This application is based upon and claims the benefit of priorities from the prior Japanese Application No. 2006-296620, file on Oct. 31, 2006, and the prior Japanese Application No. 2007-274192, file on Oct. 22, 2007; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a semiconductor light emitting device.
2. Background Art
The semiconductor light emitting device, which emits mixed color of blue light from a nitride semiconductor light emitting element and yellow light obtained through wavelength conversion by phosphors, is finding wide application in displays, illuminations, and display backlights.
The semiconductor light emitting element includes a quantum well structure made of compound semiconductor thin films. It is not easy to control the composition and thickness of a heterojunction of compound semiconductor thin films having a thickness of approximately several nm. Hence the emission wavelength of a semiconductor light emitting element has a certain distribution.
On the other hand, the excitation spectrum of phosphors depends on wavelength. Consequently, if the emission wavelength of a semiconductor light emitting element varies several nm, the excitation intensity of phosphors also varies accordingly, changing the chromaticity of the mixed color.
JP-A 2005-252250 (kokai) discloses a light emitting device using an LED (light emitting diode) or LD (laser diode) with a peak wavelength of 380 to 410 nm as an excitation light source. In this device, despite some shift in its emission wavelength, the emission intensity of the red phosphor is not affected, keeping not only the brightness, but also the balance of mixing with blue and green phosphor.
According to an aspect of the invention, there is provided a semiconductor light emitting device including: a semiconductor light emitting element; a first phosphor which absorbs light emitted from the semiconductor light emitting element and emits first wavelength-converted light; and a second phosphor which absorbs light emitted from the semiconductor light emitting element and emits second wavelength-converted light, the first phosphor having a first excitation spectrum region where excitation intensity increases with increasing wavelength around a peak wavelength of the semiconductor light emitting element, and the second phosphor having a second excitation spectrum region where excitation intensity is flat or decreases with respect to increasing wavelength around the peak wavelength of the semiconductor light emitting element.
According to another aspect of the invention, there is provided a semiconductor light emitting device including: a semiconductor light emitting element; a first phosphor which absorbs light emitted from the semiconductor light emitting element and emits first wavelength-converted light; and a second phosphor which absorbs light emitted from the semiconductor light emitting element and emits second wavelength-converted light, the first phosphor having a first excitation spectrum region where excitation intensity increases with increasing wavelength around a peak wavelength of the semiconductor light emitting element, the second phosphor having a second excitation spectrum region where a excitation intensity is flat or decreases with respect to increasing wavelength around the peak wavelength of the semiconductor light emitting element, excitation intensity resulting from mixing the first phosphor and the second phosphor having a third excitation spectrum region which is flat or increases with respect to increasing wavelength, and an upper limit of the peak wavelength falling within the third excitation spectrum region.
According to another aspect of the invention, there is provided a semiconductor light emitting device including: a semiconductor light emitting element; a first phosphor which absorbs light emitted from the semiconductor light emitting element and emits first wavelength-converted light; a second phosphor which absorbs light emitted from the semiconductor light emitting element and emits second wavelength-converted light; a third phosphor which absorbs light emitted from the semiconductor light emitting element and emits third wavelength-converted light; and a fourth phosphor which absorbs light emitted from the semiconductor light emitting element and emits forth wavelength-converted light, the first phosphor having a first excitation spectrum region where excitation intensity increases with increasing wavelength around a peak wavelength of the semiconductor light emitting element, the second phosphor having a second excitation spectrum region where excitation intensity is flat or decreases with respect to increasing wavelength around the peak wavelength of the semiconductor light emitting element, the third phosphor having a fourth excitation spectrum region where excitation intensity increases with increasing wavelength around a peak wavelength of the semiconductor light emitting element, the fourth phosphor having a fifth excitation spectrum region where excitation intensity is flat or decreases with respect to increasing wavelength around the peak wavelength of the semiconductor light emitting element, the first excitation spectrum being deferent from the fourth excitation spectrum, and the second excitation spectrum being different from the fifth excitation spectrum.
An embodiment of the invention will now be described with reference to the drawings.
An electrode provided on the upper surface of the semiconductor light emitting element 10 is connected to a second lead 44 by a bonding wire 25. The first lead 40 and the second lead 44, made of metal, are buried with a thermoplastic resin 42, for example.
In the upper portion of the thermoplastic resin 42, a second recess 50 is provided so as to continue to the first recess 19. Inside the thermoplastic resin 42, a sloped reflector 46 is provided. The reflector 46 and the inner sidewall 20 of the first recess 19 reflect upward the emitted light from the semiconductor light emitting element 10 and the light wavelength-converted by SOSE phosphor 21 and YAG phosphor 22. A mixed color can be obtained by the semiconductor light emitting element 10 and the wavelength-converted light.
