EPROM stands for erasable programmable read-only memory and is a type of solid-state memory chip. EPROM is a type of non-volatile ROM memory that retains data after the power has been switched off. EPROM ICs are used within circuits to add memory capabilities.
The basic transistor device in an EPROM chip is different from the normal MOS transistor. A floating gate avalanche injection MOS transistor is used. There are two gates to this MOS transistor and one gate is a floating gate, meaning that it has no electrical connection to that gate.
EPROM varies from other types of programmable ROM memory as, when you erase data, the entire memory is removed. EEPROM is another memory chip with erasable data. However, EEPROM chips can erase blocks of data instead of the entire chip only.
EPROM ICs can be programmed and then erased by using UV light. They feature a transparent quartz window on top of the package. The memory in an EPROM device is erased by applying UV light to the window at the top of the package. General indoor lighting will not contain enough UV light to effect it. However, outdoor light could cause memory to be erased. Therefore many chip windows are protected and covered by a label.
Once the chip is exposed to UV light, the electrons in the floating gate get excited. The high energy that they acquire allows them to overcome the silicon dioxide barrier and move over to the silicon substrate or the control gate. Once they do this you are reducing the threshold voltage. This can take up to 30 minutes, after which the electrons move out from the floating gate and then it is ready for reprogramming.