  We demonstrate controllable shift of the threshold voltage and the turn-on
voltage in pentacene thin film transistors and rubrene single crystal field
effect transistors (FET) by the use of nine organosilanes with different
functional groups. Prior to depositing the organic semiconductors, the
organosilanes were applied to the SiO2 gate insulator from solution and form a
self assembled monolayer (SAM). The observed shift of the transfer
characteristics range from -2 to 50 V and can be related to the surface
potential of the layer next to the transistor channel. Concomitantly the mobile
charge carrier concentration at zero gate bias reaches up to 4*10^12/cm^2. In
the single crystal FETs the measured transfer characteristics are also shifted,
while essentially maintaining the high quality of the subthreshold swing. The
shift of the transfer characteristics is governed by the built-in electric
field of the SAM and can be explained using a simple energy level diagram. In
the thin film devices, the subthreshold region is broadened, indicating that
the SAM creates additional trap states, whose density is estimated to be of
order 1*10^12/cm^2.
