Abstract:
A method of operating a memory device adapted for determining a program/erase state of a memory cell in the memory device. The method includes applying a drain operation voltage to a drain of the memory cell so that the memory cell generates a working voltage. The working voltage is a function of the drain operation voltage. Then, the working voltage to the drain operation voltage is differentiated to obtain a slope of the working voltage to the drain operation voltage. The program/erase state of the memory cell is determined according to the slope.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the priority benefit of Taiwan application serial no. 95127930, filed Jul. 31, 2006. All disclosure of the Taiwan application is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a memory device and operating method thereof, and more particularly, to a memory device and operating method thereof having a program/erase state defined differently from that of the conventional. 
   2. Description of Related Art 
   Nitride storage non-volatile memory (NVM) is a type of memory that allows many times of data writing, reading and erasing operations. Furthermore, the stored data will be retained even after power to the device is removed. With these advantages, nitride storage NVM has been broadly applied in personal computer and other electronic devices. 
   A typical nitride storage NVM uses a charge-trapping layer instead of the polysilicon floating gate in a conventional NVM. The charge-trapping layer is fabricated by silicon nitride, for example. The silicon nitride charge-trapping layer is sandwiched between an upper silicon oxide layer and a lower silicon oxide layer, thereby forming an oxide/nitride/oxide (ONO) composite layer. The most common NVM devices are silicon/oxide/nitride/oxide/silicon (SONOS) devices and metal/oxide/nitride/oxide/silicon (MONOS) devices. 
   The conventional method of determining program or erase state of a memory cell having the ONO structure includes applying an operating voltage on the drain, the gate and the source of the memory cell so that a threshold voltage (or a working voltage) is produced in the charge-trapping layer of the memory cell. Through hot electron or hot hole injection, the memory cell can be erased or programmed. 
     FIG. 1  is a graph with curves showing the relation between the working voltage Vth and the drain operation voltage VD of a memory cell. As shown in  FIG. 1 , the curves  101  and  103  represent the relationship between the operating voltage Vth and the drain operation voltage VD for the erase and the program state of a brand-new memory cell. According to  FIG. 1 , it can be clearly seen that, as the voltage VD at the drain of the memory cell is about 0.24V, the operating voltage difference OM 1  between the curves  101  and  103  is about 3V (3.6V−1.6V). Conventionally, this operating voltage difference is used to define the program/erase state of the memory cell. Obviously, when the voltage VD at the drain is about 1.8V, the operating voltage difference OM 2  between the curves  101  and  103  is reduced to about 0.46V (1.66V−1.2V) due to the drain inductance barrier lowering (DIBL) effect. Therefore, operating at a higher drain voltage will lead to a retraction of the operating boundary. 
   In addition, it can be observed from  FIG. 1  that when the memory device has completed one cycle of the program/erase operations (set to  1000  operations in  FIG. 1 ), the curve  101  will migrate from its original location up to the curve  105  and the curve  103  will migrate from its original location up to the curve  107 . From the standpoint of the drain operation voltage Vd set to 0.24V, the operating boundary (OM 3 ) of the program and erase state has retracted from the original 3V to 2V (4.8V−2.8V). Thus, after the memory device has performed a cycle of operations, a retraction of the operating boundary will occur. Therefore, the memory device is more vulnerable to produce erroneous actions. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is to provide a memory device capable of preventing erroneous actions in a memory cell as a result of an increase in the drain operation voltage of the memory cell or completion of an erase/program cycle. 
   In addition, the present invention is to provide a method of operating a memory device capable of maintaining a fixed operating boundary for a memory cell either under an increase in the drain operation voltage or after an erase/program cycle. 
   To achieve these and other advantages and in accordance with the purpose of the invention, the memory device includes at least a memory cell disposed between a word line and a bit line. The present invention also includes a comparator with a first compare input terminal and a second compare input terminal. The first compare input terminal is grounded through a first passive device. Similarly, the second compare input terminal is grounded through a second passive device. In addition, the present invention includes a first switching circuit and a second switching circuit. The first switching circuit is selected to apply either a first operating voltage or a second operating voltage to the drain of the memory cell so that the memory cell generates a working current. The second switching circuit directs the working current produced by the memory cell to the first compare input terminal or the second compare input terminal through the bit line. 
   In the embodiment of the present invention, the first passive device and the second passive device may be capacitors or resistors. 
   The present invention also provides a method of operating a memory device including applying a first drain operation voltage to the drain of a memory cell so that the memory cell generates a first working current. A second drain operation voltage is applied to the drain of the memory cell so that the memory cell generates a second working current. Then, the first working current is transformed into a first working voltage and the second working current is transformed into a second working voltage. The difference between the second working voltage and the first working voltage is divided by the difference between the second operating voltage and the first operating voltage to obtain a partial differential value. The present invention utilizes this partial differential value to determine the program/erase state of the memory cell. 
