Dynamic Transistor-Storage Element

1. Field of the Invention

This invention relates to a dynamic transistor storage element of the type in which, in addition to the transistors of the storage element, at least one MI.sub.1 I.sub.2 S storage transistor is provided for receiving the data stored in the storage element.

2. Description of the Prior Art

Dynamic transistor storage elements are known in the art. For example, the German published application No. 2,148,896 describes, among other things, a one-transistor storage element which comprises a capacitor and a field effect transistor. For reading out data from a one-transistor storage element, the transistor of the storage element, which is connected on the one hand to the bit line and on the other hand to a capacitor, is rendered conductive by way of its gate terminal which is connected to a selection line. This causes the data stored in the form of a charge in the storage capacitor to flow onto the bit line via the selected field effect transistor.

Dynamic storage elements are also known in the art which have three transistors. For example, the publication "New 4,096-bit MOS Chip Is Heart Of Fast, Compact Computer Memory," Electronics, Dec. 18, 1972, Pages 97 et seq describes a storage element of this type.

The publication of L. W. Terman: "MOSFET Memory Circuits," Proceedings of the IEEE, Vol. 59, No. 7, July 1971, pp. 1044 to 1058 describes a four-transistor storage element. This is a dynamic storage element comprising two selection transistors and two switching transistors having feedback. In each case, one selection transistor is connected, on the one hand, to a bit line and, on the other hand, to a junction of the storage element. The selection transistors are commonly operable by way of a word line.

A disadvantage of such dynamic storage elements resides in the fact that the stored data must be regenerated at intervals of approximately 1 to 100 ms. A further disadvantage resides in the fact that when the supply voltage is disconnected, the data stored in the storage elements is lost.

Therefore, the primary object of the present invention is to provide a storage element, as mentioned in the introductory portion of this paper, in which the stored data can be stored for a longer period of time.

This object is realized with the provision of a storage element, as described generally in the foregoing, which is characterized by the feature that at least one MI.sub.1 I.sub.2 S storage transistor is provided for receiving the data storage in the storage element.

An advantageous feature of a storage element constructed in accordance with the invention consisting that the mode of functioning of the dynamic one-transistor storage element is not disturbed in normal operation by the MI.sub.1 I.sub.2 S transistor which is additionally connected, in accordance with the invention, and that the data is only restored into the MI.sub.1 I.sub.2 S transistor in the event of long storage periods.

Advantageously, the data input in this manner into the MI.sub.1 I.sub.2 S transistors can be stored for a long period of time which can be extended up to ten years.

In accordance with another advantage of the invention, data can be written-in by elements, by rows, or also in matrix form.

Advantageously, the data can be erased row-by-row, in which case the data can be intermediately stored in the regenerator amplifiers.

Other objects, features and advantages of the invention, its organization, construction and operation, will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, on which:

FIG. 1 is a schematic circuit diagram of a dynamic two-transistor storage element constructed in accordance with the invention;

FIG. 2 is a schematic circuit diagram of a three-transistor storage element constructed in accordance with the invention;

FIG. 3 is a schematic circuit diagram of a four-transistor storage element constructed in accordance with the invention;

FIG. 4 is a schematic circuit diagram of a dynamic circuit diagram of a dynamic shift register constructed in accordance with the invention.

Referring to FIG. 1, a transistor 1 of a two-transistor storage element is connected to a reference potential, here ground, by way of a capacitor 2. Preferably, the transistor 1 is a MOS field effect transistor. The transistor 1 is connected, at a point 8, to the capacitor 2 as illustrated in FIG. 1. The capacitor 2 is also connected to a terminal 5 which, as mentioned above, is preferably connected to ground. The gate terminal of the transistor 1 is connected to a word line 7. The transistor 1 is also connected to a bit line 4 as illustrated in FIG. 1.

In accordance with the invention, a MI.sub.1 I.sub.2 S transistor 3, preferably a MNOS transistor, is connected between the junction 8 and the word line 7 in the manner illustrated, and the gate terminal of the MNOS storage transistor 3 is connected to a gate line 6.

