Trap-enhanced electric fields in semi-insulators: The role of electrical and optical carrier injection

We report extremely large field enhancement near the anode of an electrically biased metal/semi-insulator/metal structure. The large anode field results from a trap-enhanced space-charge region and is large enough to cause injection of holes at the anode. Our numerical simulations confirm this interpretation and show that for typical semi-insulating GaAs, large trap-enhanced fields (TEF) are to be expected. The TEF effect, contrary to that observed in doped materials, is enhanced by optical injection of carriers near the anode, and can be exploited for the efficient generation of ultrafast THz radiation.