Flash memory is able to retain data without power through the use of floating gate transistors. These transistors have a special structure that allows them to store an electrical charge even when power is removed. This gives flash memory its non-volatile nature, allowing it to act as long-term data storage.
How Flash Memory Works
Flash memory is made up of cells called floating gate transistors. Each cell contains a source, drain, control gate, and floating gate. The floating gate is electrically isolated, allowing it to hold a charge for extended periods without power.
Writing Data
To write data to a flash cell, a high voltage is applied to the control gate while the source and drain are connected to ground. This causes electrons to tunnel through the thin oxide layer onto the floating gate, charging it negatively. This negative charge gets trapped on the floating gate, allowing it to store a bit value of 1.
Erasing Data
To erase the data in a flash cell, a high positive voltage is applied to the source while the control gate is grounded. This causes the electrons to tunnel off the floating gate, discharging it back to a neutral state. This resets the cell back to a bit value of 0.
Reading Data
To read the bit value stored in a flash cell, a voltage is applied to the control gate while the source is grounded and drain is connected to a sense amplifier. If the floating gate is charged, the electric field from the control gate is not strong enough to turn on the channel and allow current to flow. This represents a bit value of 1. If the floating gate is discharged, current is allowed to flow, detecting a 0 bit value.
Charge Retention in Floating Gates
A key property of floating gates is that they can retain an electrical charge for long periods without power. This non-volatile storage ability is what allows flash memory to act as permanent data storage. There are several reasons floating gates can store charge indefinitely:
High Insulator Resistance
The floating gate is completely surrounded by highly resistive insulators with resistances on the order of 1015 ohms. This prevents the trapped charge from leaking off the floating gate.
Lack of Physical Connection
The floating gate is physically isolated within the transistor cell, with no direct electrical connection. This prevents a discharge path from forming.
Silicon-Oxide Interface
The interface between the silicon floating gate and the silicon oxide insulator has a very high barrier energy of around 3.2 electronvolts. This prevents electrons from crossing the interface without high voltages applied.
Thick Oxide Layer
The oxide layer surrounding the floating gate is around 10nm thick. This thickness prevents electrons from tunneling through the oxide during normal operation.
Data Retention Time
Thanks to these properties, floating gate transistors can retain data without power for exceptionally long periods of time. Typical flash memory cells have data retention times between 10 to 100 years at room temperature. However, retention times can vary based on the following factors:
Temperature
Higher temperatures provide more energy to trapped electrons, increasing the probability of discharge by tunneling. Retention times are cut in half for every 10°C rise in temperature.
Number of Program/Erase Cycles
As flash memory cells go through repeated program/erase cycles, charge can get trapped in the oxide surrounding the floating gate. This reduces the insulating properties and allows charge to leak off the floating gate faster.
Oxide Thickness and Quality
Thinner oxides around the floating gate reduce retention time by allowing easier tunneling. Lower quality oxides with more defects also reduce insulating performance.
Initial Floating Gate Charge
The more negative charge stored on the floating gate initially, the longer it takes to discharge completely. Typically multiple electrons are stored to represent a bit value of 1.
Flash Memory Types
While the basic operating physics of floating gate transistors are the same across flash memory types, there are architectural differences between the main flash memory categories:
NOR Flash
- Cells arranged in parallel
- High read speeds
- Slow write/erase times
- Used for code storage
NAND Flash
- Cells arranged in series strings
- Slow read speeds
- Fast write/erase times
- Used for data storage
3D/Vertical NAND
- Cells stacked vertically in layers
- Increased density
- Used in SSDs
Comparison to Other Memory Types
| Memory Type | Volatile? | Memory Density | Read/Write Speed | Endurance |
|---|---|---|---|---|
| SRAM | Yes | Low | Very fast | Infinite |
| DRAM | Yes | Medium | Fast | Infinite |
| Flash | No | Very high | Slow | 10k-100k cycles |
Compared to other main memory types like SRAM and DRAM, flash offers much higher memory density and is non-volatile, but has slower read/write performance and limited endurance. This makes it ideal for long-term mass storage applications rather than temporary data storage during computation.
Applications of Flash Memory
Some common applications that take advantage of flash memory’s properties include:
USB Flash Drives
Also known as thumb drives, USB flash drives use NAND flash memory to store data. They provide portable, non-volatile storage for documents, photos, media, and other files.
Memory Cards
SD, MicroSD, and CompactFlash cards use flash memory to store photos, videos, and other data on cameras, phones, handheld gaming devices, and more. Their small size and non-volatile storage make them ideal for consumer electronics.
Solid State Drives (SSDs)
SSDs use flash memory chips instead of spinning hard disks for storage. Flash’s fast access times, small size, shock resistance, and low power allow SSDs to excel in roles like PCs and laptop storage.
eMMC Storage
Embedded MultiMediaCard (eMMC) flash provides onboard solid state storage for many embedded systems. It integrates the flash memory and controller into a single chip package.
Conclusion
In summary, flash memory is able to maintain data without power through the use of floating gate transistors that can store electrical charge for years. The isolated floating gates prevent charge leakage, while the thick insulating oxide layers prevent unwanted tunneling. Lower temperatures and fewer program/erase cycles also improve data retention time. Compared to other memory types, flash offers much higher density and non-volatility, making it perfect for portable storage devices like USB drives, memory cards, SSDs, and embedded systems storage.
