  We conduct comparative research on the density of states of electron- and
hole- carrier trap levels (DTR(E)), dispersing inside the energy gap of a
rubrene single crystal in a field effect transistor (FET) struction with Ca and
Au hetero-electrodes for an ambipolar carrier injection mode, by using
polymeric protection-layer materials on a Si substrate. Three different types
of polymeric materials, poly(methyl-methacrylate) (PMMA), poly(styrene) (PS)
and poly(chloro-styrene) (PCS) are employed. From the temperature (T)-dependent
source-drain current and gate voltage (ISD-VG) transfer characteristics, the
values of DTR(E) are evaluated. PS exhibits the most efficiently-balanced
ambipolar carrier transport, which is superior to PMMA that is most typically
used as the standard protection layer on a SiO$_2$/doped-Si substrate.
Discussions are made in the framework of a carrier multiple trap and release
(CMTR) model.
