Abstract:
The invention relates to a semiconductor memory having a non-volatile two-transistor memory cell which has an N-channel selection transistor and an N-channel memory transistor. The drive circuitry for the cell includes a P-channel transfer transistor. A transfer channel is connected to a row line leading to the memory cell. This enables the voltages required for programming to be obtained with relatively little technological complexity.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending International Application PCT/DE98/01970, filed Jul. 14, 1998, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention lies in the semiconductor technology field. More specifically, the invention relates to a semiconductor memory having at least one non-volatile, dual-transistor memory cell which has the following features: 
     an N-channel selection transistor and an N-channel memory transistor; 
     the N-channel selection transistor has a selection gate and two selection channels, the selection gate being connected to a row line leading to the memory cell; 
     the N-channel memory transistor has a memory gate or a control gate and two memory channels; 
     a second memory channel and a first selection channel are connected to one another, the other memory channel or respectively the other selection channel being connected to a column line leading to the memory cell; 
     whereby the semiconductor memory has at least one transfer transistor with a first and a second transfer channel, and the first transfer channel is connected to the memory gate. 
     In the generic semiconductor memories, the individual transistors are implemented in FET technology on a semiconductor substrate. The memory transistor thereby has a floating gate, with the result that it can be programmed, by the application of suitable voltages to the channels and to the gate, in such a way that it can assume a desired state permanently or in a non-volatile fashion. 
     In order to read the memory cell, a memory channel and a selection channel are connected to one another, the other free memory channel or respectively the other free selection channel being connected to a column line leading to the memory cell. In this case, the selection transistor is driven in such a way that it turns on. If a current then flows in the event of a voltage being applied to the corresponding column line, then the memory transistor has been programmed to “conducting”, or written to, in a previous step. If no current flows in the event of the voltage being applied to the column line with the selection transistor turned on, then the memory transistor has been programmed to “nonconducting”, or erased, in a previous step. 
     EP 0317 443 A1 discloses a two-transistor memory cell comprising a selection transistor and a floating gate transistor. A special voltage is applied to the gate of the floating gate transistor for driving purposes. 
     In the case of the memories of the generic type, the fact that the voltages required for programming have to be generated with high technological complexity is particularly problematic. Furthermore, in the course of programming one memory cell, faults are frequently produced in other memory cells which are not currently selected for programming. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a semiconductor memory device with non-volatile two-transistor memory cells, which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which allows programming of the semiconductor memory in a fault-free manner with little technological complexity. 
     With the above and other objects in view there is provided, in accordance with the invention, a semiconductor memory with at least one memory cell, comprising: 
     an N-channel selection transistor having a selection gate connected to the row line, a first selection channel connection connected to the column line and a second selection channel connection; 
     an N-channel memory transistor having a memory gate, a first memory channel connection connected to the second selection channel connection, and a second memory channel connection; 
     the semiconductor memory further includes at least one P-channel transfer transistor having a first transfer channel connection connected to the memory gate and a second transfer channel connection connected to the row line leading to the memory cell. 
     In other words, the objects of the invention are attained by virtue of the fact that the transfer transistor is designed as a P-channel transfer transistor, the second transfer channel, unlike in the prior art, not being connected to an external control gate voltage but rather to the row line leading to the memory cell. 
     The invention is based on the insight that in the case of the circuits of the generic type, it is necessary to overcome a threshold voltage loss in the transfer transistor, so that higher transfer gate voltages have had to be provided. This problem can be solved by designing the transfer transistor as a transistor with a reduced threshold voltage, but this has to be bought at the expense of increased technological complexity. 
     With the way in which the transfer transistor is designed and connected up according to the invention, a transfer gate threshold voltage no longer needs to be overcome in order to program the memory transistor, with the result that reliable programming is possible with little technological complexity. 
     The invention is furthermore based on the insight that the control gate voltage “floats” in an undefined freewheel in the memory cells that are not currently being driven, on account of the particular way in which the transfer transistor is connected up in the prior art, which can lead to capacitive overcoupling of the programming voltages. Such capacitive overcoupling no longer occurs in the memory cells of the semiconductor memory according to the invention, since each memory gate is at a defined state during the programming of the semiconductor memory according to the invention. 
