Abstract:
A random access memory having a multiplicity of memory cells having logic states that can be changed by a control voltage. At least some of the memory cells include an additional device that can be activated by means of an enforced control voltage that is different from the control voltage, in order to impress a defined logic state on the memory cell.

Description:
BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0001]    The invention relates to a random access memory (RAM) including a multiplicity of memory cells that each have a memory device with a memory content which establishes a logic state that can be changed by a control voltage.  
           [0002]    In numerous applications, an electronic device, such as a personal computer, for example, requires a RAM having a defined memory content in order to start. In the case of a personal computer, this defined memory content is, for example, the operating system BIOS (Basic Input/Output System). During use, the memory content is generally overwritten and modified. Under special circumstances, for example, whenever rebooting of the device is required, the original memory state must be restored in the RAM. This has previously been accomplished by performing the time-consuming operation of rewriting the RAM from an external source. In the case of a personal computer, the rewriting would be performed from a hard disk. In the case of certain types of devices, for example, in portable applications, such as cellphones for instance, this rewriting operation usually cannot be accomplished by the user alone.  
           [0003]    There is therefore a need to rewrite the contents of a RAM to the original memory state in a manner that can be accomplished at any time and at a high speed without a great amount of time having to be expended.  
         SUMMARY OF THE INVENTION  
         [0004]    It is accordingly an object of the invention to provide a RAM which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.  
           [0005]    With the foregoing and other objects in view there is provided, in accordance with the invention, a random access memory that includes a plurality of memory cells. Each one of the plurality of the memory cells has a first memory device with a memory content defining a logic state. Each one of the plurality of the memory cells receives a first control voltage for changing the memory content of the first memory device. At least some of the plurality of the memory cells have a second memory device with a memory content defining a logic state. Each one of the plurality of the memory cells which include a second memory device receive a second control voltage for impressing the logic state of the second memory device on the first memory device. The second control voltage is an enforced control voltage that is different from the first control voltage.  
           [0006]    In accordance with an added feature of the invention, the RAM includes a plurality of word lines and a plurality of bit lines, and each one of the plurality of the memory cells is formed as a DRAM memory cell. The second memory device of one of the plurality of the memory cells having a second device is connected between one of the plurality of the word lines and one of the plurality of the bit lines, and the second control voltage is greater than the first control voltage.  
           [0007]    In accordance with an additional feature of the invention, the second memory device of one of the plurality of the memory cells includes a diode with an adapted breakdown voltage.  
           [0008]    In accordance with another feature of the invention, the second memory device of one of the plurality of the memory cells includes a transistor with an adapted turn-on voltage.  
           [0009]    In accordance with a further feature of the invention, each one of the plurality of the memory cells has a second memory device. The second memory device of each one of the plurality of the memory cells can optionally be connected between one of the plurality of the word lines and one of the plurality of the bit lines.  
           [0010]    In accordance with a further added feature of the invention, there is provided, a contact hole for connecting the second memory device of a selected one of the plurality of the memory cells between one of the plurality of the word lines and one of the plurality of the bit lines.  
           [0011]    In accordance with a further additional feature of the invention, there is provided, an electronic fuse for connecting the second memory device of a selected one of the plurality of the memory cells between one of the plurality of the word lines and one of the plurality of the bit lines.  
           [0012]    In accordance with a concomitant feature of the invention, each one of the plurality of the memory cells is formed as an FeRAM cell. One of the plurality of the memory cells includes a line, a first ferroelectric capacitor that defines the first memory device, a second ferroelectric capacitor that defines the second memory device and that is connected in parallel with the first ferroelectric capacitor, and a transistor. The second ferroelectric capacitor is connected between the transistor and the line. The first ferroelectric capacitor stores a logic state by applying a first activation voltage. The second ferroelectric capacitor stores a logic state by applying a second activation voltage. The first activation voltage is smaller than the second activation voltage. The logic state of the second ferroelectric capacitor is read out with the second control voltage, which is greater than the second activation voltage, in order to impress the logic state of the second ferroelectric capacitor on the first ferroelectric capacitor. The logic state of the first ferroelectric capacitor is variable with a control voltage that is greater than the first activation voltage and smaller than the second activation voltage. The logic state of the second ferroelectric capacitor is invariable.  
           [0013]    In other words, “hidden bits” are used to store a defined memory content in the RAM itself in an easy and quickly recoverable manner. The invention specifically envisages storing an additional hidden bit at least in some of the memory cells of the RAM. This hidden bit may be defined, for example, by wiring or mask programming or else by the state of an additional ferroelectric storage capacitor.  
           [0014]    Generally, the hidden bit is provided by an additional memory device in the memory cell of the RAM which can be activated by an enforced control voltage, which differs from the control voltage of the memory cell. What is important here is that an originally established memory content of the RAM can be recovered in a simple way by applying one and the same enforced control voltage to the memory cells concerned. The enforced control voltage ensures that the memory contents of the memory cells can be reset to the original contents at a higher speed than was previously possible by time-consuming rewriting of the memory content of the memory cells. The only precondition is that the enforced control voltage differs in a defined way from the control voltage with which the logic state of the memory cells of the RAM is usually changed or defined.  
           [0015]    The modified RAM having hidden bits can be a DRAM or an FeRAM (Ferro-Electric Random Access Memory).  
           [0016]    In the case of the DRAM, the additional memory device establishing the hidden bit includes an additional component between the word line and the bit line of the DRAM memory cell. The additional component is activatable by means of an enforced control voltage which is greater than the normal control voltage on the bit line. This additional component may either be a diode with an adapted breakdown voltage or a transistor with an adapted turn-on voltage.  
