RANDOM ACCESS MEMORY

Abstract: A bipolar random access memory array including "end of write shut down circuit means" coupled to the write circuit means is disclosed. The "end of write shut down circuit means" is activated by and only functions as the written cell switches state. The "end of write circuit means" is coupled between the opposite bit line (BL or BR) and preferably the write transistor of a write circuit of the write circuit means. The use of "the end of write circuit means" improves the overall operation of the memory and in particular the write operation thereof.

Description: The invention relates generally to random access memory according to the preamble of the claims. In many prior art, particularly described in US-A-4,092,551 and US-A-4,319,344, random access memory arrays the selected cell to be written is brought into the selected state by the proper voltages on its word/drain lines and first/second bit lines. It is known that maintaining a high voltage state on the selected first or second bit line for an indeterminate time can cause disturb conditions in subsequently selected cells. The prior art has addressed this problem in a variety of ways, each of which has its own drawbacks. One method is to use large voltage swings to increase noise immunity. But this requires higher supply voltages and uses more power. Other methods impose timing restrictions which impact performance and place additional burden on the user. The primary object of the invention is providing a random access read/write memory array having circuit means for anticipating the completion, or near completion, of the write operation and in response thereto promptly terminating the write operation, whereby the operation of the random access memory is materially enhanced. A further primary object of the invention is to provide an improved random access read/write memory array employing "write complete circuit means" (i.e. "end of write shut down circuit means") and unclamped CTS type memory cells of small area, high stability, low power requirements, high reliability (fewer components) and materially enhanced "write operation" time and has relative immunity to the "alpha particle problem". The solution of the objects is described in the characterizing part of claim 1. Improvements are characterized by claims 2 to 5. The solution to the problem in accordance with the invention is an improved random access memory which includes the provision of "self-limiting write circuit means". The "end of write shut down circuit means" is activated by and only functions as the written cell switches state. The "end of write circuit means" is coupled between the opposite bit line and preferably the write transistor of a write circuit of the write circuit means. The use of the end of write circuit means improves the overall operation of the memory and in particular the write operation. The concern over alpha particle sensitivity is fairly recent but a detailed explanation of bipolar RAM sensitivity to alpha particle bombardment can be found in the references set forth below. The modified CTS cell is at least an order of magnitude less sensitive to alpha particle errors than a comparable cell made with the same technology. It is to be appreciated that the practice of applicants invention is not limited to a particular memory cell circuit such as the complementary transistor switch (CTS) memory cell or the unclamped complementary transistor switch (CTS) memory cell. Applicants invention may be practiced with any one of a number of memory cell circuits known to the prior art. The complementary transistor switch (CTS) memory cell is well known to the art. See for example: (1) U.S. Patent No. 3,863,229 entitled "SCR (or SCS) Memory Array With Internal and External Load Resistors", granted January 28, 1975 to J. E. Gerbach and of common assignee with the subject application; (2) AC Write Scheme For Bipolar Random-Access Memories Using Schottky Coupled Cells" by J. A. Dorler et al, IBM TDB Vol. 23, No. 11, April 1981; or (3) "A 1024-Byte Random Access Memory Using a Complementary Transistor Switch (CTS) Cell" by J. A. Dorler et al, IBM Journal of Research and Development, Vol. 25, No. 3, May 1981, pages 126-134. The unclamped complementary transistor switch (CTS) type memory cell is disclosed in the publication "Saturated CTS Memory Cell Using A PNP Load" by J. A. Dorler et al, IBM TDB, February 1984, Vol. 26, No. 9, pages 4720-1. The foregoing and other objects, features and advantages of the invention will be more apparent from the following more particular description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. Figure 1 is a circuit diagram of the prior art complementary transistor switch (CTS) memory cell. Figure 2 is a circuit diagram of an unclamped complementary transistor switch (CTS) type memory cell which is employed in the preferred embodiment of the improved random access memory in accordance with the invention. Figure 3 depicts a structural layout of a half portion of the unclamped complementary transistor switch (CTS) type memory cell of Figure 2. Figure 4 is a circuit diagram