A sealing resin 23 such as silicone resin mixed with SOSE and YAG phosphor 21, 22 is provided in the first recess 19 of the first lead 40 and above the semiconductor light emitting element 10. It is assumed that the semiconductor light emitting element 10 is made of a nitride material having a peak wavelength of 440 to 490 nm.
The semiconductor light emitting element 10 includes a quantum well structure made of compound semiconductor thin films. It is not easy to control the composition and thickness of a heterojunction of compound semiconductor thin films having a thickness of approximately several nm. Around the peak wavelength, the wavelength may vary ±5 nm or more. Furthermore, the peak wavelength of the semiconductor light emitting element 10 varies to the long-wavelength side due to temperature increase.
On the other hand, the excitation intensity of the SOSE phosphor 21 emitting yellow light decreases with increasing wavelength. Thus, if the peak wavelength of the semiconductor light emitting element 10 varies to the long-wavelength side, and the associated decrease in excitation intensity of SOSE phosphor 21 is added thereto, then the chromaticity variation of the semiconductor light emitting device increases. In this embodiment, SOSE and YAG phosphor 21, 22 are mixedly disposed. Hence the chromaticity variation can be reduced even if the peak wavelength of the semiconductor light emitting element 10 varies to the long-wavelength side.
Next, the operation for mixed phosphors is described in detail.
As shown in
The wavelength at which the emission intensity of the semiconductor light emitting element 10 is maximized is called the peak wavelength. On the other hand, the wavelength corresponding to a single wavelength perceived by a human eye is called the dominant wavelength, which may be different from the peak wavelength.
The emitted light from the semiconductor light emitting element 10 is incident on the phosphor. Part of the incident light is absorbed by the phosphor, and the rest is reflected. Because of the wavelength dependence of the absorption spectrum, the spectrum of the reflected light is different from the original spectrum of the semiconductor light emitting element 10. The dominant wavelength of the mixed light of this reflected light and the emitted light of the semiconductor light emitting element 10 not incident on the phosphor varies depending on the excitation and absorption spectrum shape of the phosphor. For example, if the excitation spectrum of the phosphor is similar in shape to its absorption spectrum, the dominant wavelength of the mixed light varies to the short-wavelength side relative to the peak wavelength in the region where the spectrum increases with wavelength, and varies to the long-wavelength side relative to the peak wavelength in the region where the spectrum decreases with increasing wavelength.
Here, the composition formula of SOSE is illustratively expressed by (Me1-yEuy)2SiO4: Eu2+ (where Me is at least one alkaline-earth metal element selected from Ba, Sr, Ca, and Mg, and 0<y≦1).
On the other hand, as shown in
The semiconductor light emitting device according to this embodiment having an excitation spectrum illustrated in
The semiconductor light emitting element is assumed to be a blue LED (light emitting diode) illustratively made of an InGaAlN-based material and having a peak wavelength in the range of 440 to 490 nm. The peak wavelength determined within this range in accordance with the specification varies, illustratively in the range of ±10 nm around the peak wavelength, due to temperature variation and manufacturing parameter dispersion. Part of the emitted light from the semiconductor light emitting element 10 is absorbed by phosphors 21, 22 used in this embodiment and is wavelength-converted into yellow light by excitation. Because blue light is absorbed by phosphors 21, 22, the emission intensity of the semiconductor light emitting device around the peak wavelength is as shown by the dashed line B, lower than the emission intensity A of the semiconductor light emitting element 10 shown by the dotted line.
The dashed line C in
Next, the operation of this embodiment is described with reference to the chromaticity diagram.
In
Here, if the peak wavelength of the semiconductor light emitting element 10 is varied to the long-wavelength side, the blue light varies to the long-wavelength side along the spectral trace, and the point WH of the mixed color accordingly varies in the direction of arrow V1 in
On the other hand, as shown in
In
More specifically, potential variation in the direction V1 of the peak wavelength of the semiconductor light emitting element 10 is compensated for by the reverse variation V4, and the variation of the point WH is reduced. In this case, in the increasing excitation spectrum region R, the intensity of emission from the yellow phosphor increases, and the emission intensity in the yellow wavelength band is higher on D than on B and C, causing variation in the direction V5. However, this effect is as small as the difference between the solid line D and the dashed line C in
Next, the wavelength variation due to the temperature variation of the semiconductor light emitting element 10 is described. Variation in the peak wavelength of the semiconductor light emitting element 10 is attributed to the temperature variation of the device as well as to the variation of device structure parameters.
The dashed line C in
In general, the emission intensity of phosphor decreases as the temperature increases.
