Computers use random access memory (RAM) to store and retrieve information in a way that is easily and instantly accessible. Two types of RAM are available for computers: dynamic RAM (DRAM) and static RAM (SRAM). Each of these has its own distinct advantages and drawbacks. SRAM has the advantage of speed, and DRAM is far less expensive. Most computers use both types, but DRAM is far more common and does the majority of the work.
A dynamic RAM chip holds millions of memory cells, each comprised of a transistor and a capacitor. Each of these cells is capable of holding 1 bit of information, which is read by the computer as either 1 or 0. To determine the reading of a bit, the transistor checks for a charge in the capacitor. If a charge is present, then the reading is 1; if not, the reading is 0. Cells are arrayed in a square configuration, with rows and columns numbering in the thousands.
The problem with dynamic RAM chips is that the capacitor leaks energy very quickly and can hold a charge for only a fraction of a second. A refresh circuit is needed to maintain the charge in a capacitor and retain the information. This refreshing process takes place hundreds of times every second and requires that all cells must be accessed, even if the information is not needed. As each line of cells is read, the computer’s central processing unit (CPU) re-writes each bit of information, recharging the capacitors as needed.
Static RAM chips, on the other hand, use a different technology. Memory cells flip-flop between 0 and 1 without the use of capacitors, meaning that no refreshing process is needed and access takes place only when the information is required. Without the need to constantly access all information, SRAM is much faster than DRAM. Generally speaking, these chips are far more energy efficient, but this is only because of their limited need to access memory, and the rate of consumption rises with heavy use.
The biggest drawback to SRAM is space. Each transistor on a dynamic RAM chip can store one bit of information, but four to six transistors are required to store a bit using SRAM. This means that a dynamic RAM chip will hold at least four times as much memory as a static RAM chip of the same size, making SRAM much more expensive. DRAM is more commonly used for personal computer memory, and SRAM chips are preferred when energy efficiency is a concern, such as in cars, household appliances, and handheld electronic devices.