel to LEDs having a high operating voltage so as to operate at the same external voltage.

[Mode for Invention] The present invention will now be described in detail in connection with preferred embodiments with reference to the accompanying drawings. For reference, like reference characters designate corresponding parts throughout several views. Referring to FIG. 3, an n-type AIGalnP clad layer 21 , GalnP/AIGalnP quantum well (QW) active layer 22, a p-type AIGalnP clad layer 23 and a p-type layer 24 are sequentially formed on a GaAs substrate 20, in the same way as a conventional method. In order to vertically integrate an additional diode, a p-type AIGalnP layer 25, an n-type AIGalnP layer 26 and an n-type contact layer 27 are sequentially formed on the p-type layer 24. The additionally formed layers 25, 26 and 27 should have a greater band gap than a band gap corresponding to a wavelength formed in the active layer so as not to absorb light generated from the active layer. As such semiconductor materials, there are AIGalnP, AIGaAs, GaAsP and GAP. The compositions of the materials can be adjusted under the conditions for adjusting a voltage and preventing the absorption of light. A Zener diode shown in FIG. 4 is preferable to determine the property of the additional diode. The Zener diode can be formed by adjusting the impurity concentrations of the n-type layer 26 and the p-type layer 25. A reverse breakdown voltage Vz of the Zener diode is determined by the semiconductor compositions and impurity concentrations of the p-type layer 25 and the n-type layer 26. If a forward voltage is applied to an upper electrode 28 on an element where the additional diode is integrated, a reverse voltage is applied to the additional diode. The additional diode is turned on at a voltage higher than the breakdown voltage Vz and its voltage is maintained at the breakdown voltage Vz. If a forward voltage is higher than the breakdown voltage Vz is applied, no current flows into the p-n junction of the LED until the applied voltage in the p-n junction of the LED reaches the threshold voltage, but current flows if the applied voltage exceeds the threshold voltage. Therefore, the LED operates normally. As illustrated in FIG. 5, an operating voltage of the entire element is recognized as a voltage of the operating voltage of the existing LED diode plus the reverse breakdown voltage of the additional diode. That is, the operating voltage of the existing diode is raised by the breakdown voltage Vz. If this new technique is applicable to a GalnP/AIGalnP-based red LED, the operating voltage is raised up to about 3.0 volts from 2.0 volts. The operating voltage can be raised by adjusting the semiconductor compositions and impurity concentrations of the additionally integrated diode. While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.

Claims

1. A light emitting diode compirising: a substrate; a first plurality of semiconductor layers which are formed on the substrate and have an active layer for emitting red light, and a second plurality of semiconductor layers which are formed on the first plurality of semiconductor layers and form a diode for raising an operating voltage of the first plurality of semiconductor layers.

2. The light emitting diodo of claim 1 , wherein the first plurality of semiconductor layers includes an n-type AIGalnP clad layer 21 , a GalnP/AIGalnP quantum well active layer 22, a p-type AIGalnP clad layer 23 and a p-type semiconductor layer 24, which are sequentially formed on the substrate 20, wherein the second plurality of semiconductor layers includes a p-type AIGalnP layer 25, an n-type AIGalnP layer 26 and an n-type contact layer 27, and wherein an ohmic contact electrode 29 is connected to the substrate 20 and an ohmic contact electrode 28 is connected to the n-type contact layer 27.

3. The light emitting diodo of claim 1 , wherein the diode formed by the second plurality of semiconductor layers is a Zener diode.

4. The light emitting diodo of claim 1 , wherein the diode formed by the second plurality of semiconductor layers has a reverse breakdown voltage of 3 volts or less.

5. The light emitting d iodo of claim 1, wherein the band gaps of the second plurality of semiconductor layers are larger than the band gap of the active layer so as not to absorb light generated from the active layer.

6. The light emitting diodo of claim 1 , wherein an operating voltage of the light emitting diode is 2.5 - 4.5 volts.

7. A light emitting device comprising: a red light emitting diode (LED) having a first plurality of semiconductor layers including an active layer for emitting red light, and a second plurality of semiconductor I ayers which a re g rown o n the first p lurality of semiconductor layers and form a diode for raising an operating voltage of the first plurality of semiconductor layers; a blue LED connected in parallel to the red LED; and a green LED connected in parallel to the red LED and to the blue LED.

8. The light emitting device of claim 7, wherein the red, blue and green LEDs are assembled into one package to produce white light.

9. A light emitting device comprising: a first light emitting diode (LED) having a first plurality of semiconductor layers including an active layer for emitting a first color, and a second plurality of semiconductor layers which are grown on the first plurality of semiconductor layers and form a diode for raising an operating voltage of the first plurality of semiconductor layers; a second LED connected in parallel to the first LED, for emitting a second color; and a t hird L ED c onnected in p arallel to t he f irst L ED a nd t o the second LED, for emitting a third color; wherein the first, second and third LEDs are assembled into one package and emit light combined by the first, second and third colors by one operating voltage.