This invention relates to semiconductor memory devices and more particularly to an improved high-density MOS random access, dynamic read/write memory.
One of the most widely used memory devices for computers is the MOS dynamic RAM of the 64K-bit size, such as the device illustrated in U.S. Pat. No. 4,239,993, issued to McAlexander, White and Rao, assigned to Texas Instruments. As memory density in RAM devices of this type increases to the 256K-bit and 1-Megabit sizes, however, power dissipation in the integrated circuit becomes more of a problem. Power dissipation may be reduced by avoiding activation of circuits when not necessary; the active pull-up function is one example of such a circuit.
In the dynamic RAM of U.S. Pat. No. 4,239,993, the initial sensing voltage for the differential sense amplifier is established by connecting the storage capacitance to one bit line and a dummy capacitance to the other bit line. The sense amplifier is then activated, and the bit line with the lower voltage will be pulled to ground level by the sense amplifier. For reading a `1`, the storage cell bit line remains high, and for reading a `0` the dummy cell side stays high. The high side does, however, lose some charge during sensing, so that it is not at the full Vdd potential. Thus, after sensing is complete, active pull-up circuits on the bit lines are activated to bring the high bit lines back up to a full Vdd level, so that the storage cells will be left with a full Vdd level for a stored `1`. Power is wasted in this step, however, in sense amplifiers that sensed a `0` level, since the active pull-up is activated on the dummy cell side of the sense amplifier and the bit-sense line is brought to a full Vdd. Any charge spent charging the dummy cell is wasted since all of the dummy cells are discharged to ground in the precharge cycle.
It is the principal object of this invention to provide improved sense amplifier circuitry for high-speed, high-density, low-power dynamic random access memory devices, particularly for reducing the power needed for the active pull-up function. Another object is to provide sense amplifier circuitry for a semiconductor dynamic memory device which uses active pull-up circcuits that are not operated when not needed. A further object is to provide an improved sense amplifier for a dynamic RAM circuit which conserves power in restoring or writing a full supply level into the selected storage cell.