Question

What is a CMOS memory cell and its use in memory arrays?

Answer 1

A Complementary Metal-Oxide-Semiconductor (CMOS) memory cell is a fundamental building block of digital memory devices used to store binary data (0s and 1s). It consists of two distinct types of transistors: a PMOS (P-channel Metal-Oxide-Semiconductor) transistor and an NMOS (N-channel Metal-Oxide-Semiconductor) transistor. These transistors work together to retain and read the stored data.

Here's a basic explanation of how a CMOS memory cell works and its use in memory arrays:

Structure of a CMOS Memory Cell:

PMOS Transistor: This transistor is constructed using P-type semiconductor material. Its gate terminal is connected to the memory cell's bitline (the wire that connects multiple memory cells together).
NMOS Transistor: This transistor is constructed using N-type semiconductor material. Its gate terminal is connected to the memory cell's complementary bitline (the bitline that carries the inverted value of the original bitline).

Storing Data:
The CMOS memory cell stores data by using the transistors' conductive properties. When a certain voltage is applied to the wordline (a control line for selecting a specific memory cell), it allows data to be written or read from the memory cell.

To write a '0' into the cell, the voltage on the bitline is set to a high level (logic '1') and the voltage on the complementary bitline is set to a low level (logic '0'). This turns on the NMOS transistor and turns off the PMOS transistor, creating a conductive path from the cell to the ground, effectively storing a '0'.
To write a '1' into the cell, the voltages on the bitline and complementary bitline are reversed. This turns on the PMOS transistor and turns off the NMOS transistor, creating a conductive path to the supply voltage (Vdd), storing a '1'.

Reading Data:
To read data from the CMOS memory cell, the voltage on the wordline is activated, and the voltages on the bitline and complementary bitline are sensed. Depending on the stored data ('0' or '1'), one of the transistors will conduct more than the other, creating a voltage difference that can be detected. This voltage difference is then amplified and interpreted as the data stored in the memory cell.

Use in Memory Arrays:
CMOS memory cells are densely packed in memory arrays to create larger storage capacities. Memory arrays are organized as rows and columns of memory cells, where each cell can be individually addressed and accessed. By activating specific wordlines and bitlines, a particular memory cell can be selected for reading or writing.

Overall, CMOS memory cells are used in various types of digital memory devices such as Static Random-Access Memory (SRAM) and Dynamic Random-Access Memory (DRAM) due to their low power consumption, high integration density, and non-volatile properties.