The basic element of a semiconductor memory is the memory cell.
The cell has three functional terminals capable of carrying an electrical signal. The select terminal, selects a memory cell for a read or write operation.
The control terminal indicates read or write.
For writing, the other terminal provides an electrical signal that sets the state of the cell to 1 or 0.
For reading, that terminal is used for output of the cell’s state.
Major types of semiconductor memory
- Random access memory (RAM)
- Read only memory (ROM)
- Programmable ROM (PROM)
- Erasable PROM (EPROM)
- Electrically erasable PROM (EEPROM)
- Flash memory
The most common is referred to as random-access memory (RAM).
RAM is able to read data from the memory and to write new data into the memory easily and rapidly. Both the reading and writing are accomplished through the use of electrical signals.
RAM is volatile. If power is interrupted, then the data are lost.
A dynamic RAM (DRAM) is made with cells that store data as charge on capacitors. The presence or absence of charge in a capacitor is interpreted as a binary 1 or 0.
a static RAM (SRAM) is a digital device where binary values are stored using traditional flip-flop logic-gate configurations.
SRAM stores each bit in a bi-stable memory cell. Each cell is implemented with a six-transistor circuit.
DRAM stores each bit as charge on a capacitor. This capacitor is very small—typically around 30 femtofarads, that is, 30 × 10−15 farads.
SRAM is used for cache memory and DRAM is used for main memory.
SRAM cells use more transistors than DRAM cells, and thus have lower densities, are more expensive, and consume more power.
A read-only memory (ROM) contains a permanent pattern of data that cannot be changed. A ROM is nonvolatile; and no power source is required to maintain the bit values in memory.
A programmable ROM (PROM) is nonvolatile, the writing process is performed electrically and may be performed later than the original chip fabrication.
A read-mostly memory, which is useful for applications in which read operations are far more frequent than write operations.
EPROM, EEPROM, and flash memory are common forms of read mostly memory.
A electrically erasable programmable read-only memory (EEPROM) is a read-mostly memory that can be written into at any time without erasing prior contents; only the byte or bytes addressed are updated.
16Mbits DRAM is organized as four square arrays of 2048 by 2048 elements. The elements of the array are connected by both horizontal (row) and vertical (column) lines. Each horizontal line connects to the Select terminal of each cell in its row; each vertical line connects to the Data-In/Sense terminal of each cell in its column.
Types of DRAMs
- Fast Page Mode DRAM (FPM DRAM)
- Extended Data Out DRAM (EDO DRAM)
- Synchronous DRAM (SDRAM)
- Double Data Rate Synchronous DRAM (DDR SDRAM)
- Rambus DRAM (RDRAM)
- Video RAM (VRAM)