Chapter 15 - Memory ICs

Abstract

Memory devices are a special class of integrated circuits that are used to store binary data for later use. There are two main categories of semiconductor memories: Read-Only Memory (ROM) and Read-Write Memory (RWM). The brain of the computer is the Central Processing Unit (CPU), which is where all of the number crunching and decision making are performed. ROMs are also classed as being nonvolatile, because their data remains when power is removed from the system. The smallest unit of memory, called a cell, can be used to store a single bit of data: that is, a logic 0 or a logic 1. A number of cells physically grouped together are classed as a word, and all the cells in a word are typically written to, or read from, at the same time. The core of a memory device is made up of a number of words arranged as an array. ROM devices are said to be mask-programmed (or sometimes mask-programmable) because the data they contain is hard-coded into them during their construction. PROMs were initially intended to be used as memories to store computer programs and constant data values. However, design engineers also found them useful for implementing simple logical functions such as lookup tables and state machines. An Erasable Programmable Read-Only Memory (EPROM) transistor has the same basic structure as a standard MOSFET transistor, but with the addition of a second polysilicon floating gate isolated by layers of oxide.