The peak wavelength of the semiconductor light emitting element 10 varies to the long-wavelength side due to temperature increase. For example, in the operating range, it may vary approximately 5 nm to the long-wavelength side compared to at room temperature. Because the excitation intensity of yellow phosphor decreases due to temperature increase as shown in
The solid line D in
Furthermore, by the excitation spectrum of
The foregoing describes the case where absorption and excitation in the phosphor are similar in spectral shape. However, the wavelength dependence of the excitation spectrum can be different from that of the absorption spectrum. More specifically, the intensity of emission from phosphor depends on the quantum efficiency. Hence, in the YAG phosphor, for example, the absorption spectrum can include a region increasing with wavelength, whereas the excitation spectrum can be made flat by increasing the composition ratio of Ce activator to decrease the quantum efficiency. Consequently, in the partially enlarged chromaticity diagram of
Next, the case of using oxynitride phosphor is described. The SOSE phosphor can be replaced by oxynitride phosphor. The oxynitride phosphor is illustratively expressed by the composition formula MeSi2O2N2:Eu (where Me includes at least one selected from Ca, Sr, and Ba) or Me2Si5N8:Eu (where Me includes at least one selected from Ca, Sr, and Ba). By mixing such oxynitride phosphor with YAG, a semiconductor light emitting device with reduced chromaticity variation can be obtained.
In this modified example, the yellow-green and orange phosphors are mixed. As shown in
The yellow-green phosphor can be obtained by appropriately selecting the mixing ratio between SOSE and YAG phosphor, and each constitution. The orange phosphor can be obtained by appropriately selecting the mixing ratio between SOSE and YAG phosphor, and each constitution. The emission spectrum resulting from the yellow-green phosphor and the orange phosphor differs from that illustrated in
In this modified example, the yellow-green and orange phosphor have a YAG excitation spectrum region where excitation intensity increases with increasing wavelength around a peak wavelength of the semiconductor light emitting element, and a SOSE excitation spectrum region where excitation intensity is flat or decreases with respect to increasing wavelength of the semiconductor emitting element, respectively. Hence the chromaticity variation can be reduced with respect to the peak wavelength fluctuation of the semiconductor light emitting element by the same mechanism as explained with reference to
In this embodiment and its modification example, the mixed excitation spectrum may include fluctuations existing within a narrower region than FWHM (Full Width Half Maximum) of the semiconductor light emitting element in the flat excitation region Q or in the increasing excitation region R. In such a case, FWHM is approximately 20 nm, for example.
In the above examples, the variation range of the center wavelength of the semiconductor light emitting element is 440 to 490 nm, and the phosphor is yellow phosphor, yellow-green, and orange. However, the invention is not limited thereto, but encompasses semiconductor light emitting elements and phosphors with other emission wavelengths.
The embodiment of the invention has been described with reference to the drawings. However, the invention is not limited to these examples. For instance, the semiconductor light emitting element and phosphors constituting the semiconductor light emitting device can be modified by those skilled in the art with regard to their material, shape, and emission characteristics, and any such modifications are encompassed within the scope of the invention as long as they do not depart from the spirit of the invention.
Number | Date | Country | Kind |
---|---|---|---|
2006-296620 | Oct 2006 | JP | national |
2007-274192 | Oct 2007 | JP | national |
Number | Name | Date | Kind |
---|---|---|---|
6621211 | Srivastava et al. | Sep 2003 | B1 |
6939481 | Srivastava et al. | Sep 2005 | B2 |
7138965 | Shiiki et al. | Nov 2006 | B2 |
7221083 | Oaku et al. | May 2007 | B2 |
7573072 | Setlur et al. | Aug 2009 | B2 |
20030222268 | Yocom et al. | Dec 2003 | A1 |
20040135504 | Tamaki et al. | Jul 2004 | A1 |
20050194604 | Sakuma et al. | Sep 2005 | A1 |
20050199897 | Setlur et al. | Sep 2005 | A1 |
20050253500 | Gotoh et al. | Nov 2005 | A1 |
20060022208 | Kim et al. | Feb 2006 | A1 |
20060076883 | Himaki et al. | Apr 2006 | A1 |
20070029565 | Masuda et al. | Feb 2007 | A1 |
20070075620 | Hildenbrand | Apr 2007 | A1 |
20070132366 | Yabe et al. | Jun 2007 | A1 |
20070241666 | Jang et al. | Oct 2007 | A1 |
20080017831 | Tamatani et al. | Jan 2008 | A1 |
20080035943 | Slutsky et al. | Feb 2008 | A1 |
20080036364 | Li et al. | Feb 2008 | A1 |
Number | Date | Country |
---|---|---|
2005-252250 | Sep 2005 | JP |
WO 2006098450 | Sep 2006 | WO |
WO 2006106883 | Oct 2006 | WO |
Number | Date | Country | |
---|---|---|---|
20080129190 A1 | Jun 2008 | US |