The present invention relates to a gallium nitride (GaN)-based light-emitting device and, particularly, to a GaN-based light-emitting device having an electrode with good reflectivity of light.
GaN-based light-emitting devices are getting more attention for the reason that they can emit special frequencies of light, such as blue light and green light. The material of substrate used for GaN-based light emitting devices is limited by the property of GaN and is mainly chosen from the following material: sapphire, silicon carbide (SiC), gallium nitride (GaN), gallium phosphide (GaP) and glass. Among the above material, sapphire substrate is widely used. Since sapphire substrate is an insulator, the two electrodes of the light-emitting device must be placed on the same side of the GaN layers. It is the characteristic of so-called lateral devices. U.S. Pat. No. 5,563,422, U.S. Pat. No. 5,578,839 and U.S. Pat. No. 5,583,879 have disclosed a series of methods for manufacturing GaN-based III-V Group compound semiconductor lateral light-emitting devices using sapphire substrate. The light-emitting device manufactured therefrom is shown in FIG. 1, wherein an light-emitting diode (LED) die 10 is fixed to a lead frame 12 first, and the two electrodes 13 and 14 on the upper surface of the die 10 must be soldered with gold (or aluminum) wires 15 and 16 respectively so as to be connected to the two electrode of the lead frame. However, the soldering pad 17 on the upper surface of the die 10 will block the light, and thus the light-emitting area is reduced and the light emitted is not uniform. In addition, in the prior light-emitting device, a light-transmitting electrode 13 is disclosed for providing the effect of current spreading to enhance the light-emitting efficiency of the device. However, since the electrode 13 must be very thin to become light-transmitting, its lateral resistance will be great and thus its effect of current spreading will be very limited. Besides, since the electrode 13 is placed over the main light-emitting surface, even the electrode 13 is light-transmissive, it still will reduce the light-emitting efficiency of the device.
U.S. Pat. No. 4,476,620 discloses a flip-chip GaN-based light-emitting device as shown in FIG. 2, wherein the two electrodes 21 and 22 of an LED die are directly stuck on certain places of a lead frame 23. In such a flip-chip GaN-based light-emitting device, the emitted light can directly pass through the transparent substrate (such as a sapphire substrate) to the outside. There is no soldering pad in the main light-emitting surface of the flip-chip light-emitting device, and thus the light-emitting surface as well as the light-emitting efficiency will not be affected by the soldering pad. However, only about half the light emitted from the prior flip-chip GaN-based light-emitting device can directly pass through the transparent substrate to the outside. The other half of the light is directed to the electrodes (21 and 22) and the lead frame 23. Therefore, the prior device cannot achieve an effective result. Furthermore, the prior flip-chip GaN-based light-emitting device disclosed in U.S. Pat. No. 4,476,620 does not provide the current spreading effect, and thus merely the place where the electrode is located can have an effective light-emitting result while the other place cannot achieve a good light-emitting result.
In view of the above, how to substantially improve the light-emitting efficiency of a GaN-based light-emitting device is still a problem for the industry.
The main object of the present invention is to substantially improve the light-emitting efficiency of a GaN-based light-emitting device. The light-emitting device according to the present invention is a flip-chip light-emitting device. Therefore, there is no soldering pad in its main light-emitting surface. Additionally, the light-emitting device according to the present invention has an electrode that has good reflectivity of light and can provide effective current spreading effect Therefore, the light directed to the electrode can be reflected by the electrode to the outside through the transparent substrate, and thus substantially increase the light-emitting efficiency of the whole device. Further, the electrode can have sufficient size and thickness to provide effective current spreading effect, such that the light-emitting diode can achieve its best light-emitting result.
A flip-chip light-emitting device according to a first embodiment of the present invention comprises a transparent substrate, a semiconductor stacked structure arranged over a main surface of the transparent substrate wherein the stacked structure comprises an n-type GaN-based III-V Group compound semiconductor layer adjacent to said main surface and a p-type GaN-based III-V Group compound semiconductor layer adjacent to the n-type semiconductor layer, a first electrode being in electrical contact with the n-type semiconductor layer, and a second electrode being in electrical contact with the p-type semiconductor layer, wherein the second electrode has good reflectivity of light and covers most of the outer surface of the p-type semiconductor layer. In an alternative embodiment of the present invention, the positions of the n-type and p-type semiconductor layers are switched, and the electrode being in contact with the n-type semiconductor layer has good reflectivity of light and covers most outer surface of the n-type semiconductor layer.