U. Glaeser

FIGURE 2.23 Different configurations of a DRAM cell: (a) 4-transistor DRAM cell, (b) 3-transistor DRAM cell,
(c) 1-transistor DRAM cell.
on the charge stored on its gate capacitance. Two more transistors are contained in each cell, one used as
read access switch and the other used as write access switch. This cell is faster than the 4-transistor DRAM
cell; however, every cell needs two control and two I/O (bit) lines making the area advantage insignificant.
The widely popular DRAM cell is the single transistor DRAM cell shown in Fig. 2.23(c). It stores data
as charge in an explicit capacitor. There is also one transistor which is used as the access switch. This
structure consumes significantly less area than a static RAM cell. The cell has one control line (word
line) and one data line (bit line). The cells can be selected using the word line, and the charge in the
capacitor can be modified using the bit line.
Read Only Memories (ROMs)
ROM arrays are simple memory circuits, significantly simpler than the RAMs, which we discussed in the
preceding section. A ROM can be viewed as a simple combinational circuit, which produces a specified
output value for each input combination. Each input combination corresponds to a unique address or
location. Storing binary information at a particular address can be achieved by the presence or absence
of a connection from the selected row to the selected column. The presence or absence of the connection
can be implemented by a transistor. Figure 2.24 illustrates a 4 × 4 memory array. At any time, only one
word line among A1, A2, A3, and A4 is selected by the ROM decoder. If an active transistor exists at the
cross point of the selected row and a data line (D1, D2, D3, and D4), the data line is pulled low by that
transistor. If no active transistor exists at the cross point, the data line stays high because of the PMOS
load device. Thus, absence of an active transistor indicates a “1” whereas the presence of an active transistor
indicates a “0”.
ROMs are most effectively used in devices, which need a set of fixed values for operation. The set of
values are predetermined before fabrication and a transistor is made only at those cross-points where
one is desired. If the information that is to be stored in the ROM is not known prior to fabrication, a
transistor is made at every cross-point. The resulting chip is a write-once ROM. The ROM is programmed
© 2002 by CRC Press LLC