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DRAM vs NAND Flash: Volatility, Speed, Capacity and Uses Compared

DRAM stores data as charge on a capacitor. It reads and writes quickly, but loses its data when power is turned off. NAND flash stores data by trapping electrons in a layer surrounded by insulating film, so data survives power-off and the cost per bit can be kept low, but reading and writing are far slower than DRAM and the number of rewrites is limited. That is why PCs and smartphones use DRAM as working main memory and NAND as storage such as SSDs.

AspectDRAMNAND flash
Storage mechanismOne transistor and one capacitor (1T1C). Stores data as the presence or absence of charge on the capacitorA transistor with an adjustable threshold voltage. Stores data as the amount of electrons held in a floating gate or charge-trap layer
When power is offData is lost (volatile)Data is retained (non-volatile)
RefreshRequired (typically all rows within 64 ms)Not required, but charge slowly leaks away, so data retention time is limited
Read/write unitRandom access in fine-grained units (in practice, burst transfers of tens of bytes)Reads and writes in pages (a few KB to a dozen or so KB); erases in even larger blocks
OverwritingData can be overwritten directlyCannot be overwritten; a block must be erased before writing
Access timeTens of nanosecondsReads take roughly tens of microseconds; writes take longer, roughly hundreds of microseconds
Endurance (rewrite cycles)No practical limitLimited (roughly 50,000–100,000 cycles for SLC, a few thousand for TLC, about 1,000 for QLC)
How capacity is increasedScaling, and three-dimensional capacitors (deep, tall capacitor structures)3D NAND that stacks cells vertically, and multi-level cells that store several bits per cell
Cost per bitHighFar lower than DRAM
Typical usesMain memory in PCs and servers, LPDDR in smartphones, GDDR and HBM for GPUsSSDs, smartphone internal storage, USB flash drives, SD cards

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

DRAM
When the CPU or GPU needs to read and write working data quickly. Used as main memory to hold running programs and their data.
NAND flash
When you want to store large amounts of data cheaply and keep it without power. Used as storage for the OS, apps, photos and videos.
Together
Computers load data stored in NAND into DRAM before processing it. Some SSDs also include DRAM to temporarily hold management information.

FAQ

What is the biggest difference between DRAM and NAND?

Whether data survives when power is turned off. DRAM is volatile memory that loses its data without power; NAND is non-volatile memory that keeps it. DRAM is fast and NAND is large and cheap, so they are used for different roles.

Is this the same as the difference between "memory" and "storage" in a PC?

Broadly, yes. What a PC spec sheet calls "memory" is DRAM, and the SSD listed as "storage" is made of NAND flash.

Why does NAND have limited write endurance?

Each write and erase sends electrons through a thin insulating film, which gradually wears the film so it holds charge less well. SSD controllers extend lifetime with wear leveling, which spreads writes across the whole device, and with error correction.

Do the same companies make DRAM and NAND?

Some companies make both. Samsung Electronics, SK hynix and Micron Technology manufacture both DRAM and NAND. Kioxia focuses mainly on NAND.

Check your understanding

Which correctly describes the structure of a typical 1-bit DRAM memory cell?

  1. AOne transistor and one capacitor
  2. BSix transistors
  3. COne transistor with a floating gate
Show answer and explanation

Answer: A. One transistor and one capacitor

A DRAM cell consists of one transistor and one capacitor (1T1C), storing a bit as the presence or absence of charge on the capacitor. The capacitor is tiny—around 10 fF—so 0 and 1 differ by only tens of thousands of electrons. With so few components the cell is small (about 6F²), ideal for high density. Six transistors is the SRAM cell, and a floating-gate transistor is the flash memory cell.

Why does DRAM need periodic refresh operations?

  1. ATo increase read speed
  2. BBecause the number of writes is limited
  3. CBecause the charge stored in the capacitor is gradually lost through leakage current
Show answer and explanation

Answer: C. Because the charge stored in the capacitor is gradually lost through leakage current

Charge on the capacitor gradually leaks away through the cell transistor's subthreshold and junction leakage. Refresh reads and rewrites each cell before its data becomes unreadable; every row must be refreshed within 64 ms up to DDR4, and within 32 ms in DDR5. During refresh, the bank cannot be accessed and power is consumed, a burden that grows with capacity. A limit on write cycles is a characteristic of flash memory.

Which correctly describes a property of DRAM?

  1. AIt is volatile memory that loses data when the power is turned off
  2. BIt is non-volatile memory that retains data when the power is turned off
  3. CIt is read-only memory that cannot be rewritten
Show answer and explanation

Answer: A. It is volatile memory that loses data when the power is turned off

DRAM stores data as charge and is volatile: when power is removed, the charge is not maintained and the data is lost. Unlike SRAM, also volatile, it needs refresh even while powered. Chilled right after power-off, however, DRAM can retain data for seconds to minutes—exploited by the "cold boot attack" to read encryption keys from memory. Flash memory and similar devices are non-volatile.

  1. Compared with NOR flash, which is a characteristic of NAND flash memory?
  2. What is the unit in which data is erased in NAND flash memory?
  3. Which correctly describes 3D NAND flash memory?
  4. In the memory semiconductor market, which two products account for most of the revenue?

Practice DRAM questions

Key terms explained

1T1C cell
The DRAM memory cell that stores one bit with one transistor and one capacitor. The presence or absence of charge on the capacitor represents 0 or 1.
Refresh
The periodic reading and rewriting of DRAM data before the charge in its capacitors is lost through leakage.
NAND flash memory
Non-volatile memory in which memory cells are connected in series. It is dense and easy to scale to large capacities, and is used in SSDs and memory cards.
3D NAND
NAND flash in which memory cells are stacked vertically in many layers to increase capacity. It was introduced to get past the limits of planar scaling, and layer counts now exceed 200.
Nonvolatile memory
Memory that retains stored data when power is removed, such as flash memory and MRAM. DRAM and SRAM are volatile.
Wear leveling
A technique that spreads writes so rewrites do not concentrate on particular flash blocks, extending the memory's life.

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