Semiconductor Memories

Semiconductor memories are classified in different ways. A distinction is made between read-only (ROM) and read-write (RWM) memories. The contents RWMs can be changed in a short time for a virtually unlimited number of times and contents of ROMs are mostly useful for frequent reading and occasional writing. Since RWM memories use active circuitry (transistors) to store the information, they belong to the class of called volatile memories. This is because the data would be lost when the supply voltage is turned off. Read-only memories, on the other hand, encode information by the presence or absence of devices. Their data cannot be modified and they belong to the class of nonvolatile memories. That means the stored data is lost by the disconnection of supply voltage.

Table 1: Classification of  Semiconductor Memories

Based on the access pattern, RWMs are classified as random access class and serial memories. FIFO (first-in-first-out) is an example for serial memories. Most memories belong to the random access class, which means memory locations can be read or written in random order. One would expect memories of this class to be called RAM (random access memory); nevertheless for historic reasons, RAM has been reserved for random access RWM memories. That means though most ROM units also provide random access, but the acronym RAM should not be used for them.

Types of Semiconductor Memories:

• Volatile Memories: Static Random Access Memory (SRAM) and Dynamic Random Access Memory (DRAM) are volatile memories. SRAM is used as a cache memory in computers since it offers the fastest write/read (~8ns) speed among all memories. A DRAM cell consists of one transistor and one capacitor and it is based on the charge stored in a capacitor.
• Non-Volatile Memories: Non-volatile memory is computer memory that can get back stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy disks, and magnetic tape), optical discs, and early computer storage methods such as paper tape and punched cards.