Semiconductor Memory

Definition: Semiconductor memory is the main memory element of a microcomputer-based system and is used to store program and data. The main memory elements are nothing but semiconductor devices that stores code and information permanently.

The semiconductor memory is directly accessible by the microprocessor. And the access time of the data present in the primary memory must be compatible with the operating time of the microprocessor. Thus semiconductor devices are preferred as primary memory.

ROM, PROM, EPROM, EEPROM, SRAM, DRAM are semiconductor (primary) memories. The fabrication of semiconductor memories is done through CMOS technology.

The semiconductor memory offers high operating speed and has the ability to consume low power. Also, these are fabricated as IC’s thus requires less space inside the system.

Block Diagram of Semiconductor Memory

As we have already discussed that semiconductor memories are nothing but primary memory formed of semiconductor devices.

Basically, an IC of a semiconductor memory consists of n number of address lines and m data lines. Thereby providing the total memory capacity of 2ⁿ × m bits. This implies that it holds 2ⁿ memory locations and each location can store data up to m-bit.

The functional block diagram representation of semiconductor memory is given below:

As we can see that the block diagram consists of a row and a column address decoder along with memory array and I/O buffer. These buffers hold the data for a certain period of time.

The total number of address lines i.e., n is divided into q and r and are separately provided as input to row and a column decoder. The output from these two decoders forms a matrix array having size 2^q × 2^r having 2ⁿ crossing points. These crossing points are referred to as memory cells. And each memory cell holds the ability to store one-bit of binary data. So, whenever the processor sends an address to the memory IC then the row and column decoder accordingly select one line, which correspondingly selects a memory cell from the matrix.

In this way, the memory cells are selected by the address sent by the processor. Further, the data can be read or written in a particular selected memory cell according to the generated control signal.

Initially, the memory cells of semiconductor memory were fabricated of passive components like resistor and capacitor. After that diodes were also used. But with the advent of new technologies, bipolar and MOS transistors took the places of diodes, resistors and capacitors.

And now-a-days the memory cells are made using CMOS and HMOS technology that possesses high operational speed with low power consumption.

Let us now move further and understand how semiconductor memories are classified?

Types of Semiconductor Memory

Basically, the semiconductor memory is classified as volatile and non-volatile memory.

Volatile memories are those memories that store the data temporarily. More specifically we can say that data is stored in volatile memory only till the duration power supply to the IC is ON. And once the supply gets OFF then the stored data gets lost.

As against in non-volatile type of memory, the data retained in the memory even if the power supply is OFF. Thus we can say that in non-volatile memory the data is stored on a permanent basis.

Let us now move further and understand the further classification of non-volatile and volatile memory.

Non-Volatile Memory

ROM: It stands for Read-Only Memory. It is a memory array that is permanently programmed by the manufacturer or programmer only once. Hence its data cannot be changed by the processor once it is programmed.

Thus the processor can only read the data present in this memory hence called read-only memory or fixed memory.

The process of loading the data in the ROM is known as programming. The way in which ROM is programmed further classifies it.

• Custom programmed ROM (ROM): These memories are programmed by the manufacturer at the time of its fabrication. Hence is stored permanently.
• Programmable ROM (PROM): PROM’s are programmed by the user but these offers re-programmability after erasing previously loaded contents. There exist two methods by which the contents of the ROM are erased.

EPROM: It stands Erasable Programmable Read-Only Memory. The EPROM offers re-programming, by erasing the previously stored data by making use of ultraviolet rays. The memory-erasing time lies between 10 to 30 minutes.

Basically, the electrons in the isolated gate of MOS transistor of memory cells get removed when irradiated with ultraviolet rays. Thereby allowing removal of stored data in the memory cell through the control gate.

Further, in order to reprogram the EPROM, the memory chip is inserted in the PROM programmer socket. A PC provides interfacing to the PROM programmer and the programmer installs the information to be loaded in the chip from the personal computer.

EEPROM: It is an abbreviation used for electrically erasable programmable read-only memory. Initially, the data in E²PROM is erased by applying external voltage at the erase pin of the chip. But this somewhat increases the complexity of the overall system.

So, the latest versions provide incorporation of supply voltage within the chip.

Volatile Memory

RAM: RAM stands for Random Access Memory. It is a volatile memory that means the data is stored temporarily until the power supply is ON. This memory is used for short term storage of data. At the time of read operation, RAM exhibits non-destructive nature.

This means that while reading, the data present in the memory location will not be destroyed. But writing data in a memory location where some data is already present will destroy the previously stored data. Or we can say that the newly entered data will replace the previous data.

RAM is majorly classified into two categories:

• SRAM: It is an abbreviation for Static Random Access Memory. SRAM has an array of flip-flops that are used to store the data. The memory cells consist of flip flops that hold the data until the power supply is on.
• DRAM: It is an acronym for Dynamic Random Access Memory. It is also a read/write memory that stores the data in the form of charges in the capacitor and transistor pair present in the memory cell.
  However, the stored charge takes some milliseconds to get dissipated therefore periodic refreshing of the elements is required, in order to have random access feature.

So, this is all about semiconductor memory and its types.