By PURPLELEC | 26 September 2023 | 0 Comments
Flash memory storage principle
To explain the storage principle of flash memory, we still have to start with EPROM and EEPROM.
EPROM means that its contents can be erased by special means and then rewritten. Its basic unit circuit (storage cell) often uses a floating gate avalanche injection MOS circuit, referred to as FAMOS. It is similar to a MOS circuit. Two high-concentration P-type regions are grown on an N-type substrate, and the source S and drain D are respectively derived through ohmic contacts. There is a polysilicon gate floating in the SiO2 insulating layer between the source and drain, with no direct electrical connection to the surrounding area.
This kind of circuit indicates whether the floating gate is charged or not to store 1 or 0. After the floating gate is charged (for example, negative charge), a positive conductive channel is induced between the source and drain just below it, so that The MOS tube is turned on, which means 0 is stored. If the floating gate is not charged, no conductive channel will be formed, and the MOS tube will not conduct, that is, 1 will be stored.
The working principle of the EEPROM basic memory unit circuit is shown in the figure below. Similar to EPROM, it generates a floating gate on top of the floating gate of the EPROM basic unit circuit. The former is called the first-level floating gate, and the latter is called the second-level floating gate. An electrode can be led out to the second-level floating gate, so that the second-level floating gate is connected to a certain voltage VG. If VG is a positive voltage, a tunnel effect occurs between the first floating gate and the drain, causing electrons to be injected into the first floating gate, which is programming. If VG is made a negative voltage, the electrons in the first-level floating gate are forced to dissipate, that is, erased. It can be rewritten after erasing.
The basic unit circuit of flash memory is similar to EEPROM and is also composed of double-layer floating gate MOS tubes. But the first layer of gate dielectric is very thin and acts as a tunnel oxide layer. The writing method is the same as that of EEPROM. A positive voltage is applied to the second-level floating gate to allow electrons to enter the first-level floating gate. The reading method is the same as EPROM. The erasing method is to apply a positive voltage to the source and use the tunnel effect between the first-level floating gate and the source to attract the negative charges injected into the floating gate to the source.
Because the source is erased by applying a positive voltage, the sources of each unit are connected together. In this way, the flash memory cannot be erased by byte, but erased in full or in blocks. Later, with the improvement of semiconductor technology, flash memory also realized the design of single transistor (1T), which mainly added floating gate and select gate to the original transistor.
A floating shed that stores electrons is formed on a semiconductor where current is conducted in one direction between the source and drain. The floating gate is wrapped with a silicon oxide film insulator. Above it is the select/control gate that controls the conduction current between source and drain. The data is 0 or 1 depending on whether there are electrons in the floating gate formed on the silicon substrate. If there are electrons, it is 0, if there are no electrons, it is 1.
As its name suggests, flash memory is initialized by deleting data before writing. Specifically, electrons are extracted from all floating gates. All data will be returned to "1".
When writing, only write when the data is 0, and do nothing when the data is 1. When writing 0, a high voltage is applied to the gate and drain electrodes, increasing the electron energy conducted between the source and drain electrodes. In this way, electrons will break through the oxide film insulator and enter the floating gate.
When reading data, a certain voltage is applied to the gate electrode. If the current is large, it is 1, and if the current is small, it is 0. When the floating gate has no electrons (data is 1), and a voltage is applied to the drain while a voltage is applied to the gate electrode, a current is generated due to the movement of a large number of electrons between the source and drain. In the state where the floating gate has electrons (data is 0), the number of electrons conducted in the channel will decrease. Because after the voltage applied to the gate electrode is absorbed by the floating gate electrons, it is difficult to affect the channel.
Leave a Reply
Your email address will not be published.Required fields are marked. *