     In the system according to the invention, a logic signal converted to high voltage can be applied to the transfer gate of the transfer transistor. For this purpose, it is expedient to use a logic signal which also drives the respective programming state of the memory cell. In this case, the provision of an inverter, which is complicated to produce, for driving the transfer gate is avoided due to the design of the transfer transistor as a P-channel transfer transistor, since a P-channel transfer transistor turns off when the gate is driven, and vice-versa. In principle, however, the transfer transistor with such an inverter can also be designed as an N-channel transfer transistor. 
     The arrangement according to the invention makes it possible, without any losses and without further special measures, for the full programming voltage to be switched via the channel of the transfer transistor to the memory gates. 
     It should be understood that the invention can also be realized with a memory in which the memory and selection transistors are designed as P-channel transistors if the transfer transistor is then designed as an N-channel transistor. However, such an arrangement is more uncommon, but may afford advantages if so-called “hole conduction” is desired for the transfer of charge carriers. 
     In accordance with an added feature of the invention, a control line is connected to the transfer gate for driving the transfer transistor via the control line. 
     In accordance with an additional feature of the invention, there is provided an N-channel discharge transistor having a discharge gate connected to the control line, and a first discharge channel connection connected to the memory gate. 
     In accordance with another feature of the invention, the N-channel discharge transistor has a second discharge channel connection connected to ground. 
     In this development of the invention the drive circuit has an N-channel discharge transistor, which has a discharge gate and a first and a second discharge channel, the first discharge channel being connected to the memory gate, the second discharge channel being connected to ground, and the discharge gate being connected to that control line via which the transfer transistor is driven. 
     Such a discharge transistor ensures, during the programming of the memory cell, that the memory gate is at a defined potential, in particular at ground potential, during a programming operation. It is precisely when the transfer transistor is turned off that it is thereby ensured that the memory gate is at a potential of 0 V in a defined manner. 
     In accordance with a further feature of the invention, the at least one memory cell is one of a plurality of memory cells arranged in rows and columns, and wherein: 
     within the rows, the selection gates of a plurality of the memory cells are connected in parallel, and the memory gates of a plurality of memory cells are connected in parallel; and 
     within the columns, the first memory channels and respectively the second selection channels are connected in parallel. 
     In accordance with again an added feature of the invention, a drive circuit of at least one of the columns has a P-channel block selection transistor with a block selection gate, a first block selection channel connection connected to the row line leading to the memory cell, and a second block selection channel connection connected to the first transfer channel connection. 
     In accordance with a concomitant feature of the invention, a block selection control line is connected to the block selection gates such that the block selection transistors can be driven via the block selection control line. 
     In other words, the semiconductor memory according to the invention is organized in rows and columns, where, within the rows, the selection gates and the memory gates of a plurality of memory cells are connected in parallel, and where, within the columns, the first memory channels and respectively the second selection channels are connected in parallel. This enables a memory according to the invention to be arranged in rows and columns in a particularly simple manner. 
     In this case, at least one column is provided whose drive circuit has the transfer transistor connected up according to the invention. The drive circuit may additionally have in each case a P-channel block selection transistor having a block selection gate and having two block selection channels, a first block selection channel being connected to a row line leading to a memory cell, and a second block selection channel being connected to the first transfer channel. This enables the semiconductor memory to be divided into individual blocks for programming the memory cells, which is particularly advantageous since this means that it is no longer necessary to program specific states for an entire row of the semiconductor memory, but rather only for a block selected from said row. As a result, it is also possible, in particular, to erase an individual block. For this purpose, a block selection control line is provided, which is connected to the block selection gates in such a way that the block selection transistors can be driven via the block selection control line. 
     The invention also generally relates to a drive circuit for driving at least one memory cell with a transfer transistor which is connected up as above. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a semiconductor memory with non-volatile two-transistor memory cells, 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. 
    
