Describe the architecture and operation of semiconductor RAM (Random Access Memory) memories such as DRAM (Dynamic RAM) and SRAM (Static RAM)?

SRAM (static RAM)

An SRAM cell is a bistable flip-flop made of six transistors. Two cross-coupled inverters keep one of two stable states for 1 and 0, and two access transistors connect the cell to a pair of bit lines (b and b').

        word line
   ------+----------+------
         |          |
      [T5]        [T6]
   b ----+--[ flip-flop ]--+---- b'
          (T1 to T4)

To read, the word line is made high. The access transistors turn on and the cell state appears on the bit lines, where a sense amplifier detects it. To write, the data and its complement are put on the bit lines, and the word line forces the flip-flop into the new state. The data stays as long as power is on, and no refresh is needed. SRAM is very fast but costly, and it takes a large area. It is used in cache.

DRAM (dynamic RAM)

A DRAM cell has one transistor and one capacitor. The charge on the capacitor stands for the bit: charged is 1 and discharged is 0.

   bit line ----[ T ]----+
                 |       |
   word line ----+     ===  C
                         |
                        GND

To write, the word line turns on the transistor, and the bit line charges or discharges the capacitor. To read, the transistor is turned on and the charge is shared with the bit line. A sense amplifier detects the small change. This read destroys the stored charge, so the data is written back at once. The charge also leaks, so each row must be refreshed every few milliseconds. DRAM is slower, but cheap and dense. It is used as main memory. The address is given in two parts, the row address (RAS) and then the column address (CAS), which saves pins.

Comparison