In the following, the mode of operation of a dynamic two-transistor storage element constructed in accordance with the invention will be described. If, for example, the data "1" has been written into the storage element, the capacitor 2 is charged and the source terminal (the junction 8) of the MNOS storage transistor 3 carries a negative potential, e.g. approximately -15 V, and if, however, simultaneous to the write-in process, or just afterwards, a voltage U.sub.7 of, e.g. approximately -15 V is connected to the word line 7, and a voltage pulse U.sub.6 of, e.g. approximately -30 V is connected to the gate line 6, the threshold voltage of the MNOS storage transistor 3 is not shifted. If, on the other hand, a "0" is stored, the threshold voltage of the MNOS storage transistor 3 is displaced in the direction of negative voltage values, as during the write-in process the source terminal carries ground potential.

For reading out of information--assuming a p-channel MNOS storage transistor 3--a voltage -U.sub.7 ' of, e.g. approximately -15 V is connected to the word line 7, and a read-out voltage -U.sub.L of, e.g. approximately -8 V is connected to the gate line 6. If the MNOS storage transistor possesses a high negative threshold voltage, it is blocked and the capacitor 2 remains uncharged. If, on the other hand, the threshold voltage of the MNOS capacitor 3 is only slightly negative, it becomes conductive on the connection of a gate voltage of -U.sub.L and charges the capacitor 2 virtually to the voltage -U.sub.L. Then, the voltage -U.sub.L likewise occurs on the bit line 4, since the transistor 1 is conductive. This voltage on the bit line 4 corresponds to a stored "1".

If the MNOS storage transistor 3 is not to be operated, the potential O V is connected to the gate line 6. Then, the MNOS storage transistor 3 advantageously has no influence on the normal operation of the dynamic one-transistor storage element.

When a n-channel MNOS transistor is employed as a storage transistor 3, and a switching transistor 1, for write-in and read-out of data, the corresponding potentials of the opposite polarity are connected to the gate line 6, and to the word line 7, and to the bit line 4.

In FIG. 2, the transistors 1*, 2* and 3* and a capacitor 11* form a dynamic three transistor storage element. Here, the transistors 2* and 3* are connected in series in the manner illustrated on the drawing and are located between a bit line 6* and a terminal 9*. Preferably, the terminal 9* carries ground potential. The transistor 1* is connected, on the one hand, to the bit line 6* and, on the other hand, to the gate terminal of the transistor 3*. The gate terminals of the transistors 1* and 2* are connected to each other. The gate terminals of the transistors 1* and 2* are connected to the bit line 5* which serves as a word line. In accordance with the invention, the MNOS storage transistor 4* is connected to the dynamic three transistor storage element in the manner illustrated on the drawing. Here, the MNOS storage transistor 4* is connected by its source terminal, in a manner illustrated, at a junction point 10* to the drain terminal of the transistor 3* and to the source terminal of the transistor 2*. The drain terminal of the MNOS storage transistor 4* is connected to the read-out line 7* and the gate terminal of this MNOS switching transistor 4* is connected to the gate line 8*. Preferably, the voltage supply U.sub.DD is connected to the line 6* (-10 V).

In the following, the mode of operation of the dynamic three transistor storage element, in accordance with the invention, is explained. If a "1" is input into the capacitor 11* of the storage element, the transistor 3* is in the conductive state. This means that the source terminal of the transistor 4* carries the potential of the terminal 9*--preferably, ground potential. When a "0" is stored, on the other hand, the transistor 3* blocks, which means that the source terminal of the MNOS storage transistor 4* is free of potential. If, now when a p-channel MNOS storage transistor 4* is employed, a negative voltage pulse is connected to the gate line 8* and to the read-out line 7*, when a "1" is stored the threshold voltage of the MNOS transistor 4* shifts towards negative voltage values, whereas when a "0" is stored, the threshold voltage of this transistor remains at its original value. This is due to the fact that when a "1" is stored, the full negative voltage is active across the gate of the MNOS storage transistor 4*, whereas this is not the case when a "0" is stored. Preferably, a negative voltage pulse of approximately -30 V/10 .mu.s is connected to the gate line 8* and to the read-out line 7*.

