  We use a frequency-dependent electro-optic technique to measure the hole
mobility in small molecule organic semiconductors, such as 6,13
bis(triisopropylsilylethynyl)-pentacene. Measurements are made on semiconductor
films in bottom gate, bottom contact field-effect transistors (FETs.) Because
of the buried metal layer effect the maximum response, due to absorption in the
charge layer, will be for a dielectric film ~ 1/4 of a wavelength (in the
dielectric) (e.g. ~ 1 micron thick in the infrared.) Results are presented for
FETs prepared with both spin-cast polymer and alumina dielectrics prepared by
atomic layer deposition. At low frequencies the results are fit to solutions to
a non-linear differential equation describing the spatial dependence of flowing
charge in the FET channel, which allows us to study multiple crystals forming
across one set of drain-source contacts. FETs prepared on alumina dielectrics
show interesting deviations from the model at high frequencies, possibly due to
increased contact impedance.
