OECTs(Organic electrochemical transistors) are used in flexible electronics and bioelectronics due to its low operating voltage and high transconductance. However, in the case of hydrophobic semiconductors (ex. P3HT) with high mobility, the performance of OECT is lowered because of its difficulty of ion penetration/transport. Here, a porous semiconductor film is fabricated using a phase separation method between P3HT and other polymers (PS, PBA). In OECT using this method, a contact area between an electrolyte and a semiconductor are increased, thereby enabling fast intercalation/de-intercalation, high transconductance and capacitance. This method improves the performance of OECT by increasing the contact area between electrolyte and semiconductor. This suggests an effective route of enhancing OECT properties without adding a hydrophilic function. Finally, the phase separation method is also being applied to high-sensitivity chemical sensors.