    
     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 
     FIG. 1 is a schematic circuit diagram of a first embodiment of the semiconductor memory according to the invention; 
     FIG. 2 is a schematic circuit diagram of a second embodiment of the semiconductor memory according to the invention; and 
     FIG. 3 is a schematic circuit diagram of a third embodiment of the semiconductor memory according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a circuit diagram of a first semiconductor memory according to the invention. The memory is implemented on a semiconductor substrate. Only a partial region of the semiconductor memory having four memory cells Z 1 , Z 2 , Z 3  and Z 4  is illustrated. The memory cells Z 1 , Z 2 , Z 3  and Z 4  can be driven via two row lines AG 1 , AG 2  and via two column lines SP 1 , SP 2 . 
     In order to drive the memory cells Z 1 , Z 2 , Z 3  and Z 4 , use is made of a drive circuit having a transfer transistor TT 1 , a discharge transistor ET 1 , a transfer transistor TT 2  and a discharge transistor ET 2 , which are driven via a writing signal line WRITX. A signal which is converted to high voltage and is generated from a logic signal which controls the writing operation is present on the writing signal line WRITX. 
     The transfer transistor TT 1  and the transfer transistor TT 2  are produced as P-channel transistors using FET technology, whereas the discharge transistor ET 1  and the discharge transistor ET 2  are designed as N-channel transistors using FET technology. 
     The memory cell Z 1  has a selection transistor AT 1  and a memory transistor ST 1 . The selection transistor AT 1  is produced as a conventional N-channel transistor using FET technology, while the memory transistor ST 1  is implemented as an N-channel transistor with a so-called “floating gate”. A first selection channel of the selection transistor AT 1  is connected to the column line SP 1 , while a second selection channel of the selection transistor AT 1  is connected to a first memory channel of the memory transistor ST 1 . A second memory channel of the memory transistor ST 1  is connected to a common source line SOURCE. 
     A selection gate of the selection transistor AT 1  is connected to the row line AG 1 . Likewise, a second transfer channel of the transfer transistor TT 1  is connected to the row line AG 1 . A first transfer channel of the transfer transistor TT 1  is connected to a memory gate KG 1  of the memory transistor ST 1 . The gate of the memory transistor ST 1  which is associated with the memory gate KG 1  is in this case designed as a so-called “floating gate”. 
     A transfer gate of the transfer transistor TT 1  is connected to the writing signal line WRITX. A discharge gate of the discharge transistor ET 1  is likewise connected to the writing signal line WRITX. A first discharge channel of the discharge transistor ET 1  is connected to the memory gate KG 1 , while a second discharge channel of the discharge transistor ET 1  is connected directly to ground. 
     The memory cell Z 3  is connected in parallel with the memory cell Z 1  with respect to the row line AG 1 . For this purpose, the memory cell Z 3  has a selection transistor AT 3 , which is designed as an N-channel transistor using conventional FET technology, and a memory transistor ST 3 , which is designed as an N-channel transistor with a “floating gate”. A first selection channel of the selection transistor AT 3  is connected to the column line SP 2 , while a second selection channel of the selection transistor AT 3  is connected to a first memory channel of the memory transistor ST 3 . A second memory channel of the memory transistor ST 3  is connected to the common source line SOURCE. The selection gate of the selection transistor AT 3  is connected in parallel with the selection gate of the selection transistor AT 1  and is connected to the row line AG 1 . 
