As a characteristic feature of conventional conjugated polymers, it has been generally agreed that conjugated polymers exhibit either high hole transport property (p-type) or high electron transport property (n-type). Although ambipolar properties have been demonstrated from specially designed conjugated polymers, only a few examples have exhibited ambipolar transport properties under limited conditions. Furthermore, there is, as yet, no example with 'equivalent' hole and electron transport properties. We describe the realization of an equivalent ambipolar organic field-effect transistor (FET) by using a single-component visible-near infrared absorbing diketopyrrolopyrrole (DPP)-benzothiadiazole (BTZ) copolymer, namely poly[3,6-dithiene-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1 ,4-dione-5', 5 ''-diyl-alt-benzo-2,1, 3-thiadiazol-4,7-diyl] (PDTDPP-alt-BTZ). PDTDPP-alt-BTZ shows not only ideally balanced charge carrier mobilities for both electrons (mu(e) = 0.09 cm(2) V-1 s(-1)) and holes (mu(h) = 0.1 cm(2) V-1 s(-1)) but also its inverter constructed with the combination of two identical ambipolar FETs exhibits a gain of similar to 35 that is much higher than usually obtained values for unipolar logic.