The microstructure and programming mechanism of an electrically programmable antifuse is described. The device consists ofn+pn+junctions in polycrystalline silicon programmed by applying voltage pulses to the twon+terminals. High voltage electron microscopy and optical microscopy reveal the micro‐structure of programmed antifuses. Devices with Al metallization can be programmed with a single long pulse. The final resistance is about 20&OHgr; and a metallic aluminum spike is formed between the two contacts. With Al and TiW (barrier) metallization, programming occurs after a series of short pulses by migration ofn+dopant. This results in an ohmic contact between the twon+regions with a final resistance of approximately 300&OHgr;.