corresponding to the structural layout of the half portion of the unclamped complementary transistor switch (CTS) type memory cell of Figure 3. Figure 5 is a circuit schematic of a random access read/write memory array in accordance with the preferred embodiment of the invention. Figure 6 is a detailed circuit diagram of the write circuit and write complete circuit employed in random access read/write memory array of Figure 5 and in accordance with the preferred embodiment of the invention. Figure 7 discloses idealized waveforms to be viewed in conjunction with Figures 5 and 6 and the hereinafter explanation of the "read operation" and the "write operation" of the random access memory array in accordance with the invention. Refer to figures 6 and 7 during the following explanation of the read and write operation. For a cell to be read or written it must be in a fully selected state, cells in a half selected state are immune from read or write operations. A cell is selected in the word dimension by lowering the voltage on the word and drain line (WL and DL) it is connected to. A cell is selected in the bit dimension by raising the voltage on the bit lines (BL and BR) it is connected to. A cell that is connected to a selected pair of bit lines and selected word/drain lines is fully selected, and its content may be read or altered. Reading the content of a cell is accomplished by sensing a differential voltage or current in the bit lines. For example, if the cell in figure 6 contains a "0", which could be defined by transistor NPNL conducting, it could be read by the bit decode circuit raising the voltage of node BIT, and the word decode lowering the voltage of node DL. If transistor NPNL was conducting then in fully selected state collector current would flow thru resistor RBL and diode DL, the base current for NPNL is supplied thru resistor RBR and diode DR. Transistor PNPL is slightly conducting while PNPR and NPNR are nonconducting. The differential current that flows in the bit lines is determined by the beta of the NPN transistor in the cell. Typically the beta value is 100 and the voltage difference between the bit lines or the differential current between transistors TBSL and TBSR can easily be sensed. When the bit decode selects the proper bit column to read, it also does one other thing; it enables the write circuit. If a pulse occurs on the WC line, which is connected to all write circuits, new data will be written into the selected cell. Refer to figures 6 and 7 for an example how a "1" would be written into a cell that contains a "0". The W1 and W0 lines are always out of phase with each other, for the case of writing a "1" the W1 line would be high and W0 low. The cell is brought into the fully selected state by the word and bit decoders. The write circuit is enabled by raising the BS line. The writing is accomplished by a pulse on the WC line. When node WC goes high current will flow thru resistor RL and transistor TCL because W1 is high and W0 is low. Resistors RL and RR regulate the amount of current thru transistors TCL or TCR. When a PN junction is forward biased it is well known that it stores charge in its depletion layer. This charge is what is used to write the cell. When WC falls transistor TWC turns off. Node C will rise to the positive supply thru resistor RC. The charge that was stored in the base-collector junction of transistor TCL gets coupled onto node L. The rise in voltage of node L turns on transistor TBL and raises the voltage of the left bit line BL above the right bit line BR. It should be noted that this technique will raise node L above the positive supply VC. The cell is written when rising left bit line becomes high enough to turn on transistor NPNR. At this point in time the current in the bit line switches from being the collector current of transistor NPNL to base current of transistor NPNR. The state change of the cell, from 0 to 1, corresponds to an impedance change seen on the bit line by transistor TBL. Because bit line BL is now in a high impedance state there is no good discharge path for node L. If this condition were to remain then the contents of the next selected cell in that bit column could be altered. To remedy that condition diodes D1 and D2 are connected from node L to the opposite bit line that was raised. At the time that BL changed from supplying the collector current of NPNL to supplying the base current of transistor NPNR, bit line BR changed from supplying base current of NPNL to collector current of NPNR. As the current in BR increases its voltage drops to the point where diodes D1 and D2 conduct and discharge node L. The base and collector currents of transistor NPNR are eventually supplied by transistors TBSL and TBSR respectively as the write circuit shuts itself off. This self limiting feature of the write circuit makes possible high speed write operation and very low power dissipation. It can be appreciated that the write circuit uses power only when the write pulse is high, typically a low duty cycle. At all other times and for all unselected bit columns the write circuit dissipates zero power.

