  We investigate the effect of a small leakage current through the gate
insulator on the stability of organic single-crystal field-effect transistors
(FETs). We find that, irrespective of the specific organic molecule and
dielectric used, leakage current flowing through the gate insulator results in
an irreversible degradation of the single-crystal FET performance. This
degradation occurs even when the leakage current is several orders of magnitude
smaller than the source-drain current. The experimental data indicate that a
stable operation requires the leakage current to be smaller than $10^{-9} \
\mathrm{A/cm}^2$. Our results also suggest that gate leakage currents may
determine the lifetime of thin-film transistors used in applications.
