  We have used an electro-optic technique to measure the position-dependent
infrared absorption of holes injected into a thin crystal of the organic
semiconductor, 6,13-bis(triisopropylsilylethynyl)-pentacene incorporated in a
field-effect transistor. By applying square-wave voltages of variable frequency
to the gate or drain, one can measure the time it takes for charges to
accumulate on the surface, and therefore determine their mobility.
