SSD – Solid State Drives

What is a SSD?

A SSD, Solid State Drive (or Solid State Disk) consists of primarily a controller and NAND Flash Memory to store data. The key difference between SSDs and traditional HDDs (Hard Disk Drive) is that SSDs store data on interconnected flash memory semiconductor chips rather than on a magnetic disk. With no physical movement in the process of reading/writing data to these chips, this allows SSDs to be more robust, quicker, quieter and with less latency than their predecessors of electromechanical disks. SSDs deliver non-volatile memory, which means the data is retained in the chips even with power removed and even do not have to be located on the system’s motherboard as they can be mounted on a PCI/PCIe card or within a HDD-sized case which is installed in an appropriate slot within the computer or externally, usually in the notebook-class form factor. That being said, many ultra-fast laptops have their SSDs on the motherboard to save precious internal space. Although many think of SSDs as the same as USB Flash Memory Sticks/Drives, they are different in the type of flash memory chips that are utilised and the form factors in which they are available, reflected in the large price differences between the two storage devices.

Types of SSD:
: Single-level cells can only be one of two voltage states, allowing a single bit of data storage per cell.
MLC SSD: Multi-level cell; allows four voltage states per cell to be used, meaning two bits can be stored on each cell using the same number of transistors. In MLC-3 there are eight states and MLC-4 has sixteen.
SSHD: Solid state hybrid drives or hybrid drives; Consists of a large (traditional) hard disk drive with a small amount of NAND flash solid-state drive which acts as a cache.

How SSDs work?
A SSD basically consists of two main components, the Controller and the NAND Flash Memory, which along with the other minor components are mounted onto a PCB Board, and then housed within a casing to form a Solid State Drive. Also of importance, but device specific is the SSD Firmware. The flash memory, usually NAND, physically stores the data, whilst the Controller is the brain of the operation managing data storage, access, performance and the health of the SSD. Newer to the market SSDs can feature capacitors or batteries meaning they easily out-perform the current SSDs.

SSD Controller
The SSD controller is an embedded processor chip which performs the task of executing the firmware code and acts as a bridge between the flash memory and the host, performing functions that are required and requested by the host in order to access the data stored in the memory chips. The main functions of a controller are: reading, writing, Error Correcting code (ECC), data encryption, over-provisioning, garbage collection, bad block management, wear-leveling, RAISE (Redundant Array of Independent Silicon Elements) and all communications with the host.

NAND Flash Memory
The term NAND relates to the Boolean operator of NOT AND, and in SSDs are integrated circuits for data storage. Slower than DRAM (Dynamic random access memory) chips, NAND based SSDs are usually usually multi-level cells, but their characteristic of being a non-volatile storage of data (unlike DRAMs which need constant power) allow for their portability, and has boosted their popularity over past few years. SSD Data is stored in a large array of cells produced from floating-gate transistors. Insulated by an oxide layer are two gates, the Control Gate (CG, top) and the Floating Gate (FG, bottom). Electrons flow freely between the CG and the Channel when a voltage is applied to either entity, attracted in the direction to which the voltage is applied. To program a cell, a voltage is applied at the CG, attracting electrons upwards. The floating gate, which is electrically isolated by an insulating layer, traps electrons as they pass through on their way to the CG. They can remain there for up to years at a time under normal operating conditions. To erase a cell, a voltage is applied at the opposite side (the Channel) while the CG is grounded, attracting electrons away from the floating gate and into the Channel. The level of electrical charge that’s applied to the cell then determines the state of the cell.

SSD Applications:
Big Data
Cloud Computing
Consumer electronics
Embedded Equipment
Energy: Oil & Gas, Grid Optimisation
Industrial Automation
Industrial Computing
Industrial Manufacturing
IT Applications
Military electronics
Multimedia electronics
Server / Desktop  Virtualization
Software Acceleration
Visual Computing

Manufacturers of SSDs List:
OCZ Storage Solutions – Toshiba
Tegile Systems

Angelbird Technologies
Corsair Memory
Kingston Technology
Lexar Media, Inc
PLX Technology / Avago
Solidata Technologies
Violin Memory

This is part of the Wikielectronics, a Glossary of Terms and Components used in Electronics Systems, Design and Manufacture. We welcome your comments, thoughts and additions to this article – or indeed if you have a component or area of electronics design that you would like to submit an article into our Wiki, please email this to the site editor: and we’ll take a look!


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