On read-out of data, i.e. when the data stored in the MNOS storage transistor 4* is to be read back into the dynamic three transistor storage element, the read-out line 7* is connected to ground potential, whereas the gate line 8* is connected to the read-out voltage -U.sub.L. In dependence upon the nature of the stored data, the bit line 6* is now connected, by way of the transistor 2* and by way of the MNOS transistor 4* to ground potential, or to supply voltage, when the storage transistor 4* blocks. In dependence upon whether the storage transistor 4* is conductive or is blocked, a read-out current either flows by way of the transistor 2* and via the storage element 4* onto the line 7*, or does not so flow.

When the data is restored out of the dynamic three transistor storage element into the MNOS storage transistor 4*, the data is inverted.

A further advantage is achieved in that the transistor 1* is designed as a MNOS transistor. However, the threshold voltage of the MNOS transistor 1* is to be made more negative than the threshold voltage of the transistor 2*, in order to prevent simultaneous write-in by way of the transistor 1* during read-out. Therefore, one does not require two different oxide thicknesses, as are required in the storage element described in the publication "New 4,096 bit MNOS Chip Is Heart Of Fast, Compact Computer Memory," Electronics, Dec. 18, 1972, Pages 97 to 103.

When a MNOS-n-channel transistor is employed as a storage transistor 4*, corresponding potentials of the opposite polarity are connected to the gate line 8* and to the read-out line 7* for write-in and read-out operations.

In a further development of the invention, the dynamic three transistor storage element can also be constructed in such a manner that the gate line of the transistors 1* and 2* are separately extracted. This has the advantage that these transistors can be operated separately during write-in and read-out operations.

An advantage of the three transistor storage element, in accordance with the invention, resides in the fact that its mode of operation is in no way disturbed by the MNOS transistor in accordance with the invention and that only in the event of long storage periods is the data restored into the MNOS storage transistor.

In FIG. 3 of the drawings, the conventional dynamic four transistor storage element is formed from the transistors 1', 2', 3' and 4'. The transistors 1' and 4' represent the selection transistors. The transistor 1' is connected, on the one hand, to a bit line 7' and, on the other hand, to the junction point 11' of the storage element. Correspondingly, the transistor 4' is connected, on the one hand, to the junction point 12' of the storage element and, on the other hand, to the bit line 8'. The transistors 1' and 4' are commonly operable by way of a word line 9', and therefore, the gate terminals of these transistors are connected to the word line 9'. The transistors 2' and 3' represent the switching transistors of the dynamic storage element. The source zones of the transistors 2' and 3' are connected to the terminal 14'. A drain zone of the switching transistor 2' is connected to the junction point 11' of the flip-flop formed in this circuit and the drain zone of the transistor 3' is connected to the junction point 12' of this flip-flop. The gate terminal of the switching transistor 2' is connected to the junction point 12' of the storage element and the gate terminal of the switching transistor 3' is connected to the junction point 11' of the storage element.

Preferably, MOS transistors are employed as selection transistors for the transistors 1' and 4' and as the switching transistors 2' and 3'.

The MI.sub.1 I.sub.2 S transistors 5' and 6' illustrated in FIG. 3, which are preferably MNOS transistors, are additionally connected to the dynamic four transistor storage element in a manner illustrated on the drawing. On the one hand, the MNOS storage transistor 5' is connected to the junction 11' of the storage element and, on the other hand, to the line 13'. The MNOS storage transistor 6' is, on the other hand, connected to the junction point 12' of the storage element and, on the other hand, is also connected to the line 13'. The two MNOS storage transistors 5' and 6' are commonly operable by way of the gate line 10' and, therefore, their gate terminals are connected to the gate line 10'. Preferably, the source terminals of the switching transistors 2' and 3' which are connected to the terminal 14' are connected to ground potential.

