Analysis of photon capturing in light emitting diode with different surfaces

The external efficiency of today´s LED is still very small, typically less than 10%, although the internal efficiency can be very high (/spl sim/100%). This is because when light have been generated in the active layer inside the diode, they must be liberated to the outside in order to be of any use. Because the refractive index of semiconductor material is usually quite high (/spl sim/3.5), more than three times as air, therefore the critical angle /spl theta//sub c/ given by Snell´s law (from semiconductor to air) is very small, typically /spl sim/17/spl deg/. Recently, LED´s having external quantum efficiency as high as 30% has been reported; this was achieved by adding many interesting features including fabricating a coarse texturing on the semiconductor surface with a back reflector, where the escape probability of photons can be increased substantially. However, the mechanism of achieving such a high efficiency due to the textured surface is not yet fully understood. Two programs have been coded to simulate the external efficiency of semiconductor LED-a 3-dimensional LED program without back reflector and a 2-dimensional program with back reflector.