  We report on a giant persistent photoconductivity (PPC) induced
semiconductor-to-conductor like transition in zinc-tin-oxide (ZTO)
photo-thinfilm transistors (TFT). The active ZTO channel layer was prepared by
remote-plasma reactive sputtering and possesses an amorphous structure. Under
subbandgap excitation of ZTO with UV light, the photocurrent reaches as high as
~10 -4 A (a photo-to-dark current ratio of ~10 7) and remains close to this
high value after switching off the light. During this time, the ZTO TFT
exhibits gigantic PPC with long-lasting recovery time, which leads the ZTO
compound to undergo a semiconductor-to-conductor like transition. In the
present case, the conductivity changes over six orders of magnitude, from ~10-7
to 0.92 {\Omega} -1cm-1. After UV exposure, the ZTO compound can potentially
remain in the conducting state for up to a month. The underlying physics of the
observed PPC effect is investigated by studying defects (deep-states and
tail-states) by employing a discharge current analysis (DCA) technique.
Findings from the DCA study reveal direct evidence for the involvement of
sub-gap tail-states of the ZTO in the giant PPC, while deep-states contribute
to mild PPC.
