The electronic properties of organic field effect transistors limit the efficiency of integrated circuits build on basis of printed organic semiconductors. In order to control the mobility of high efficient semiconductors, like rubrene, tetracene, tetracyanoquinodimethane (TCNQ), copper phthalocyanine and many others, single-crystal field-effect transistors have been prepared on surfaces of single crystals and characteristics have been measured. The highest mobility has been measured on rubrene single crystals. The mobility of as-grown crystals measured by air-gap field effect transistor is in the range of 10 cm2/Vs but falls below 1 cm2/Vs during reduction. It was observed that the measured mobility depends on the dielectric used for field effect transistors.