  Understanding charge transport in organic semiconductors in large electric
fields is relevant to many applications. We present transport measurements in
organic field-effect transistors based on poly(3-hexylthiophene) and
6,13-bis(triisopropyl-silylethynyl) pentacene with short channels, from room
temperature down to 4.2 K. Near 300 K transport in both systems is well
described by thermally assisted hopping with Poole-Frenkel-like enhancement of
the mobility. At low temperatures and large gate voltages, transport in both
materials becomes nearly temperature independent, crossing over into
field-driven tunneling. These data, particularly in TIPS-pentacene, show that
great caution must be exercised when considering more exotic (e.g.,
Tomonaga-Luttinger Liquid) interpretations of transport.
