1. Field of the Invention
The present invention relates to a semiconductor light emitting diode, and more particularly to an AlGaInP light emitting diode with improved illumination.
2. Description of the Prior Art
The light emitting diode (LED) has been studied and developed for over forty years from DC 1960. From a conventional light emitting diode to a current high brightness light emitting diode, the light emitting diodes have been broadly used, such as traffic signals, car indicator lamps, colorful outside screens and even application in illumination in the future.
Although the internal quantum efficiency of the light emitting diode can be improved to 90 percent and even more by the current epitaxial technique, the external quantum efficiency of the high brightness light emitting diode has an efficiency of only 10 percent or lower. Thus, efficiency is a very important issue. Improvements in the light generated within the light emitting diode and the brightness thereof by means of various structures and the methods for fabrication is a highly sought after result.
Referring to FIG. 1, which is a cross-sectional view of a prior (AlxGa1-x)0.5In0.5P light emitting diode 10. The (AlxGa1-x)0.5In0.5P light emitting diode 10 comprises an N type GaAs substrate 101 with a thickness about 200 μm, an N type distributed bragg reflector (DBR) 102, an (AlxGa1-x)0.5In0.5P light-emitting layer 103, a P type GaP window layer 104, a P type top electrode 105 and an N type bottom electrode 106.
The N type distributed bragg reflector (DBR) 102 can be an N type AlAs/AlxGa1-xAs based DBR, an N type AlAs/In0.5(Ga1-xAlx)0.5P based DBR, and an N type In0.5(Ga1-xAlx)0.5P based DBR. The purpose of the N type distributed bragg reflector 102 is to reflect the emitting light from the (AlxGa1-x)0.5In0.5P light-emitting layer 103.
The (AlxGa1-x)0.5In0.5P light-emitting layer 103 includes an N type (AlxGa1-x)0.5In0.5P lower cladding layer, an undoped (AlxGa1-x)0.5In0.5P active layer and a P type (AlxGa1-x)0.5In0.5P upper cladding layer. The purpose of the N type (AlxGa1-x)0.5In0.5P lower cladding layer and the P type (AlxGa1-x)0.5In0.5P upper cladding layer is for carrier injection into the (AlxGa1-x)0.5In0.5P light-emitting layer 103 and carrier confinement in the (AlxGa1-x)0.5In0.5P light-emitting layer 103. The thickness of both of the N type (AlxGa1-x)0.5In0.5P lower cladding layer and the P type (AlxGa1-x)0.5In0.5P upper cladding layer should be thicker than the diffusion length of the injection carriers to prevent the carrier diffusion from the (AlxGa1-x)0.5In0.5P light-emitting layer 103 into the cladding layers.
The P type GaP window layer 104 is used to more efficiently spread the current and facilitating the light emission from the (AlxGa1-x)0.5In0.5P light-emitting layer 103 so as to improve light extraction of the light emitting diode.
The P type top electrode 105 is formed on the central portion of the P type GaP window layer 104. The front shape of the P type top electrode 105 is generally rounded or formed of other shapes. The N type bottom electrode 106 is formed on a bottom of the N type GaAs substrate 101.
The (AlxGa1-x)0.5In0.5P light emitting diode 10 is a PN junction with a forward bias to inject holes from the P type (AlxGa1-x)0.5In0.5P upper cladding layer and electrons from the N type (AlxGa1-x)0.5In0.5P lower cladding layer into the undoped (AlxGa1-x)0.5In0.5P active layer. The undoped (AlxGa1-x)0.5In0.5P active layer emits visible light due to the recombination of the electrons and holes in this active layer.
FIG. 2 is a schematic diagram of current distribution within the (AlxGa1-x)0.5In0.5P light emitting diode 10 of FIG. 1. The P type top electrode 105 is a rounded electrode formed on the central portion of the P type GaP window layer 104. The current density under the P type top electrode 105 is highest. However, a part of light generated under the P type top electrode 105 is absorbed. Another part of the light generated under the P type top electrode 105 is reflected back into the light emitting diode chip. And, most of the light reflected back into the light emitting diode chip is absorbed. Therefore, the structure of the prior (AlxGa1-x)0.5In0.5P light emitting diode 10 adversely influence brightness performance of the light emitting diode.
Accordingly, it is an intention to develop an improved structure of a light emitting diode, which can prevent current passing through the region under a top electrode so as to resolve the drawback of the prior light emitting diode.