Memory chips are semiconductor devices used as internal storage areas within a computer. Computer memory refers to the storage of programs or data on a temporary or permanent basis. Memory devices can be classified into two distinct groups: primary and secondary. Memory chips may be grouped into either of these categories depending on the type as described below.

Primary memory is directly accessible to the central processing unit (CPU) and includes any data actively or continuously operated upon. Random access memory (RAM) is a good example of a primary memory type. Primary memory allows for fast data access but is limited to smaller storage space.

Secondary memory is not directly accessible by the CPU and is used for longer-term data storage. It is much slower to access compared with primary memory, but it allows for much greater capacity. Secondary memory devices include hard disks, solid state drives and optical drives.

Memory devices may also be grouped as volatile or non-volatile, depending on the device's behavior when power is interrupted. Volatile memory (more frequently known as temporary memory) retains information as long as it is supplied with power, but experiences the total loss of stored memory when power is cut off. Conversely, non-volatile memory retains stored data even when not powered. Because of this fact, non-volatile devices are often used for long-term (secondary) storage. As with the distinction between primary and secondary types, memory chips may be volatile or non-volatile depending on the device's technology and application.

As semiconductor devices, memory chips are composed of transistors and capacitors: the capacitors serve to store two binary logic bits (0 or 1), while transistors allow reading and writing of data to the capacitors. A memory chip is comprised of thousands of these tiny circuits, known as memory cells. Data stored in the cells can be nondestructively accessed (or "read") by means of binary memory addresses assigned to each cell. While early chips could hold only a few bits of information, the largest semiconductor memory chips can now store several gigabits, with higher capacities constantly being developed and introduced.

Chip Types

Memory chips are primarily identified and specified based on their technology type, which includes their methods for reading, writing, and storing data. Before exploring each specific type, it is helpful to examine two broader classes of memory chips distinguished by their data storage qualities.

Random access memory (RAM) is used as a generic term for any memory chip that can be written to and read from which is not a read-only (ROM) chip, as described below. Most RAM is classified as volatile; special types of non-volatile RAM (NVRAM), such as ferroelectric RAM (FRAM), are non-volatile. RAM types include:

• Dynamic RAM (DRAM)
• Static RAM (SRAM)
• Non-volatile RAM (NVRAM)
• Ferroelectric RAM (FRAM)
• Non-volatile static RAM (NVSRAM)

Read-only memory (ROM) is more difficult to modify than RAM and is normally used to read permanent data. ROM is typically non-volatile and is used to store frequently-read (but not written/modified) system software. ROM types include:

• Programmable ROM (PROM)
• Erasable programmable ROM (EPROM)
• Electrically erasable programmable ROM (EEPROM)
• Flash
• Mask ROM

The image below shows the function of both memory types in the hierarchy of a typical computer. Note that ROM is distinct from the computer's hard disk, another type of permanent storage.

Hierarchy of computer memory. Image credit: Ateno de Manila University

The table below describes specific types of memory chips and their common parameters. The attributes listed in the subtype column are described below the table.

• Byte-addressable: data can be read and written one byte at a time.
• Rewritable-when-removed: chips must be removed from the circuit board and reprogrammed externally.
• Symmetric byte-addressable: data can be read and written one byte at a time; reading and writing speeds are equal or nearly equal.
• Asymmetric block write: data is read at byte level but written at block level; writing speed is slower than reading speed.
• Non-rewritable: data on the chip can never be changed.

Specifications

Numerical Specs

When selecting memory chips, the buyer should consider the important specifications listed below, if the manufacturer provides them. Most specifications are related to a chip's speed and efficiency.

• Data rate is expressed in hertz (Hz) and represents the number of bits which can be transferred within the chip each second.
• Access time is another important spec related to a memory chip's speed. This figure represents the elapsed time (in nanoseconds) from the moment the CPU calls for data to the moment it receives the request back.
• Cycle time represents the time necessary to complete a single read/write process and to reset the chip for another cycle.
• Bandwidth refers to the amount of data a memory chip can move or process within a set time frame. It can be expressed in bits per second (bps or b/s), bytes per second (Bps or B/s), or hertz (Hz or cycles/second).

Standards

Standards related to memory chips are typically developed for specific memory types, such as EEPROM and SRAM.

Some general memory standards include:

BS EN 61964 - Pin configurations for IC memory devices

SMD 5962-08208 - FIFO microcircuit memory device 

Image credits:

Intersil Corporation | ROHM Semiconductor USA, LLC