Ambipolar transistors typically suffer from large off-current inherently due to the ambipolar conduction. Using a tri-gate transistor we show that it is possible to electrostatically switch ambipolar polymer transistors from ambipolar to unipolar mode. In unipolar mode symmetric characteristics with an on/off current ratio of larger than 10 (5) are obtained. This enables easy integration into low-power complementary logic and volatile electronic memories.