The mode of operation of the process in accordance with the invention for the operation of a dynamic four-transistor storage element will now be described. Here, it has been assumed that initially the two MNOS storage transistors 5' and 6' each store a "0" which means that the start voltage of the two MNOS storage transistors 5' and 6'--assuming p-channel transistors--amounts to approximately -2 V. Now, an item of data is to be written into the storage element by way of the bit lines 7' and 8'. For this purpose, first of all the two selection transistors 1' and 4' are rendered conductive by way of the word line 9'. This ensures that the item of data present on the bit lines 7' and 8' passes to the junction points 11' and 12' of the storage element. For example, this item of data causes the switching transistor 2' to be set into the conductive state and the switching transistor 3' into the blocking state. Now, in order to restore this data into the MNOS storage transistors, a high negative voltage pulse is connected to each of the lines 10' and 13'. For example, a voltage of -30 V is connected for approximately 10 .mu.s. Since the source terminal of the MNOS storage transistor 5' is connected to ground potential by way of the conductive switching transistor 2', the full negative voltage is active across the gate and the threshold voltage of the MNOS storage transistor 5' is displaced in the direction of negative voltage values. For example, the threshold voltage of the MNOS storage transistor 5' then amounts to approximately -12 V. As the switching transistor 3' blocks, and therefore the source terminal of the MNOS storage transistor 6' carries no ground potential, the gate insulator of this transistor is connected to only a small voltage which is not sufficient to displace the threshold voltage of the MNOS storage transistor 6'. The data carried by the bit lines 7' and 8' has therefore been restored into the two MNOS storage transistors 5' and 6' through the connection of a high negative voltage pulse to the gate line 10' and to the line 13'.

In order that this item of data contained in the MNOS storage transistors 5' and 6' may be re-input into the dynamic four transistor storage element, the supply voltage of the storage circuit is connected to the line 13'. Preferably, this voltage amounts to approximately -20 V. At the same time, the other line 10' is connected to a read-out voltage -U.sub.L having a value which corresponds approximately to the arithmetic mean of the two threshold voltages of the MNOS storage transistors 5' and 6'. This example is again based on the utilization of p-channel transistors. With this voltage -U.sub.L the MNOS storage transistor 6' becomes conductive, whereas the MNOS storage transistor 5' is placed in a blocking condition. The switching transistor 2' is rendered conductive, via the conductive MNOS storage transistor 6', in that part of the potential of the line 13' reaches the gate terminal of the switching transistor 2'. As the MNOS storage transistor 5' is in a blocking condition, the switching transistor 3' is not rendered conductive. Therefore, the data is re-input into the dynamic storage element.

As a result of the connection of a positive potential of approximately +30 V to the gate line 10', the data written in the MNOS transistors is erased.

This process is explained in detail by G. Dorda & M. Pulver in the publication Tunnel Mechanism In MNOS Structures In Physics States Solid (a) 1, 71 (1970) Pages 71-79.

As previously explained above, the storage element constructed in accordance with the invention may also be employed as a static six transistor storage element. In this case, the two MNOS transistors 5' and 6' serve as load transistors of the flip-flop circuit. The transistors 2' and 3' represent switching transistors of the flip-flop circuit. The supply voltage -U.sub.DD, which preferably amounts to approximately -30 V is present between the point 14' and the line 13'.

The erasure and write-in of data is effected in the manner described above in association with the dynamic four transistor storage element. This means that for the write-in of data, a negative potential of approximately and preferably -30 V is connected to the gate line 10', and that for the erasure of the data, a positive potential of, preferably and approximately +30 V is connected to the gate line 10'. During the read-out process, a potential of preferably and approximately -15 V is connected to the gate line 10'. This ensures that the flip-flop circuit triggers into the state which is impressed with the stored data.

The fact that the data stored in the MNOS transistors can disintegrate very rapidly when a dc voltage is connected to the gate electrodes is of no significance, since the magnitude of the start voltage of the MNOS transistors is of no further importance after the triggering of the flip-flop circuit.

When MNOS n-channel transistors are employed, the corresponding potentials of the opposite polarity are connected to the gate line 10' and to the line 13'.

Advantageously, two different items of data can be stored simultaneously in the storage element, in accordance with the invention, in which case one item data is stored in the MI.sub.1 I.sub.2 S storage transistor, whereas the other item of data is stored in the dynamic storage element.

Advantageously, the data stored in the four transistor storage element can be input into the MNOS transistors by virtue of one single pulse.

