A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies as memory to store data persistently. It is also sometimes called a solid-state device or a solid-state disk, although SSDs do not have physical disks.
SSDs can use traditional hard disk drive (HDD) interfaces and form factors, or newer form factors and interfaces that have been developed to address specific advantages of the flash memory technology used in SSDs. Traditional interfaces (e.g., SATA and SAS), and standard HDD form factors allow such SSDs to be used as drop-in replacements for HDDs in computers and other devices. Newer form factors such as mSATA, M.2, U.2, and Ruler SSD and higher speed interfaces such as NVMe over PCI Express can increase performance over HDD performance.
SSDs have no moving mechanical components. This distinguishes them from conventional electromechanical drives such as hard disk drives (HDDs) or floppy disks, which contain spinning disks and movable read/write heads. Compared with electromechanical drives, SSDs are typically more resistant to physical shock, run silently, have quicker access time and lower latency. While the price of SSDs has continued to decline over time, SSDs are (as of 2018) still more expensive per unit of storage than HDDs and are expected to remain so into the next decade.
As of 2017, most SSDs use 3D TLC NAND-based flash memory (often simply called NAND). NAND is non-volatile memory, which retains data even when power is removed. For applications requiring fast access but not necessarily data persistence after power loss, SSDs may be constructed from random-access memory (RAM). Such devices may employ batteries as integrated power sources to retain data for a certain amount of time after external power is lost. Since 2018, some SSDs have 3D QLC (4 bits per cell) NAND, which increases capacity and lowers costs, but at the expense of a lower endurance rating. For example, a 1 TB QLC NAND SSD has about the same endurance rating as a 500 GB TLC (3-bit) NAND SSD. High-performance SSDs may use SLC (1-bit) or MLC (2-bit) NAND, which can be much faster than TLC or QLC NAND, but have lower capacity and are significantly more expensive, making them better suited for caches or other applications that require very high performance.
However, all SSDs still store data in electrical charges, which slowly leak over time if left without power. This causes worn out drives (that have exceeded their endurance rating) to start losing data typically after one (if stored at 30 °C) to two (at 25 °C) years in storage; for new drives it takes longer. Therefore, SSDs are not suitable for archival storage. The only exception to this rule are SSDs based on 3D XPoint memory (sold by Intel under the Optane brand), which stores data not by storing electrical charges in cells, but by changing the electrical resistance of the cells. 3D XPoint, however, is a relatively new technology with unknown data-retention characteristics and may not be suitable for archival purposes.
Hybrid drives or solid-state hybrid drives (SSHDs), such as Apple’s Fusion Drive, combine features of SSDs and HDDs in the same unit, containing a large hard disk drive and an SSD cache to improve performance of frequently-accessed data.