DRAM, short for Dynamic Random Access Memory, is a semiconductor memory device. It requires periodic refreshing and is less stable than SRAM. However, it uses up far less silicon area. DRAM manufacturing is optimized for dense capacitor structures; they are denser by a factor of 10-20 (bits/cm2) than SRAM built using a standard process. However it has much higher latency.
x __| |__|
C ----- |
The Single-Transistor DRAM is the most dense of all DRAM designs. It is built using one transistor and one capacitor. It stores the data as charge on the capacitor rather than in a feedback loop. It is for this reason that it takes up so little space (compared to 4 NMOS + 2 PMOS transistors for the SRAM cell). However, because capacitors leak charge over time and because a read operation is destructive, the cell must be periodically refreshed to maintain data integrity. The refresh is performed by reading the cell (which automatically triggers the mechanism to rewrite the data back in).
BITLINE is set either high (or VDD) or low (or VSS) depending what needs to be written to the cell. WORDLINE is raised to high, which selects the cell, and the voltage (VDD or VSS) is forced onto the capacitor C.
There are two ways to perform a read
1. Voltage Sensing
BITLINE is first precharged to VDD/2. When WORDLINE is raised, the capacitor C shares its charge with BITLINE, causing a change in voltage ΔV that can be detected by a sense amplifier. If x was originally 0 (VSS), then BITLINE's voltage goes down a bit because some of the charge got transferred over to C; if x was originally 1 (VDD), then BITLINE's voltage goes up a bit because some charge got transferred over from C. This transfer of charge means that value at x gets destroyed when the read is performed. As such, the sensing circuitry needs to restore the cell back to its previous value.
2. Current Sensing
Both BITLINE and WORDLINE are raised to high. If the cell draws little to no charge, then it must have stored a 1. Otherwise, it must have stored a 0. The amount of charge that gets transfered is measured by the sensing circuitry. Again, because the read is destructive, the sensing circuitry needs to restore the cell back to its previous value.
CMOS VLSI Design 4th Edition, Neil H. E. West, David Money Harris
ECE451 VLSI Systems and Design (Farid N. Najm) Lecture Notes