Negative luminescence is a physical occurrence by which a semiconductor device gives out lesser thermal radiation when an electric current passes through it, than it does during thermal equilibrium. Operating negative luminescent devices – such as those manufactured by using germanium, indium antimonide (InSb) and mercury cadmium telluride (HgCdTe) – when studied by thermal cameras, look colder than their environments.
Why it happens
Semiconductor materials exhibit negative luminescence very easily. When the temperature of a semiconductor material is increased, it is known that the energy possessed by the ‘valence band’ electrons in a semiconductor atom is increased. Valence band electrons may subsequently jump into the ‘conduction band’ after crossing the forbidden energy gap. This results in the so-called ‘electron-hole’ pairs to be formed when an electric field is applied to drive the electrons. An electronic current thus begins to pass. In this way, electrons and holes are prevented from getting a chance to combine again and return the energy they receive, whether in the form of infrared rays or photons of light. Incoming energy, as mentioned above, as a corresponding rise in temperature, can be in the form of infrared radiation or light waves. This explains the phenomenon of negative luminescence. The phenomenon of negative luminescence cannot be considered as an exceptional case to Kirchoff’s Law of Thermal Radiation. According to the law, energy emission and absorption in heated objects has to be the same provided the condition of thermal equilibrium is met. However, as thermal equilibrium is not present when an electric current is passed through a semiconductor as described above, the phenomenon of negative luminescence appears to be a violation of Kirchoff’s Law.
The concept of negative luminescence is not entirely new. It was in the 1960s that this effect was first seen by Russian scientists. The phenomenon of negative luminescence in semiconductors was studied by researchers in countries like West Germany, Ukraine and the United States. It was observed in both medium (3 - 5 μm) and low (8 - 12 μm) wavelength infrared sections of the electromagnetic spectrum, using indium antimonide (InSb) in heterojunction diodes. Research continues to be conducted in university departments across the globe.