     The memory gate of the memory transistor ST 3  is connected in parallel with the memory gate of the memory transistor ST 1  and is connected to the second transfer channel of the transfer transistor TT 1 . Accordingly, the memory gate of the memory transistor ST 3  is also connected to the first discharge channel of the discharge transistor ET 1 . 
     The memory cell Z 2  has a selection transistor AT 2  and a memory transistor ST 2 . The selection transistor AT 2  is produced as a conventional N-channel transistor using FET technology, while the memory transistor ST 2  is designed as an N-channel transistor with a so-called “floating gate”. A first selection channel of the selection transistor AT 2  is connected to the column line SP 2 , while a second selection channel of the selection transistor AT 2  is connected to a first memory channel of the memory transistor ST 2 . A second memory channel of the memory transistor ST 2  is connected to the common source line SOURCE. 
     A selection gate of the selection transistor AT 2  is connected to the row line AG 2 . Likewise, a second transfer channel of the transfer transistor TT 2  is connected to the row line AG 2 . A first transfer channel of the transfer transistor TT 2  is connected to a memory gate KG 2  of the memory transistor ST 2 . The gate of the memory transistor ST 2  which is associated with the memory gate KG 2  is in this case designed as a so-called “floating gate”. 
     A transfer gate of the transfer transistor TT 2  and a discharge gate of the discharge transistor ET 2  are connected to the writing signal line WRITX. A first discharge channel is connected to the memory gate KG 2 , while a second discharge channel is connected directly to ground. The memory cell Z 4  is connected in parallel with the memory cell Z 2  with respect to the row line AG 2 . For this purpose, the memory cell Z 4  has a selection transistor AT 4  (implemented as an N-channel transistor using conventional FET technology) and a memory transistor ST 4  (implemented as an N-channel transistor with a “floating gate”). A first selection channel of the selection transistor AT 4  is connected to the column line SP 2 , while a second selection channel of the selection transistor AT 4  is connected to a first memory channel of the memory transistor ST 4 . A second memory channel of the memory transistor ST 4  is connected to the common source line SOURCE. The selection gate of the selection transistor AT 4  is connected in parallel with the selection gate of the selection transistor AT 2  and is connected to the row line AG 2 . The memory gate of the memory transistor ST 4  is connected in parallel with the memory gate of the memory transistor ST 2  and is connected to the second transfer channel of the transfer transistor TT 2 . Accordingly, the memory gate of the memory transistor ST 4  is also connected to the first discharge channel of the discharge transistor ET 2 . 
     The memory cells Z 1 , Z 2  are connected in parallel with respect to the column line SP 1 , while the memory cells Z 3 , Z 4  are connected in parallel with respect to the column line SP 2 . 
     The three states of “erase”, “write” and “read” are explained below for the memory cell Z 1 . In this case, no signal is applied to the column line SP 1  in the “erase” state, since this is not necessary for the latter state. Only during the writing and during the reading out of the content of the memory cell Z 1  is a signal applied to the column line SP 1 . However, this is not explained in any further detail here since this is of secondary importance for the essence of the invention. 
     The states of the row lines AG 1 , AG 2 , of the memory gates KG 1 , KG 2  and of the writing signal line WRITX for the individual operating states are represented in the table below: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 AG1 
                 KG1 
                 AG2 
                 KG2 
                 WRITX 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Erase 
                 Up 
                 Up 
                 0 
                 0+Utp 
                 0 
               
