Flash Memory: Tunneling and Erasing
Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10 to 13 volts, is applied to the floating gate. The charge comes from the column, or bitline, enters the floating gate and drains to a ground.
This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A special device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is above the 50 percent threshold, it has a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. A blank EEPROM has all of the gates fully open, giving each cell a value of 1.
The electrons in the cells of a flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip, which can then be rewritten. Flash memory works much faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block or the entire chip, and then rewrites it.
You may think that your car radio has flash memory, since you're able to program the presets and the radio remembers them. But it's actually using flash RAM. The difference is that flash RAM has to have some power to maintain its contents, while flash memory will maintain its data without any external source of power. Even though you've turned the power off, the car radio is pulling a tiny amount of current to preserve the data in the flash RAM. That is why the radio will lose its presets if your car battery dies or the wires are disconnected.