EEPROMs an Upgrade from EPROMsBy Megan Tung
EPROMAn EPROM is an erasable programmable read-only memory chip that retains its data when its power supply is switched off. Programming is done through hot electron injection. This process is when electrons or ‘holes’ gain high kinetic energy after being accelerated by a strong electric field. The high kinetic energy causes the hot electrons to overcome a potential barrier, causing them to get injected and trapped in the gate oxide, making the device unstable. High voltages are applied to the select gate and drain connections of the cell transistor.
The select gate of the transistor is pulsed “on” causing a large drain current flow. The high voltage on the gate connection attracts electrons that penetrate the thin gate oxide and are stored on the floating gate. EPROMs can be erased by exposing it to a strong ultraviolet light source, meaning they can be rewritten many times (unlike PROMs). EPROMs are not suitable for storing information that will change frequently because the chip will need to be removed from the device it is in in order to be reprogrammed.
- Storing BIOS (basic input/output systems) information. BIOS is non-volatile firmware used to perform hardware initialization during the booting process, and to provide runtime services for operating systems and programs
- Storing basic software for modems, video cards, and other peripherals
- Early CPU’s made use of an EPROM for the storage of programming the data
EEPROMEEPROMs are electrically erasable programmable read-only memory chips, a type of non-volatile memory used to store relatively small amounts of data. EEPROMs allow individual bytes to be erased and reprogrammed, unlike EPROMs. They can maintain its state through power cycles (on/off), like a PROM or EPROM.
The capacity of EEPROMs is determined by the number of transistors on the silicon die. Each byte (word) of data stored requires 8-bits (or transistors programmed to 1 or 0 state). For example, a 256-bit device stores 32 KB of data. Each bit of storage has two transistors; one is the floating gate and the other is the control gate. The floating gate’s only link to the row (wordline) is through the control gate. When the link between the floating and control gate is in place, the cell has a value of 1.
To change the value to 0 (programming data on the chip), a process called tunneling is required. Tunneling alters the placement of electrons in the floating gate. An electrical charge is applied to the floating gate. The charge comes from the column (bitline), enters the floating gate and drains into ground. The charge causes the floating-gate transistor to act like an electron gun. The electrons are pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. Now there is a barrier of electrons between the floating gate and the control gate. If the flow through the gate is greater than 50% of the charge, the cell has a value of 1. When the charge passing through drops below 50%, the value of the cell is 0, meaning data is being stored on the cell. A blank EEPROM has all of its gates fully open, giving each cell a value of 1.
The content is erased by applying an electric signal to the chip, so the chip does not need to be removed from the computer to be erased and reprogrammed. Most EEPROMs have a max rewrite number of roughly 1 million; however, this can vary depending on the chip. The data on the chip (when stored at room temperature) usually lasts around 10-20 years, sometimes more.
- Reprogrammable calibration data for test equipment
- Storing data in a remote keyless access system
- Storing data on microcontroller boards
- Used in devices like digital potentiometers, digital real-time clocks to save calibration or similar data that is required when the power is switched off or removed
Megan Tung is a summer intern at Jameco Electronics, attending the University of California, Santa Barbara (UCSB). Her interests include photography, music, business, and engineering.