               
                   
                 Write 
                 Up 
                 0 
                 0 
                 0 
                 Up 
               
               
                   
                 Read 
                 U1 
                 U1 
                 0 
                 0+Utp 
                 0 
               
               
                   
                   
               
             
          
         
       
     
     In this case, the voltage “Up” designates the programming voltage (e.g. 18 V), the voltage “U1” designates a read-out voltage and the voltage “Utp” designates the positive absolute value of the threshold voltage of a p-channel transistor (approximately 1 V). 
     As the table clearly shows, a programming voltage Up is applied to the row line AG 1  during the erasure of that row of the memory in which the memory cell Z 1  is situated. As a result, the first transfer channel of the transfer transistor TT 1  is likewise at the level of the programming voltage Up. The voltage 0 V is present on the writing signal line WRITX, with the result that the transfer transistor TT 1  is in the turned-on state, since it is a P-channel transistor. By contrast, the discharge transistor ET 1  is designed as an N-channel transistor, with the result that the signal 0 V of the writing signal line WRITX, which signal is present at the discharge gate of the discharge transistor ET 1  moves the latter into a turned-off state. Consequently, the programming voltage Up is present at the memory gate KG 1 , which forces the “floating gate” of the memory transistor ST 1  into the “erased” state. 
     The memory cell Z 2  remains unaffected by the operations on the row line AG 1  and on the writing signal line WRITX insofar as the memory gate KG 2  is always in a defined state at 0 V+Utp in accordance with the potential of the row line AG 2 . 
     Since the memory cells Z 3  and Z 4  are connected in parallel with the memory cells Z 1  and Z 2  with respect to the row lines AG 1 , AG 2 , they behave in a corresponding manner to the memory cells Z 1  and Z 2 . Therefore, in the course of the “erase” state, all those memory cells which are addressed by the row line AG 1  are erased. 
     The memory cell Z 2  and the memory cell Z 4  are erased in a corresponding manner to the erasure of the memory cells Z 1  and Z 3 . 
     When a value is written to the memory cell Z 1 , the value Up is applied to the row line AG 1  and to the writing signal line WRITX. On account of the state of the writing signal line WRITX, the N-channel discharge transistor ET 1  turns on, while the P-channel transfer transistor TT 1  turns off. As a result, the potential of ground, namely 0 V, is present at the memory gate KG 1 . By the application of a suitable signal to the column line SP 1 , the memory transistor ST 1  is written to, since the selection transistor AT 1  is turned on since the signal Up is present at the selection gate. 
     It will be understood, in this context, that the memory cell Z 2  remains unaffected by the operations in the memory cell Z 1  insofar as the memory gate KG 2  always maintains the defined value of 0 V in accordance with the potential of ground switched through the discharge transistor ET 2 . 
     When a value is read from the memory cell Z 1 , the value U 1  is applied on the row line AG 1 , while the signal 0 is applied to the writing signal line WRITX. As a result, the memory gate KG 1  is at the potential U 1  in a defined manner, while the selection transistor AT 1  is in the turned-on state. The state of the memory transistor ST 1  can then be read out by the application of a suitable voltage to the column line SP 1 . 
     By the application of a suitable voltage to the column line SP 2 , in this operating mode, the memory state of the memory transistor ST 3  of the memory cell Z 3  can be read out, since the selection transistor AT 3  is likewise in the turned-on state. The memory cells Z 2  and Z 4  remain unaffected by the state of the memory cells Z 1  and Z 3  insofar as the memory gate KG 2  is always in a defined state at 0 V+Utp, since the memory gate KG 2  always maintains the defined value of 0 V+Utp in accordance with the potential of ground switched through the discharge transistor ET 2 . 
     The column lines SP 1  and SP 2  are wired up with corresponding standard values both in the course of writing and in the course of reading. 
     FIG. 2 shows a circuit diagram of a further semiconductor memory according to the invention, which is designed on a semiconductor substrate. FIG. 2 illustrates only a partial region of the semiconductor memory having four memory cells Z 11 , Z 12 , Z 13  and Z 14 . The memory cells Z 11 , Z 12 , Z 13  and Z 14  can be driven via two row lines AG 1 , AG 2  and via two column lines SP 1 , SP 2 . 
     In order to drive the memory cells Z 11 , Z 12 , Z 13  and Z 14 , use is made of a drive circuit having a transfer transistor TT 11 , a discharge transistor ET 11 , a transfer transistor TT 12  and a discharge transistor ET 12 , which are driven via a writing signal line WRITX. A signal which is converted to high voltage and is generated from a logic signal which controls the writing operation is present on the writing signal line WRITX. The drive circuit furthermore comprises a block selection transistor BT 11  and a block selection transistor BT 12 . A block selection gate of the block selection transistor BT 11  and a block selection gate of the block selection transistor BT 12  are connected to a block selection signal line BLKN. A signal which is converted to high voltage and is generated from a further logic signal which controls the block-by-block programming operation is present on the block selection signal line BLKN. 
     The transfer transistors TT 11  and TT 12  and the block selection transistors BT 11  and BT 12  are produced as conventional P-channel transistors using FET technology. 
     The memory cell Z 11  has a selection transistor AT 11  and a memory transistor ST 11 . The selection transistor AT 11  is produced as a conventional N-channel transistor using FET technology, while the memory transistor ST 11  is designed as an N-channel transistor with a so-called “floating gate”. A first selection channel of the selection transistor AT 11  is connected to the column line SP 1 , while a second selection channel of the selection transistor AT 11  is connected to a first memory channel of the memory transistor ST 11 . A second memory channel of the memory transistor ST 11  is connected to a common source line SOURCE. 
