DRAM vs SRAM: Key Differences Explained

DRAM stores each bit as charge on a tiny capacitor, so it is dense and cheap but must be refreshed. SRAM stores each bit in a latch built from transistors, so it is faster and needs no refresh, but takes much more area per bit. That is why computers use DRAM for large main memory and SRAM for small, fast caches.

AspectDRAMSRAM
Storage elementOne transistor and one capacitor (1T1C). The bit is the presence or absence of charge on the capacitorUsually six transistors (6T): two cross-coupled inverters forming a latch, plus two access transistors
RefreshRequired. Charge leaks away, so every row must be read and rewritten periodically (typically within 64 ms)Not required. The latch holds its data as long as power is applied
Without powerData is lost (volatile)Data is lost (volatile)
Area per bitSmall (a cell is about 6F², where F is the minimum feature size)Large (an order of magnitude or more bigger than DRAM)
SpeedSlower. A row must be activated and later precharged, so an access takes tens of nanosecondsFast. On-chip SRAM can be accessed in about a nanosecond, a few clock cycles
Cost per bitLowHigh (because of the larger area)
Standby powerNeeds refresh even when idle (reduced by self-refresh modes)No refresh, but transistor leakage in a large array adds up
ManufacturingUses deep capacitor structures and high-k dielectric films, and is made in dedicated memory fabsBuilt in a standard logic CMOS process, so it can sit on the same chip as the processor
Typical capacitySeveral to tens of gigabits per chipKilobytes to tens of megabytes per cache level in a typical processor
Typical usesMain memory (DDR, LPDDR) and graphics memory (GDDR, HBM)CPU and GPU caches, register files and small on-chip buffers

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

When to use which

DRAM
When you need a lot of memory at a low cost per bit and can tolerate some latency: main memory in PCs, servers and smartphones, and graphics memory for GPUs.
SRAM
When speed matters most and the capacity can be small: CPU caches, register files, and small buffers where you do not want refresh circuitry.
Together
Real computers use both. A fast SRAM cache holds frequently used data and hides the latency of the slower DRAM.

FAQ

Is SRAM faster than DRAM?

Yes. SRAM holds each bit in a latch of transistors, so on-chip SRAM can be accessed in about a nanosecond. DRAM must first activate a row and takes tens of nanoseconds, which is why SRAM is used for CPU caches.

Why is DRAM cheaper than SRAM?

A DRAM cell needs only one transistor and one capacitor, while an SRAM cell usually needs six transistors. The smaller cell packs far more bits into the same silicon area, which lowers the cost per bit.

Why does DRAM need refresh?

The capacitor in a DRAM cell slowly leaks its charge, so the data must be read and rewritten periodically, typically within 64 ms. SRAM keeps its value as long as power is applied, so it needs no refresh.

Is DRAM or SRAM non-volatile?

Neither. Both lose their data when power is removed. Non-volatile memories such as flash, MRAM and ReRAM keep their data without power.

Check your understanding

Compared with SRAM, which is a characteristic of DRAM?

  1. AIt is faster than SRAM and needs no refresh
  2. BIts cells are small and the cost per bit is low, but it needs refresh
  3. CIt retains data when the power is turned off
Show answer and explanation

Answer: B. Its cells are small and the cost per bit is low, but it needs refresh

DRAM's small 1T1C cell (about 6F²) packs roughly 20 times denser than a 6T SRAM cell (about 120–150F²), lowering cost per bit. But DRAM needs refresh, and an access takes tens of ns for row activation and the like, versus about 1 ns for SRAM. Systems therefore use DRAM for main memory and SRAM for CPU caches, building a memory hierarchy that combines their strengths.

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; standards typically require every row to be refreshed within 64 ms. 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.

  1. Which correctly describes the structure of a typical six-transistor SRAM cell?
  2. Which correctly describes a property of SRAM?
  3. What is the most appropriate main use of SRAM?

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.
6T SRAM cell
The SRAM memory cell made of two cross-coupled inverters (four transistors) and two access transistors.
Refresh
The periodic reading and rewriting of DRAM data before the charge in its capacitors is lost through leakage.
Cache memory
Fast, small memory placed close to the CPU. It holds frequently used data to bridge the speed gap with main memory. Mostly built from SRAM.

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