We have designed and synthesized new polymer semiconductors containing branched oligothiophene units. These polymers form amorphous films with high thermal stability owing to strong interactions between oligothiophene side chains. On thin-film devices fabricated by spin coating of these polymers as a semiconducting layer, the field-effect mobility increased from 10(-6) to 10(-4) cm(2) V(-1) s(-1) with increasing side-chain length. The UV-vis absorption, photoluminescence and electrochemical properties of the polymers in solutions were also affected by the length of the side chains.