     A selection gate of the selection transistor AT 11  is connected to the row line AG 1 . Likewise, a second block selection channel of the block selection transistor BT 11  is connected to the row line AG 1 . A second transfer channel of the transfer transistors TT 11  is connected to a first block selection channel of the block selection transistor BT 11  and a first transfer channel of the transfer transistor TT 11  is connected to a memory gate KG 11  of the memory transistor ST 11 . The gate of the memory transistor ST 11  which is associated with the memory gate KG 11  is in this case designed as a so-called “floating gate”. A transfer gate of the transfer transistor TT 11  is connected to the writing signal line WRITX. 
     A discharge gate of the discharge transistor ET 11  is connected to the writing signal line WRITX. A first discharge channel is connected to the memory gate KG 11 , while a second discharge channel is connected directly to ground. 
     The memory cell Z 13  is connected in parallel with the memory cell Z 11  with respect to the row line AG 1 . For this purpose, the memory cell Z 13  has a selection transistor AT 13 , which is designed as an N-channel transistor using conventional FET technology, and a memory transistor ST 13 , which is designed as an N-channel transistor with a “floating gate”. A first selection channel of the selection transistor AT 13  is connected to the column line SP 2 , while a second selection channel of the selection transistor AT 13  is connected to a first memory channel of the memory transistor ST 13 . A second memory channel of the memory transistor ST 13  is connected to the common source line SOURCE. The selection gate of the selection transistor AT 13  is connected in parallel with the selection gate of the selection transistor AT 11  and is connected to the row line AG 1 . The memory gate of the memory transistor ST 13  is connected in parallel with the memory gate of the memory transistor ST 11  and is connected to the second transfer channel of the transfer transistor TT 11 . Accordingly, the memory gate of the memory transistor ST 13  is also connected to the first discharge channel of the discharge transistor ET 11 . 
     The memory cell Z 12  has a selection transistor AT 12  and a memory transistor ST 12 . The selection transistor AT 12  is produced as a conventional N-channel transistor using FET technology, while the memory transistor ST 12  is designed as an N-channel transistor with a so-called “floating gate”. A first selection channel of the selection transistor AT 12  is connected to the column line SP 2 , while a second selection channel of the selection transistor AT 12  is connected to a first memory channel of the memory transistor ST 12 . A second memory channel of the memory transistor ST 12  is connected to the common source line SOURCE. 
     A selection gate of the selection transistor AT 12  is connected to the row line AG 2 . Likewise, a second block selection channel of the block selection transistor BT 12  is connected to the row line AG 2 . A second transfer channel of the transfer transistor TT 12  is connected to a first block selection channel of the block selection transistor BT 12  and a first transfer channel of the transfer transistor TT 12  is connected to a memory gate KG 12  of the memory transistor ST 12 . The gate of the memory transistor ST 12  which is associated with the memory gate KG 12  is in this case designed as a so-called “floating gate”. 
     A transfer gate of the transfer transistor TT 12  is connected to the writing signal line WRITX. A block selection gate of the block selection transistor BT 12  is connected to the block selection signal line BLKN. 
     A discharge gate of the discharge transistor ET 12  is connected to the writing signal line WRITX. A first discharge channel is connected to the memory gate KG 12 , while a second discharge channel is connected directly to ground. 
     The memory cell Z 14  is connected in parallel with the memory cell Z 12  with respect to the row line AG 2 . For this purpose, the memory cell Z 14  has a selection transistor AT 14 , which is designed as an N-channel transistor using conventional FET technology, and a memory transistor ST 14 , which is designed as an N-channel transistor with a “floating gate”. A first selection channel of the selection transistor AT 14  is connected to the column line SP 2 , while a second selection channel of the selection transistor AT 14  is connected to a first memory channel of the memory transistor ST 14 . A second memory channel of the memory transistor ST 14  is connected to the common source line SOURCE. The selection gate of the selection transistor AT 14  is connected in parallel with the selection gate of the selection transistor AT 12  and is connected to the row line AG 2 . The memory gate of the memory transistor ST 14  is connected in parallel with the memory gate of the memory transistor ST 12  and is connected to the second transfer channel of the transfer transistor TT 12 . Accordingly, the memory gate of the memory transistor ST 14  is also connected to the first discharge channel of the discharge transistor ET 12 . 
     The memory cells Z 11 , Z 12  are connected in parallel with respect to the column line SP 1 . The memory cells Z 13 , Z 14  are connected in parallel with respect to the column line SP 2 . 
     The three states of “erase”, “write” and “read” are explained below for the memory cell Z 11 . In this case, no signal is applied to the column line SP 1  in the “erase” state, since this is not necessary for the latter state. Only during the writing and during the reading out of the content of the memory cell Z 11  is a signal applied to the column line SP 1 . However, this is not explained in any further detail here since this is of secondary importance for the essence of the invention. 
     The states of the row lines AG 1 , AG 2 , of the memory gates KG 11 , KG 12  and of the writing signal line WRITX for the individual operating states are represented in the table below: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 AG1 
                 KG11 
                 AG2 
                 KG12 
                 WRITX 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Erase 
                 Up 
                 Up 
                 0 
                 0+Utp 
                 0 
               
