  We explore the effect of molecular size on injected carrier transport. We
have fabricated and characterized field effect transistors (FETs) on optically
transparent single crystals of tetracene, the molecule of which is comprised of
four benzene rings (BR). These FETs exhibit effective channel mobility up to
0.15 cm^2/Vs, intrinsic free carrier mobility of ~ 75 cm2/Vs, and on/off ratios
up to 2*10^7. The present results show 1) the possibility of FET behavior in a
transparent crystal with low intrinsic carrier density and 2) a weak dependence
of performance on number of BRs, when compared to similar pentacene (5 BR)
devices.
