The highly photosensitive characteristics of organic thin-film transistors (OTFTs) made using soluble star-shaped oligothiophenes with four-armed pi-conjugation paths, 4(HPBT)-benzene and 4(HP3T)-benzene molecules having a relatively high quantum yield, are reported. 4(HPBT)-benzene-based organic phototransistors (OPTs) exhibited high photosensitivity (similar to 2500-4300 A W-1) even with low optical powers (similar to 6.8-30 mu W cm(-2)) at zero gate bias. The measured photosensitivity of the devices was much higher than that of inorganic single-crystal Si-based phototransistors, as well as that of other OPTs reported earlier. With the highly photosensitive characteristics of the 4(HPBT)-benzene-based OPTs, a high ratio of the on and off current switching of similar to 4 x 10(4) With low optical power and low gate bias was observed. The slow relaxation of the photoinduced charges and charge-trapping phenomena at the interface could lead to a reproducible memory operation for 4(HPBT)-benzene-based OPTs.