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What is DRAM (Dynamic Random Access Memory) ?

Semiconductor memory is currently the most widely used and largest memory device in the field of storage. DRAM (Dynamic Random Access Memory) is a kind of semiconductor memory, the main principle is to use the amount of charge stored in the capacitor to represent whether a binary bit is 1 or 0.

The advantage of DRAM compared to SRAM is that it is simple in structure, and each bit of data only needs a capacitor and a transistor to process, compared to six transistors usually required for one bit on an SRAM memory chip. Therefore, DRAM has a very high density, higher capacity per unit volume and therefore lower cost. This article focuses on the basics of DRAM.

What is DRAM

Introduction of DRAM
Introduction of DRAM

DRAM (Dynamic Random Access Memory, specifically SDRAM) is the largest memory in the market and the largest revenue share in the memory segment.

DRAM uses a transistor (MOSFET) plus a capacitor to store one bit (1bit) of data. The smallest structure of DRAM is the “cell”, which contains a capacitor and a transistor. Among them, the capacitor is used to store data, and the transistor is generally an N-channel MOSFET (metal-oxide semiconductor field effect transistor), which is used to control the switch of capacitor charge and discharge, and a metagrid can store 1bit of data.

The DRAM basic memory cell consists of a transistor and a capacitor (1T1C), and because the memory cell is relatively simple, DRAM can achieve high storage density and capacity, which is widely used in host memory.

What’s the type of DRAM

What’s the type of DRAM
Types of DRAM

DRAM can be mainly divided into DDR (Double Data Rate) series, LPDDR (Low Power Double Data Rate) series, GDDR (Graphics Double Data Rate) series, HBM series. Among them, DDR is mainly used in servers and PCs, LPDDR is mainly used in mobile phones, GDDR is mainly used in image processing, and HBM is mostly used in high-end GPUs and AI chips.

Their transmission speed (bandwidth) order is roughly HBM>GDDR>DDR. It should be understood that bandwidth is a very important parameter in DRAM products, which means the amount of data transmission per second, directly related to the computing speed of the system, especially when AI develops rapidly, the bandwidth requirements for DRAM products are getting higher and higher, which directly leads to the application and development of HBM.

From the perspective of the market, DRAM can be divided into mainstream DRAM and niche DRAM, among which the manufacturers in the mainstream DRAM market are Samsung, Micron, Hynix and Changxin memory; The niche DRAM market is relatively more fragmented, with companies such as NANYA, Winbond and GigaDevice and Beijing Junzheng in addition to Samsung, Micron, and Hynix (which is gradually exiting the market).

How does DRAM work

How does DRAM work
How does DRAM work

Before answering this question, let’s first clarify what “stored data” is? Set two relatively stable states in the machine, one of which is 0 and the other is 1, and the two relatively stable states for the machine are 0 and 1. Being able to maintain two states stably for a period of time, and both states can be written and read, is the essence of storing data.

Each memory cell of DRAM is implemented with a transistor and small capacitors. If the write bit is “1”, the capacitor is charged: if the write bit is “0”, the capacitor is not charged. When readout, a transistor is used to read the charge state of the capacitor connected to it. If the capacitor is charged, the bit is “1”; If the capacitor is not charged, this bit is “O”.

Because of the leakage resistance of the capacitor, each bit unit must be continuously and periodically charged to maintain the original data loss, which is called refresh.

What are the characteristics of DRAM

Since only one transistor and small capacitor are used in each memory bit, the level of integration is high. In terms of the memory capacity of a single chip, DRAM can far exceed SRAM. In terms of chips of the same capacity, DRAM is also significantly less expensive than SRAM. These two advantages make DRAM a major part of your computer’s memory. DRAM’s row-and-column address time-sharing control and the need for refresh control make it a bit more complex than SRAM’s interfaces. DRAM generally has slower access speeds than SRAM.

What’s the structure of DRAM memory cell

The structure of DRAM memory cell
The structure of DRAM memory cell

DRAM Memory Cell Circuit

WL(X): Word Line, X Address Address;

BL (Y): Bit Line, Y Address (Column Address) and Data In/Out (Data In/Out);

Transistor : Metal oxide semiconductor field effect (MOS) transistor switch;

Capacitor: The charge storage unit is the capacitor.

Note: Each cell in the memory chip has a separate address in a combination of byte and bit lines. Taking the mainstream 4GB single-sided 8-chip memory stick in 2016 as an example, each memory chip has 4G independent addresses.

What is the difference between SRAM and DRAM

The difference between SRAM and DRAM
The difference between SRAM and DRAM

1. Fast speed
2. No need to refresh to keep data
3. No MCU with special interface
4. Small capacity, 256Kb-16Mb
5. Low integration, high price per unit capacity
6. Low static power consumption, large operating power consumption

1. Slower
2. Refresh is required to keep the data
3. MCU with external storage controller required
4. Large capacity, 16Mb-4Gb
5. High integration, low price per unit capacity
6. Low power consumption for operation


DRAM accounts for the largest proportion of the memory market size, with a market size of $60.3 billion in 2019, accounting for 33.24% of the memory. Among semiconductor storage, DRAM is obviously the largest share, accounting for 58% of the market share.

DRAM’s shift from 2D architecture to 3D architecture is one of the major trends in the future. 3D DRAM is to stack the memory cell (Cell) above the logic cell to achieve more output per unit wafer area, here mainly by changing the capacitor design, from planar capacitors to deep groove capacitors to stacked capacitors, compared to ordinary planar DRAM, 3D DRAM can effectively reduce the unit cost of DRAM.



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