  We investigate rubrene single-crystal field-effect transistors, whose
stability and reproducibility are sufficient to measure systematically the
shift in threshold voltage as a function of channel length and source-drain
voltage. The shift is due to space-charge transferred from the contacts, and
can be modeled quantitatively without free fitting parameters, using Poisson's
equation, and by assuming that the density of states in rubrene is that of a
conventional inorganic semiconductor. Our results demonstrate the consistency,
at the quantitative level, of a variety of recent experiments on rubrene
crystals, and show how the use of FET measurements can enable the determination
of microscopic parameters (e.g., the effective mass of charge carriers).
