A modeling of the electrical injection and the optical output of a typical high brightness LED indicates an important influence by top layer thickness.Not only the high reflective index of the top layer which limits the inner emitting angle of less than 16.6°

but also light area where the light emitted directly under the electrode will be total reflected back or be absorbed .As increasing the thickness at the top layer

the current spreads via diffusion and electrical density distributed uniformly on the whole emitting area

where most of the emitting area has been outside of the shade of the electrode.The thicker the top layer is

the easier for current spread and light to emit out.A method of high brightness light emitting diodes precisely design was provided in this paper.A current density function

which is symmetrical because the electrode is round

was deduced with perfect p n model.From the calculated curve of current density distribution

the one of 50μm top layer is more uniform than that of 5μm one.On other words

as increasing the thickness at the top layer

more current was spread outside of the shade of the electrode.LED optical output can be divided into edge emitting and surface emitting.A thick top layer is very useful for increasing LED edge emitting and decreasing no light area in surface emitting where was shaded by electrode.The calcu lated result shows that LED with a top layer between 49 and 98μm will make maximal optical output.The processes of electrical injection and optical out as basis

external quantum efficiency of LED with any thickness of top layer can be calculated.The theoretical calculated curve indicates external quantum efficiency vs.the thickness of LED top layer rise the slope sharply before 20μm

and then become a slight slope.If the top layer is thick enough

current in active layer distribution uniform completely

maximal optical output

and internal quantum efficiency being 1

the maximal external quantum efficiency of this type of LED is about 12.1%.