The mode of operation of the dynamic four transistor storage element in normal operation is advantageously not disturbed by the additionally connected MI.sub.1 I.sub.2 S transistors. The data is only restored into these MI.sub.1 I.sub.2 S transistors in the event of long storage periods.

Referring to FIG. 4, the conventional, known dynamic shift register is formed by the transistors 1" to 6". The shift register basically comprises two inverters. The inverter 10" comprises the transistors 1" and 2" and the inverter 20" comprises the transistors 4" and 5". These inverters are connected in a manner illustrated in FIG. 4 via a transistor 3". By way of the line 12" and by way of the terminals 21" and 51" the supply voltage U.sub.DD is connected to the inverters 10" and 20". Preferably, the line 12" is connected to ground potential. The transistors 2" and 5" serve as load elements for the circuit. Preferably, the transistors 1" and 6" are MOS field effect transistors, the transistors 2" and 5" being connected in a known manner as static or dynamic load resistors.

The data passes by way of the terminal 11" into the first inverter 10" which comprises the transistors 1" and 2", and the data is advanced with the aid of a pulse train .phi.31" which is connected to the terminal 31" of the transistor 3", to the second inverter 20" which comprises the transistors 4" and 5". From the latter circuit, the data is advanced to a terminal 62" by way of the transistor 6" which is pulsed by a pulse train .phi.61" connected to the terminal 61". Additional inverters are connected to terminal 62" which forms the output of the shift register.

In accordance with the invention, MNOS storage transistors are additionally connected, in the manner illustrated in FIG. 4, to the known dynamic shift register. To the first inverter 10" a MNOS storage transistor 7" is connected in such a manner that its source terminal is connected to the source terminal of the transistor 3" and its drain terminal is connected to a line 9". The gate terminal of the MNOS storage transistor 7" is connected to a gate line 13". As illustrated in FIG. 4, the MNOS storage transistor 8" is connected to the second inverter 20" in such a manner that its source terminal is connected to the drain terminal of the transistor 6" and its drain terminal is connected to the line 9". The gate terminal of the MNOS storage transistor 8" is, in turn, connected to the gate line 13".

If, for example, the data "1" is input into the dyanmic storage element, the transistor 1" is rendered conductive by way of the terminal 11" which forms the input of the shift register. This ensures that the source terminal of the MNOS storage transistor 7" carries ground potential. If, on the other hand, the data "0" is input into the shift register, the transistor 1" is blocked by way of the terminal 11". This results in the fact that the source terminal of the MNOS storage transistor 7" carries the supply voltage potential -U.sub.DD. When the data has been written into the shift register, in accordance with the invention, a voltage pulse is connected to the line 9" and to the gate line 13". Assuming the use of p-channel MNOS storage transistors, a single voltage--preferably and approximately -30 V--is connected for approximately 10 .mu.s. In dependence upon whether the source terminal of the MNOS storage transistor 7" is connected to ground potential, or to the potential of the supply voltage, either the threshold voltage of this MNOS transistor is displaced toward a negative value, or is not so displaced.

On read-out of information, as a result of the connection of the read-out voltage -U.sub.L to the gate line 13", the data from the MNOS transistor 7" is re-input into the shift register. In this case, the supply voltage -U.sub.DD is connected to the line 9". During this operation, the data stored in the shift register is inverted.

When the pulse train .phi.31" has caused the data to pass by way of the transistor 3" to the second storage element, the foregoing statements in respect of the MNOS storage transistor 7" also apply to the MNOS storage transistor 8".

When n-channel MNOS transistors are employed, for the write-in and read-out of data the corresponding potentials of the opposite polarity are connected to the line 9" and to the gate line 13".

Advantageously, it is also possible to utilize quite generally MI.sub.1 I.sub.2 S transistors instead of the MNOS storage transistors. Preferably, one employs MNOS transistors having a dielectric which consists of a SiO.sub.2 layer having a thickness of approximately 1 to 3 nm and of a Si.sub.3 N.sub.4 layer having a thickness of approximately 30 to 60 nm.

The data stored in the dynamic shift register is advantageously input into the MNOS storage transistors in accordance with the invention by one single pulse.

Although I have described my invention by reference to particular illustrative embodiments thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.