Minority-carrier injection into semi-insulators

Abstract

A previous paper was concerned with the injection of minority carriers into relaxation semiconductors without traps. The present paper considers traps in their dual role as recombination centers and charge-storage localities. It is shown that the presence of traps changes the boundary between majority-carrier depletion ($\Delta n < 0$) and the majority-carrier augmentation ($\Delta n > 0$), moving the system more towards the latter, compared with expectations based on the trap-free case. Trapping of injected minority carriers also leads to an increase of majority-carrier diffusion in opposition to the current, and the containment of this diffusion calls for higher-than-normal electric fields. As a consequence, the injection region can have a higher-than-normal resistance, qualitatively as predicted by Van Roosbroeck, but for quite different reasons. Computed contours show the corresponding charge concentration and field profiles. The resistance enhancement effect is expected to be most prominent in lifetime semiconductors of high trap density; conditions for its appearance (if any) in relaxation semiconductors are discussed. Questions relating to the definition of relaxation semiconductors are also discussed.