Claims: Random access memory array having a plurality of memory cells connected between a pair of bit lines, word lines and decoders, circuit means connected to said plurality of memory cells for selecting any predetermined one of said plurality of memory cells, write circuit means connected to said pair of bit lines for writing data into said selected predetermined one of said plurality of memory cells, characterized by an end of write shut down circuit (Fig. 6) consisting of unidirectional elements (D1, D2, D3, D4) which are connected between a control electrode (L,R) of the write circuit of one bit line (BL, BR) and the other bit line (BR,BL) of the pair of bit lines (BL, BR), wherein said end of write shut down circuit deactivates the write circuit at least slightly in anticipation of the completion of the writing of data into said selected predetermined one of said plurality of memory cells. Random access memory array, as recited in claim 1, characterized in that each of said unidirectionally conductive circuit means consists essentially of at least one diode (D1 or D2). Random access memory array, as recited in claim 1, characterized in that each of said unidirectionally conductive circuit means comprises first and second serially connected Schottky diodes (D1 and D2). Random access memory array, as recited in claim 1, characterized by m pairs of bit lines (BL; BR), each pair of bit lines containing a first bit line (BL) and a second bit line (BR), each column of memory cells being associated with a discrete one of said m pairs of bit lines, and each memory cell of each column of memory cells being connected between the first bit line and the second bit line of the pair of bit lines associated with the column of memory cells; by n pairs of word/drain lines (WL, DL), each pair of word/drain lines containing a word line (WL) and a drain line (DL), each row of memory cells being associated with a discrete one of said n pairs of word/drain lines, and each memory cell of each row of memory cells being connected between the word line and drain line of the pair of word/drain lines associated with the row of memory cells; by m write "0" circuits, each of said m write "0" circuits being associated with a discrete one of said m bit line pairs (BL; BR) and connected to the first bit line of said discrete one of said m bit line pairs (BL; BR); by a first group of m end of write circuits, each one of said first group of m end of write circuits being connected between a discrete one of said m write "0" circuits and the second bit line of the bit line pair associated with said discrete one of said m write "0" circuits; by m write "1" circuits, each of said m write "1" circuits being associated with a discrete one of said m bit line pairs and connected to the second bit line of said discrete one of said m bit line pairs; by a second group of m end of write circuits, each one of said second m end of write circuits being connected between a discrete one of said m write "1" circuits and the first bit line of the bit line pair associated with said discrete one of said m write "1" circuits; by word-drain line selection circuit means, said word-drain line selection circuit means in response to receipt of a word-drain line address selecting any predetermined pair of said n pairs of word-drain lines; by bit line pair selection circuit means, said bit line pair selection circuit means in response to receipt of a bit line pair selection address selecting any predetermined pair of said m pairs of bit lines; by controllable circuit means, said controllable circuit means connected to said m write "0" circuits and said m write "1" circuits and cooperating with said word-drain line selection means and said bit-line pair selection circuit means, said controllable circuit means initiating either a "write operation" or a "read operation" whereby during a "write operation" a bit ("1" or "0") will be written into a selected memory cell and during a "read operation" a bit ("1" or "0") will be read from a selected memory cell; and by sense amplifier circuit means connected to said m pair of bit lines; said sense amplifier electrically manifesting the binary bit ("1" or "0") read during a "read operation". Random access memory array, as recited in claims 1 or 4, wherein each memory cell is a complementary transistor switch (CTS) memory cell.

Inventors: JORDY, GEORGE JOHN, MOSLEY, JOSEPH M.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: G11C11
IPC codes: G11C11