Antifuse
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Whereas a fuse is a device that is designed to permanently break an electrically conductive path (typically when the current through the path exceeds a specified limit) an antifuse is designed to permanently create a conductive path. Integrated circuits (ICs) that use antifuse technology employ a thin barrier of non-conducting amorphous silicon between two metal conductors. When a sufficiently high voltage is applied across the amorphous silicon it is turned into a polycrystalline silicon-metal alloy with a low resistance, which is conductive.
Certain programmable logic devices (PLDs) use antifuse technology to configure logic circuits and create a customized design from a standard IC design. Antifuse PLDs are one time programmable in contrast to other PLDs that are SRAM based and which may be reprogrammed to fix logic bugs or add new functions. Antifuse PLDs have advantages over SRAM based PLDs in that like ASICs, they do not need to be configured each time power is applied. They may be less susceptible to alpha particles which can cause circuits to malfunction. Also circuits built via the antifuse's permanent conductive paths may be faster than similar circuits implemented in PLDs using SRAM technology.
Antifuses may be used in programmable read-only memory (PROM). Each bit contains both a fuse and an antifuse and is programmed by triggering one of the two. This programming, performed after manufacturing, is permanent and irreversible.
The antifuse is usually triggered using an approximately 5 mA current. With a poly-diffusion antifuse, the high current density creates heat, which melts a thin insulating layer between polysilicon and diffusion electrodes, creating a permanent resistive silicon link.
Antifuses are also seen in direct mains powered Christmas tree lights (generally older or very cheap ones) that are wired in series (low voltage lights are wired in parallel). Each bulb has an antifuse (with a fairly high resistance) across it. When the bulb blows the voltage across the antifuse rises significantly and the antifuse shorts out the blown bulb allowing the series circuit to keep functioning. the antifuse is made with a coil of wire over the two vertical wires in the bulb with a high resistance coating this coating melts when it gets hot giving the antifuse action.
external links
http://home.howstuffworks.com/christmas-lights2.htm information on use of this method in christmas lights note that they avoid use of the term antifuse presumablly because of thier non-technical audiance. Template:Tech-stub