Absorption of Infrared Light by Free Carriers in Germanium

The absorption of infrared light associated with the presence of free carriers in germanium has been measured by injecting these carriers across a $p-n$ junction at room temperature. The absorption is found to be proportional to the concentration of carriers. The absorption as a function of wavelength shows the same rather sharp maxima previously observed in normal $p$-type germanium. These bands are found to change with temperature. An explanation of this absorption is offered in terms of a degenerate energy band scheme.