EPROM's, EEPROM's, and Flash EEPROM's use variations of something called a FLOATING GATE. To understand what a
floating gate is, you need to understand a bit about what an MOS transistor is.
The image above is a diagram of an MOS transistor. In this case, zero voltage is applied to the gate, which in this
picture is the metal on top of the silicon dioxide on of the silicon. The silicon has N type regions on each side of
a P type region. N type silicon conducts by means of negatively charged electrons. P type silicon conducts by means
of positive charges called "holes". In the picture above, electricity is unable to flow from one N type region to the
other, because the electrons from one N type region are not able to carry current in the P type region between.
In this image above, a positive voltage has been applied to the gate above the silicon dioxide. Because opposites
attract, the positive voltage above the silicon dioxide attracts negatively charged electrons below the silicon dioxide.
The electrons cannot get through the silicon dioxide (which is an insulator much like glass), and so they form a
negatively charged region just underneath the silicon dioxide called the N-channel. Now, there is a path for electricity
to be conducted from one N-region, through the N channel, to the other N-region.
In a floating gate technology such as in EPROM's and EEPROM's, the layer on top of the silicon dioxide is made of polycrystalline silicon (called "poly" or "polysilicon"), and there is another layer of silicon dioxide on top of that poly, and finally another conductor on top of the second layer of silicon dioxide. The layer of polysilicon sandwiched between the two layers of silicon dioxide acts as a FLOATING GATE: if a strong enough negative voltage is applied to the top conductor, the polysilicon FLOATING GATE can reach a positive voltage. The negative voltage can be removed from the top conductor, and the FLOATING GATE still has that positive voltage, since there is nowhere for the positive charges to go. The positive charges on the floating gate are enough to attract negative charges in the silicon below the bottom silicon dioxide, so current flows like in the bottom picture of a MOS transistor.
In an EPROM, an ultraviolet light can be used to cause all of the floating gates to discharge, by giving enough energy to
cause the positive charges in the floating gate to leave. EPROM's come in special packages that have a window
through which ultraviolet light can be applied:
In an EEPROM, the floating gates can be discharged electrically.
Hereare websites that gives a lot more details about EPROM's and EEPROM's: About EPROM's EDN article about Flash memories
For a book about NonVolatile memores such as EPROMS, EEPROMS, and Flash EEPROMs, visit this site: IEEE Store and search for this book: Nonvolatile Semiconductor Memory Technology: A Comprehensive Guide to Understanding and Using NVSM Devices edited by William D. Brown and Joe E. Brewer