Differences between Dynamic random-access memory and Static random-access memory

Dynamic random-access memory vs. Static random-access memory

Dynamic random-access memory (DRAM) and Static random-access memory (SRAM) are two different types of volatile semiconductor memory. Both lose their stored information when power is removed. The fundamental structural difference between them lies in the design of their memory cells. A DRAM cell consists of a transistor and a capacitor, while an SRAM cell is typically made of six transistors. This design variance leads to significant differences in performance, cost, and typical use cases.

Comparison Table

Category	Dynamic random-access memory (DRAM)	Static random-access memory (SRAM)
Data Retention Mechanism	Stores data as a charge in a capacitor; requires periodic refreshing to prevent data loss due to leakage.	Uses a flip-flop circuit (made of multiple transistors) to hold data as long as power is supplied; no refresh is needed.
Speed	Slower, with access times typically in the range of 10-100 nanoseconds.	Faster, with access times as low as 0.5-2.5 nanoseconds, making it suitable for speed-sensitive applications.
Cost per Bit	Less expensive due to its simpler cell structure.	Significantly more expensive because of its complex cell structure with more transistors.
Density	High density, allowing for larger memory capacities in a smaller physical area.	Low density, as the six-transistor cell takes up more space than a one-transistor, one-capacitor cell.
Power Consumption	Generally higher standby power consumption due to the constant need for refresh cycles.	Lower power consumption when idle, but can consume more power during active use.
Typical Applications	Main system memory in PCs, laptops, smartphones, and servers; graphics cards.	CPU cache (L1, L2, L3), processor registers, and in networking devices like routers and switches.

Venn diagram comparing Differences between Dynamic random-access memory and Static random-access memory

Internal Structure

The memory cell in a DRAM chip is the simplest type, consisting of one transistor and one capacitor. Data is stored as an electrical charge on the capacitor, where a charged state can represent a binary '1' and a discharged state a '0'. This one-transistor, one-capacitor (1T1C) structure allows DRAM to achieve very high storage densities.

In contrast, an SRAM cell is more complex. The standard design is a 6T cell, which uses six transistors to form a latch or flip-flop circuit to store one bit of data. This design does not use a capacitor, and the data is held statically as long as there is power. Because it requires more components per bit, an SRAM cell is larger and cannot be packed as densely as a DRAM cell.

Operation

Due to the nature of capacitors to leak charge over time, DRAM requires a constant refresh operation. An external memory controller must read and rewrite the data in each cell every few milliseconds to prevent data loss. This refresh cycle adds overhead and contributes to DRAM's slower access times compared to SRAM.

SRAM does not need to be refreshed. The flip-flop circuit will hold its data indefinitely as long as power is supplied, which is why it is called "static". This allows for faster and more direct access to the stored data, resulting in lower latency.

Applications

The differences in cost, speed, and density determine the primary applications for DRAM and SRAM. DRAM's lower cost and higher density make it the ideal choice for the main system memory in most computing devices, including personal computers, servers, and mobile phones.

SRAM's superior speed makes it the preferred technology for cache memory within a CPU. Processors use multiple levels of SRAM cache (L1, L2, L3) to store frequently accessed data and instructions, bridging the speed gap between the fast processor and the slower main memory (DRAM). SRAM is also used in other high-speed applications such as network switches, routers, and hard drive buffers.