Semiconductor memories are the volatile memory storages that store the program and data until the power supply to the system is ON. The cycle time of these semiconductor memories ranges from 100 ns to 10 ns. The cycle time is the time from the start of one access to the start of the next access to the memory.
In this section, we will discuss all kinds of semiconductor memories along with their essential characteristics. We will also discuss its internal organization about how the memory cells are organized inside the memory unit.
Content: Semiconductor Memory Types
What is Semiconductor Memory?
Almost all the memory units are made of semiconductor material, especially silicon. Semiconductor memories are used for storing digital data as they can be accessed faster. The development over years in semiconductor technology has improved its accessing speed, increased its capacity, and decreased the cost per bit. This has led to large yet affordable main memory.
The semiconductor memories are of two types volatile memory and non-volatile memory. Volatile memory (such as RAM) is the memory that holds the data or information until the power supply to the memory chip is ON.
Internal Memory Organization
The memory cells in the memory chip are organized in the form of an array where each particular cell can hold one bit of information. Refer to the figure below to understand the internal organization of memory.
The figure below represents the memory circuit with 16 words (W0 to W15) where each word has a word length of 8 bits (b0 to b7). So, this is referred to as 16×8 memory organization.
Memory cells of the same row or a word are connected via a common line which we refer to as a word line. On the other hand, all the memory cells of a column are connected to a sense/write circuit via two-bit lines. Further, the sense/ write circuit is connected to data input and output lines.
Semiconductor Memory Types
Semiconductor memories can be classified into two types random access memory (RAM) and read-only memory (ROM).
Random Access Memory (RAM)
RAM being volatile memory, each of its addressable locations can be accessed in a very short time. Each addressable location in RAM has a fixed accessing time independent of its location and prior accessing patterns. RAM can be classified as either static RAM or dynamic RAM.
1. Static RAM
In static RAM memories or SRAM, the content of the memory cell retains as long as the power supply to the memory chip is ON. In any situation, if the power supply to the memory chip is interrupted then the content of memory cells of static RAM is also lost.
When the power is resumed back to the memory chip there is no guarantee that the memory cells may have the same content as they have before the interruption of the power supply. This is the reason the static RAM is volatile in nature
Any cell of static RAM can be accessed very quickly hence they are faster. The CMOS SRAMs i.e. complementary metal-oxide-semiconductor memory consumes very low memory as the power is supplied through the cells only when the cell is being accessed.
2. Dynamic RAM
Though the static RAM is faster its memory cells require several transistors which makes it expensive. So, to design a less expensive and higher density RAM we have to implement it using simpler cells.
But, the fact with the simpler cell is that the simpler cell does not hold data for a long period until the data is accessed from the cell frequently either for read or write operation. The memory circuit implemented using such simpler cells is referred to as dynamic RAMs.
In dynamic RAM the information is stored in the memory cell by imposing the charge on the capacitor. But the capacitor can hold charge only for tens of milliseconds and to hold the content for a longer time the capacitor must be charged to its full value. The capacitor is charged while its content is refreshed either by reading the contents from the cell or writing new information to the cell.
Once the capacitor is charged full while refreshing its content it also starts consuming the tiny amount of current which can be measured in Pico amperes. Now you can retrieve the correct information from the cell before it gets discharged.
Read-Only Memory (ROM)
Being a non-volatile memory the ROM memories can only be read and not written. ROM memories are written once at the time of their manufacturing. Once written the ROM memories cannot be rewritten or modified. Usually, the ROM memories store small programs that are essential for efficient working of computers like system programs, subroutines, etc.
ROM memories have various forms such as PROM, EPROM, EEPROM, and flash memory.
1. Programmable ROM (PROM): PROM is written with the help of electric signals. PROM can be written by the manufacturer or it even allows the user to write on PROM but it requires some special equipment. It provides flexibility as it lets users program it.
2. Erasable Programmable ROM (EPROM): It is more flexible and user-oriented when compared with PROM. Because the content of EPROM can be erased and memory can be rewritten with new data. The memory cells of EPROM are made of transistors, to erase the content of EPROM the charge on these transistors needs to be dissolved using ultraviolet rays.
The EPROM memory chip does not allow selective erasure of the content. As when UV light is exposed to the EPROM memory chip it erases all its content. This feature supports the development phase of a digital device. The written information on EPROM lasts for a long time.
3. Electrically Erasable Programmable ROM (EEPROM)
The EEPROM can be programmed, erased, and read electrically. However, a different voltage is required for each operation and this makes the circuit even more complex. But the complexity of the circuit can be outweighed by many advantages of EEPROM.
4. Flash memory
The flash memory is similar to EEPROM and uses electrically erasing technology. Flash memory also allows selective erasure and selective writing of cells. The difference is that the flash memory can write an entire block of cells at once. The other difference is that the flash memory uses the same voltage to write, erase, and read the memory. This reduces the complexity of the circuit and even consumes less power.
So, this is all about the semiconductor memories. We have discussed its internal structure and its types. Semiconductor memory is faster and is available in a wide range of speed