From the experimental results

it is clear that luminescence intensity of the wafer reaches the maximum

when the thickness of p-type layer (

d

) in a GaPⅩⅣN light emitting diode is in a range of 12～17μm. Increasing

d

the effect of the surface recombination might be compressed

making the internal quantum efficiency to be increased. At the same time

it might increase the internal absorption

resulting in a decrease of the extraction efficiency. Combining the two factors

and using the resonable values of the relevant parameters

a rational explanation for the optimum value of

d

measured experimentally is obtained.