Erasable-programmable read-only-memory, EPROM, is a basic type of nonvolatile memory that has been around since the late 1960s. During the 1970s and into the 1990s, EPROM accounted for the majority of nonvolatile memory chips manufactured. EPROM maintained its dominance for decades and still has a healthy market share because of its simplicity and low cost: a typical device is programmed once on an assembly line, after which it functions as a ROM for the rest of its life. An EPROM can be erased only by exposing its die to ultraviolet light for an extended period of time (typically, 30 minutes). Therefore, once an EPROM is assembled into a computer system, its contents are, for all practical purposes, fixed forever. Older ROM technologies included programmable- ROMs, or PROMs, that were fabricated with tiny fuses on the silicon die. These fuses could be burned only once, which prevented a manufacturer from testing each fuse before shipment. In contrast, EPROMs are fairly inexpensive to manufacture, and their erasure capability allows them to be completely tested by the semiconductor manufacturer before shipment to the customer. Only a fullcustom mask-programmed chip, a true ROM, is cheaper to manufacture than an EPROM on a bitfor- bit basis. However, mask ROMs are rare, because they require a fixed data image that cannot be changed without modifying the chip design. Given that software changes are fairly common, mask ROMs are relatively uncommon.
An EPROM’s silicon bit structure consists of a special MOSFET structure whose gate traps a
charge that is applied to it during programming. Programming is performed with a higher than normal voltage, usually 12 V (older generation EPROMs required 21 V), that places a charge on the floating gate of a MOSFET. When the programming voltage is applied to the control gate, a charge is induced on the floating gate, which is electrically isolated from both the silicon substrate as well as the control gate. This isolation enables the floating gate to function as a capacitor with almost zero current leakage across the dielectric. In other words, once a charge is applied to the floating gate, the charge remains almost indefinitely. A charged floating gate causes the silicon that separates the MOSFET’s source and drain contacts to electrically conduct, creating a connection from logic ground to the bit output. This means that a programmed EPROM bit reads back as a 0. An unprogrammed bit reads back as a 1, because the lack of charge on the floating gate does not allow an electrical connection between the source and drain.
An EPROM’s silicon bit structure consists of a special MOSFET structure whose gate traps a
charge that is applied to it during programming. Programming is performed with a higher than normal voltage, usually 12 V (older generation EPROMs required 21 V), that places a charge on the floating gate of a MOSFET. When the programming voltage is applied to the control gate, a charge is induced on the floating gate, which is electrically isolated from both the silicon substrate as well as the control gate. This isolation enables the floating gate to function as a capacitor with almost zero current leakage across the dielectric. In other words, once a charge is applied to the floating gate, the charge remains almost indefinitely. A charged floating gate causes the silicon that separates the MOSFET’s source and drain contacts to electrically conduct, creating a connection from logic ground to the bit output. This means that a programmed EPROM bit reads back as a 0. An unprogrammed bit reads back as a 1, because the lack of charge on the floating gate does not allow an electrical connection between the source and drain.