               
                   
                 Write 
                 Up 
                 0 
                 0 
                 0 
                 Up 
               
               
                   
                 Read 
                 U1 
                 U1 
                 0 
                 0+Utp 
                 0 
               
               
                   
                   
               
             
          
         
       
     
     The block selection signal BLKN assumes the voltages 0 V (“selected”) or Up (“not selected”), depending on whether or not the block of the semiconductor memory in which the memory cells Z 1  to Z 4  are situated is selected. 
     In this case, the voltage “Up” designates the programming voltage (e.g. 18 V), the voltage “U1” designates a read-out voltage and the voltage “Utp” designates the positive absolute value of the threshold voltage of a p-channel transistor (approximately 1 V). 
     For the description below of the method of operation of the semiconductor memory, it is assumed that the signal BLKN is always at 0 V, with the result that the channels of the block selection transistors are in the turned-on state and forward the signals on the row lines to the channels of the transfer transistors TT 11  and TT 12 . 
     As the table clearly shows, a programming voltage Up is applied to the row line AG 1  during the erasure of that row of the memory in which the memory cell Z 11  is situated. As a result, the first transfer channel of the transfer transistor TT 11  is likewise at the level of the programming voltage Up. The voltage 0 V is present on the writing signal line WRITX, with the result that the transfer transistor TT 11  is in the turned-on state, since it is designed as a P-channel transistor. By contrast, the discharge transistor ET 11  is designed as an N-channel transistor, with the result that the signal 0 V of the writing signal line WRITX, which signal is present at the discharge gate of the discharge transistor ET 11 , moves the latter into a turned-off state. Consequently, the programming voltage Up is present at the memory gate KG 11 , which forces the “floating gate” of the memory transistor ST 11  into the “erased” state. 
     The memory cell Z 12  remains unaffected by the operations on the row line AG 1  and on the writing signal line WRITX insofar as the memory gate KG 12  is always in a defined state at 0 V+Utp in accordance with the potential of the row line AG 2 . 
     Since the memory cells Z 13  and Z 14  are connected in parallel with the memory cells Z 11  and Z 12  with respect to the row lines AG 1 , AG 2 , they behave in a corresponding manner to said memory cells Z 11  and Z 12 . Therefore, in the course of the “erasure” state, all those memory cells which are addressed by the row line AG 1  are erased. 
     The memory cell Z 12  and the memory cell Z 14  are erased in a corresponding manner to the erasure of the memory cells Z 11  and Z 13 . When a value is written to the memory cell Z 11 , the value Up is applied to the row line AG 1  and to the writing signal line WRITX. On account of the state of the writing signal line WRITX, the N-channel discharge transistor ET 11  turns on, while the P-channel transfer transistor TT 11  turns off. As a result, the potential of ground, namely 0 V, is present at the memory gate KG11. By the application of a suitable signal to the column line SP 1 , the memory transistor ST 11  is written to, since the selection transistor AT 11  is turned on on account of the signal Up present at the selection gate. In this case, it should be noted that the memory cell Z 12  remains unaffected by the operations in the memory cell Z 11  insofar as the memory gate KG 12  always maintains the defined value of 0 V in accordance with the potential of ground switched through the discharge transistor. When a value is read from the memory cell Z 11 , the value U 1  is applied on the row line AG 1 , while the signal  0  is applied to the writing signal line WRITX. As a result, the memory gate KG 11  is at the potential U 1  in a defined manner, while the selection transistor AT 11  is in the turned-on state. 
     The state of the memory transistor ST 11  can then be read out by the application of a suitable voltage to the column line SP 1 . By the application of a suitable voltage to the column line SP 2 , in this operating mode, the memory state of the memory transistor ST 13  of the memory cell Z 13  can be read out, since the selection transistor AT 13  is likewise in the turned-on state. The memory cells Z 12  and Z 14  remain unaffected by the state of the memory cells Z 11  and Z 13  insofar as the memory gate KG 12  is always in a defined state at 0 V+Utp, to be precise on account of the ground switched through the discharge transistor. The column lines SP 1  and SP 2  are wired up with the corresponding standard values both in the course of writing and in the course of reading. 
     FIG. 3 shows a third embodiment that is similar to the first embodiment except that the memory transistors ST 1 -ST 4  and the selection transistors AT 1 -AT 4  are designed as P-channel transistors and the transfer transistors are designed as an N-channel transistors. Such an arrangement is uncommon, but may afford advantages if so-called “hole conduction” is desired for the transfer of charge carriers.