           [0017]    To easily produce the RAM modified with hidden bits, all of its memory cells are preferably provided with the additional memory device in the form of the additional component, which can then optionally be connected to the word and bit lines. This optional connection preferably takes place by means of a contact hole, which is either made or not made.  
           [0018]    In the case of a RAM in the form of an FeRAM, by contrast with the aforementioned DRAM, the information of a hidden bit can also be modified. This is achieved by connecting the additional memory device between the word line and the bit line of the DRAM memory cell and by providing first and second ferroelectric capacitors. The first and second ferroelectric capacitors each store a bit by applying an activation voltage Vc 1  and Vc 2  (Vc 1 &lt;Vc 2 ), respectively, by means of a selection transistor. It is possible for the bit (hidden bit) of the second ferroelectric capacitor C 2  to be read out with a voltage (enforced control voltage) U&gt;Vc 2  in order to impress the defined logic state on the first capacitor during the subsequent rewriting of the memory cell. The bit of the first capacitor C 1  is variable with a voltage Vc 1 &lt;U&lt;Vc 2 , but the bit of the capacitor C 2  is invariable.  
           [0019]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0020]    Although the invention is illustrated and described herein as embodied in a random access memory, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0021]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 schematically shows a memory cell of an inventive DRAM; and  
         [0023]    [0023]FIG. 2 schematically shows a memory cell of an inventive FeRAM. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown an RAM that includes a word line WL, a bit line BL, and also a line PL (common plate line), which are linked in a conventional way to a selection transistor T. This construction of a DRAM cell corresponds to this extent to the standard cell construction. As an additional component, also referred to here as an “additional device”, a component Vr is arranged between the word line WL and the bit line BL. The additional component Vr is intended to connect through with a voltage Uv, which lies above the normal operating voltage of the bit line BL.  
         [0025]    Otherwise, the component Vr is to be high-ohmic. This component Vr may be, for example, in the form of a diode with an adapted breakdown voltage or a transistor with an adapted turn-on voltage.  
         [0026]    By breaking and making the connection between the component Vr and the word line WL or the bit line BL, the logic state of a hidden bit is established. This operation is also referred to as mask programming and is indicated in FIG. 1 by a cross between the component Vr and the bit line BL. The connection may be established in this way, for example, by means of a contact hole, which, according to the desired memory content, is either provided or omitted.  
         [0027]    Since the component Vr is normally high-ohmic, the memory cell of the DRAM shown behaves during operation in just the same way as a standard DRAM cell.  
         [0028]    The DRAM contains a multiplicity of cells modified in this way, to which it is possible during normal operation to write information that can also be erased from corresponding cells. If it is required to reestablish the original memory state, for example, for rebooting a personal computer, the state of the multiplicity of the hidden bits is rewritten in the memory cells. For this purpose, first the entire memory, i.e. the multiplicity of memory cells, is written with a logic “0”.  
         [0029]    Then, a voltage U&gt;U v  is applied to the word line WL. On account of the voltage U, the component Vr connects through and a capacitor C is written with a logic “1”. This operation applies exclusively to those memory cells which are electrically connected to the word and bit lines by the mask programming of the components Vr. The memory as a whole has the information of the hidden bits written to it, and consequently again has the original memory content.  
         [0030]    This operation may also optionally be used to write to only some of the memory cells.  
         [0031]    It is important that the writing operation according to the invention can be performed at a fast rate that is similar to that of a “refresh”, and consequently is much faster than in the case when the original information is conventionally loaded from an external source, such as a hard disk, for example.  
         [0032]    [0032]FIG. 2 shows a FeRAM memory cell that has been modified according to the invention. The FeRAM memory cell includes a word line WL, a bit line BL, and a line PL (common plate line) that are each connected in a conventional way to a selection transistor. In this regard, the construction of the FeRAM cell corresponds to the construction of a standard cell. An additional component, also referred to as an “additional device”, in the form of a second ferroelectric capacitor C 2  is arranged between the selection transistor and the line PL of the cell, in parallel with a first ferroelectric capacitor C 1  of the cell. The second ferroelectric capacitor C 2  is provided in order to store and read out the information of a hidden bit in the cell when the information is intended to impress a defined logic state on the cell. This is achieved by the fact that, for the memory cell concerned, the two ferroelectric capacitors C 1  and C 2  are provided with different voltages Vc 1  and Vc 2  (Vc 1 &lt;Vc 2 ) for storing two bits per selection transistor T. The hidden bit is written in the second capacitor C 2  with the higher Vc 2 . As long as only the first capacitor C 1 , which is used for the normal storing operation, is written to and read from with a voltage Vc 1 &lt;U&lt;Vc 2 , the information in the second capacitor C 2  remains undisturbed (U=control voltage of the FeRAM memory cell concerned). If it is intended for the hidden bit to be rewritten to the cell for the sake of recovering the original memory content, first the first capacitor C 1  is brought into a defined state (polarity of the voltage applied to the first capacitor C 1 =polarity of the read-out voltage for the second capacitor C 2 ), whereupon the second capacitor C 2  is read out with a higher voltage U&gt;Vc 2 . In the subsequently required rewriting operation, writing automatically also takes place to the first capacitor C 1  and, as a result, the information of the hidden bit is written into the memory cell. Since the internal reading out and rewriting is carried out block by block with a specific number of memory cells, this method proceeds much faster than the previously used method of loading in external data, for example, from a hard disk.