A phonon is required in the process of absorption or emission in the case of an indirect bandgap semiconductor , so indirect bandgap semiconductors are usually very inefficient emitters .
Emitters work reasonably well as absorbers also .
A phonon is required in the process of absorption or emission in the case of an indirect bandgap semiconductor , so indirect bandgap semiconductors are usually very inefficient emitters , although they work reasonably well as absorbers also .