PRS-LEDs have been demonstrated using a GaInN/GaN LED emitting in the blue and an electrically passive AlGaInP photon-recycling semiconductor emitting in the red part of the spectrum (Guo et al., 1999). The emission spectrum of the device, depicted in Fig. 21.17, shows the emission line of the primary LED at 470 nm and a second emission line at 630 nm due to absorption of the 470 nm light in the AlGaInP layer and re-emission of light at 630 nm. The recycling semiconductor used in this experiment is an AlGaInP/GaAs double heterostructure. The photon-recycling semiconductor is planar and no surface texturing was performed.

To avoid absorption of light in the GaAs substrate, the GaAs substrate of the AlGaInP

epitaxial layer was removed. Firstly, the AlGaInP/GaAs recycling semiconductor was mounted on a glass slide. Subsequently, the GaAs substrate was removed by polishing and selective wet chemical etching. Then the primary LED wafer and the photon-recycling wafer were bonded together.

The approximate theoretical luminous efficiency of several types of white LED lamps is given in Table 21.2. The data show that the dichromatic light source has the highest luminous efficiency as compared to spectrally broader emitters.

Generally, dichromatic white LEDs have a higher luminous efficacy but lower color - rendering index (CRI) compared with trichromatic white LEDs. It can be shown that there is a fundamental trade-off between color rendering and the luminous efficacy of light-emitting devices (Walter, 1971). In order to improve the general CRI of dichromatic devices such as the PRS-LED, two possibilities can be considered. Firstly, the emission lines can be intentionally broadened, e. g. by compositional grading. Secondly, a second photon-recycling semiconductor can be added thus creating a trichromatic PRS-LED. However, any broadening of the two emission lines or the addition of an emission line will decrease the luminous efficacy and luminous efficiency of the device.