Fuse - This is a two-terminal device that is normally a low resistive element and is programmed or "blown" resulting in an open or high impedance. Typical materials are nichrome and polysilicon. This is element is inherently radiation-hard. Fuse Manufacturers
Antifuse - This is a two-terminal device that is normally a high resistive element and is programmed to a low impedance. Typical programmed impedances range from 25 to 500 ohms, depending on the specific antifuse material, technology, and programming. This element is generally inherently radiation-tolerant; certain versions can be made radiation-hard. The failure mode of these elements during irradiation is rupture from a heavy ion. For a memory application, a cell's programmed state may be sensed differentially, with one element programmed (closed) and the other unprogrammed (open). An example of this structure is the UTMC PROM family.
Switch - This device consists of a memory element (either volatile or non-volatile) which controls a switch. This generally has the highest impedance of the three classes of programmable elements. The volatile, SRAM-based memory elements in use today are considered radiation-soft. EPROM, EEPROM, or SONOS (Northrop-Grumman) non-volatile elements should be relatively radiation-hard to upset. EEPROM cells have been shown to be susceptible to rupture during write cycles (high voltage present) by heavy ions.
Volatile - The memory elements lose their contents when power is removed from the device. SRAM-based devices are volatile and require another device to store their configuration program.
Non-volatile - The memory elements keep their contents when power is removed from the device. The element may be one time programmable or "reprogrammable." Examples of the former include fuses and antifuses. Examples of the latter include EPROM, and EEPROM storage elements. Programmable devices can be both non-volatile and reprogrammable.
One Time Programmable - This device can be programmed only once; it's contents can not be changed. While typically these devices are fuse or antifuse based, they can also be low-cost EPROM devices. In this case, typically used for production devices, an inexpensive package is used without a window.
Reprogrammable - These devices can have their configuration loaded more than once. SRAM-based devices may be reloaded without restriction. Many other forms of reprogrammable elements have restrictions on the number of write cycles, although they are high enough not to be of practical concern for most applications. The FRAM (ferroelectric RAM) is a non-volatile memory element which a limit to the number of read cycles; the readout mechanism of this two-terminal element is destructive and requires a write cycle to restore the contents.
