The Mechanisms of Degradation in an Optically Coupled Isolator

Degradation was observed in optically coupled isolators that were operated under both photodiode and phototransistor biasing configurations. The device "on" state current decreased during operation due to degradation of the light emitting diode radiant power output. This decrease in radiant power output was associated with an increase in the nonradiative, space charge recombination current component of the light emitting diode forward current. In addition, large and rapid increases were observed in the optically coupled isolator "off" state currents. This degradation was due to formation of a surface inversion channel in the base region of the photosensitive transistor.