A TPX system consists of a light-emitting diode (LED) (though other types of emitters are conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic control circuit. The LED is heated to a temperature higher than the PV temperature by an external heat source. If no power is applied to the LED, the system functions much like a very inefficient TPV system, but if a forward bias is applied at some fraction of the bandgap potential, an increased number of electron-hole pairs (EHPs) will be thermally excited to the bandgap energy. These EHPs can then recombine radiatively so that the LED emits light at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then delivered to the cooler PV cell over the optical coupling and converted to electricity.