  We develop a broadly applicable transport-based technique, GAte Modulated
activation Energy Spectroscopy (GAMEaS), for determining the density of states
(DOS) in the energy gap. GAMEaS is applied to field effect transistors made
from different single crystal oligomer semiconductors to extract the
free-carrier mobility, u_0, from the field effect mobility, u_eff. Samples with
a lower DOS exhibit higher u_eff. Values of u_0 up to 100 +/- 40 cm2/Vs at 300K
are observed, showing that performance can be greatly enhanced by improving
sample purity and crystal quality.
