Memory devices

Details Memory devices Memory devices

2000-09-19

2002-06-04

Nguyen, Viet Q. (Department: 2818)

Static information storage and retrieval

Read/write circuit

Flip-flop used for sensing

C365S063000, C365S189011, C365S196000

Reexamination Certificate

active

06400626

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to memory devices.
BACKGROUND TO THE INVENTION
A key component of computers, both large scale and personal computers (PCs), and many other electronic products, is the Dynamic Random Access Memory (DRAM). DRAM integrated circuits are used to store binary data, and are well known and understood. A typical DRAM stores 64 Mbit of data (where 1 Mbit is 2
20
or 1,048,576 bits). Each bit requires a memory cell.
Each memory cell consists of a capacitor and a switch. The capacitor stores charge, and the switch (which is typically a MOSFET) allows transfer of the charge to a common line (or bitline) for access. The presence of a charge, for example, can be used to indicate a logic “1”, and the lack of such a charge used to indicate a logic “0”. The signal controlling the switch is commonly called a wordline, and the data appears on the bitline.
The memory cells are arranged in an array of rows (or wordlines) and columns (or bitlines), which are normally orthogonal to each other.
Traditional DRAM have been arranged in multiple arrays for speed of data access, since reduction of the wordline length reduces the electrical delay for cell selection, and reduction of the bitline length increases the signal developed for sensing, thus making sensing faster.
The size of the signal development available for sensing is dependent on the relative capacitance of the memory cell's capacitor as compared with the bitline's parasitic capacitance. The higher the ratio of memory cell capacitance to bitline capacitance, the larger the signal.
However, in order to have a high density of memory cells, the memory cell size and hence its capacitance is limited, which means that the bitline capacitance must be reduced for safe sensing. This results in a limit on the length of the bitline and hence the number of memory cells per bitline. For this reason, memory devices are arranged into multiple arrays as mentioned above.
However, the use of multiple arrays requires multiplexing of the data from these arrays. Thus, there is a requirement for the data to pass at least one multiplexing element, and more often several multiplexing elements. Each multiplexing stage introduces an undesirable delay in the path of the data.
In order to reduce such delays, it is known to use sense amplifiers, which convert a small signal into a greater logic level. Usually, a sense amplifier is provided for each level of multiplexing.
In order to understand the advantages of the invention described below, the operation of a state of the art DRAM design will first be described.
FIG. 1
shows a DRAM memory cell
1
according to the prior art. Each memory cell comprises of a capacitor
3
, which is capable of holding a binary value in the form of charge stored on the capacitor, and a switch
5
, for example a MOSFET. The switch
5
is interposed between a bitline
7
and the capacitor
3
. The switch
5
is controlled by a wordline
9
, which is normally but not exclusively orthogonal to the bitline
7
. The wordline
9
and bitline
7
are used respectively to propagate the decoded address and the data through the array.
The capacitor
3
has a common node
11
, which is typically connected to a voltage equal to half the supply voltage (i.e. Vcc/2). Thus, storage of logic 1 requires the capacitor to store a voltage level of Vcc, and for the storage of logic zero requires 0V or ground.
Due to losses caused by leakage, the state of the memory cell
1
must be refreshed at regular intervals.
The bitline
7
is typically precharged to a voltage equal to half the supply voltage (i.e. Vcc/2) and then left to float with this voltage stored on its parasitic capacitance, immediately before a memory cell is accessed for a read operation.
As is well known, during a read operation, the wordline
9
is enabled thereby turning the switch
3
ON such that the charge is shared between the capacitance of the bitline
7
and the storage capacitor
3
of the memory cell
1
. This causes the voltage on the bitline
7
to rise above the common node voltage
11
for logic 1, or fall below the common node voltage
11
for logic zero. The sharing of the charge destroys the contents of the memory cell's capacitor
3
. This phase of the read operation is called signal development.
The next phase of the read operation is the signal amplification and restore. Here, the differential signal is amplified using, for example, a cross-coupled inverter pair. This pulls the bitline to the full supply rail (either Vcc or 0V) which allows the memory cell to be restored, or refreshed.
The limit on the size of the array, as mentioned above, is resolved by the use of multiple arrays. The bitlines of each array are connected to a data bus using MOSFET switches which are controlled by an address decode similarly to that of the wordline, and this decode is commonly referred to as column decoding.
There may be several levels of column multiplexing dependent on the number of I/O's that are required.
FIG. 2
shows a simplified three sense amplifier version of a current state of the art DRAM design.
The first stage of sense amplifiers
13
are used to sense the differential signal generated by a memory cell being read. The outputs of the sense amplifiers
13
are connected via MOSFET switches
15
to a data bus
17
. The databus
17
connects the outputs from a number of first stage amplifiers
13
to a second stage sense amplifier
19
.
Likewise, other second stage sense amplifiers, for example sense amplifier
21
, receive outputs from similar arrays of first stage sense amplifiers. The outputs from the second stage sense amplifiers
19
,
21
are connected to a third stage sense amplifier
25
, via MOSFET switches
23
and a data bus
24
. In the example shown, the third stage sense amplifier
25
provides a data I/O signal.
Traditionally, to save packaging costs the number of I/Os are limited relative to the size of the memory. Most state of the art 256 Mbit DRAMs have no more than 32 I/Os.
The disadvantage of these extra switches
15
,
23
is that the combination of data bus parasitic capacitance and switch resistance introduces an electrical delay, thereby limiting the speed of the access. It is for this reason that the sense amplifiers
19
,
25
are connected to the data buses
17
and
24
respectively, thereby speeding up the access.
However, the additional stages of sense amplifiers require complex control circuitry, thereby increasing the complexity of the DRAM. Indeed, although the example described above shows only three sense amplifier stages, it is not uncommon to use more than three stages to transfer data from the memory cell to the output.
FIG. 3
shows how a conventional DRAM design described above accesses a wordline's worth of data. As can be seen, column accessing is used to give the appearance of high bandwidth. Each column access requires approximately 60 ns to get data from the bitcell into the first stage sense amplifiers
13
. To obtain a wordline's worth of data, (ie. the data from all the cells accessed by a particular wordline), “n” column access are required, (n being typically equal to eight).
The time taken to output a wordline's worth of data in this manner is not acceptable to many high speed data processing applications, for example graphic processing systems.
In addition, the additional switches, sense amplifiers, control circuitry and data buses, all add to the complexity of the integrated circuit design layout. Furthermore, the additional circuitry reduces the density of DRAM that is possible for a given area of integrated circuit, and reduces its manufacturing yield.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a dynamic random access memory device comprising a plurality of bitlines, a plurality of wordlines arranged to cross the plurality of bitlines so as to form an array, a plurality of memory cells associated with the wordlines and bitlines, and a plurality of sense amplifiers, each sense amplifier having first and

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