   Accordingly, because the slope of the working voltage to the drain operation voltage is utilized to determine if the memory cell is in program state or erase state, the present invention prevents erroneous actions as a result of operating at a high working voltage or passing through a program/erase cycle. In other words, the memory device in the present invention is able to operate after multiple program/erase cycles or within a larger drain operation voltage range. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a graph showing the relationship between the working voltage Vth and the drain operation voltage VD of a memory cell. 
       FIG. 2A  is a graph showing the relationship between the working current Ids and the gate voltage VG for a memory cell in an erase state. 
       FIG. 2B  is a graph showing the relationship between the working current Ids and the gate voltage VG for a memory cell in a program state. 
       FIG. 3  is a circuit diagram of a memory device according to one embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 2A  is a graph showing the relationship between the working current Ids and the gate voltage VG for a memory cell in an erase state.  FIG. 2B  is a graph showing the relationship between the working current Ids and the gate voltage VG for a memory cell in a program state. As shown in  FIGS. 2A and 2B , the different curves in the same graph represent the change in working current Id when the drain operation voltage on the drain of the memory cell changes. As shown in  FIG. 2A , the application of different drain operation voltages to the drain of the memory cell causes a smaller variation in the working current Id than the same in  FIG. 2B . 
   In other words, when the drain operation voltage on the drain of the memory cell changes, the variation of the working current Id of the memory cell operating in the erase state is less than that of the memory cell operating in the program state. The present invention utilizes this particular characteristic to define the memory erase state or the program state of the memory cell. 
   As shown in  FIG. 1 , the working voltage Vth shifts when the drain operation voltage VD starts to increase and hence leads to a retraction of the operating boundary. However, the working voltage Vth is a function of the drain operation voltage VD so that the slope of the curves  101  and  103  are obviously different (can be confirmed by  FIGS. 2A and 2B ). Therefore, the present invention utilizes the differential value of the working voltage Vth to drain operation voltage VD to define the program state or the erase state of the memory cell. 
     FIG. 3  is a circuit diagram of a memory device according to one embodiment of the present invention. As shown in  FIG. 3 , the memory device  300  may include a plurality of memory cells  301 . The gate terminal of the memory cell  301  is coupled to a word line  303 , the source terminal of the memory cell  301  is coupled to a bit line  305 , and the drain terminal of the memory cell  301  is coupled to a drain operation voltage Vd 1  or Vd 2  through a switching circuit  307 . Furthermore, the memory device  300  includes a comparator  309  having compare input terminals IN 1  and IN 2  which are respectively grounded through passive devices K 1  and K 2  such as resistors or capacitors and also connected to the bit line  305  through a switching circuit  311 . 
   When the memory cell  301  is enabled, the switching circuit  307  directs the drain operation voltage Vd 1  to the drain of the memory cell  301 . The memory cell  301  generates a working current Id 1  that flows into the compare input terminal IN 1  via the bit line  305 . As the working current Id 1  flows into the compare input terminal IN 1 , it is transformed by the passive device K 1  into a working voltage Vth 1  and sent to the comparator  309 . 
   Next, the switching circuit  307  directs the drain operation voltage Vd 2  to the drain of the memory cell  301 . The memory cell  301  generates a working current Id 2  that flows into the compare input terminal IN 2  according to the drain operation voltage Vd 2 . Similarly, the working current Id 2  is transformed by the passive device K 2  into a working voltage Vth 2  and sent to the comparator  309 . 
   After receiving the working voltages Vth 1  and Vth 2 , the comparator  309  outputs the difference between the working voltages Vth 1  and Vth 2  from an output terminal Vout. Thus, the output Vout from the comparator  309  can be divided by the difference between drain operation voltage Vd 1  and Vd 2  to obtain a partial differential value, that is, the aforementioned slope. From the slope, the program or erase state of the memory cell  301  can be determined. 
   In the present embodiment, the memory cell  301  may have at least one storage area. In some other embodiments, the memory cell  301  can be a SONOS structure or a dual MONOS structure. 
   In the SONOS structure, the foregoing method cannot be used to separately operate the two storage areas because the two storage areas will interfere with each other. However, in the dual MONOS structure, the second word line can be used as a switch for the two storage areas so that the two storage areas will not interfere with each other. 
   The present invention also provides an operating mechanism capable of operating different storage areas. The memory cell  301  is assumed to have a first storage area and a second storage area. When the memory device  300  determines that the slope is smaller than a first preset value, the memory cell  301  is in an erase state. In the present embodiment, the first preset value can be 1. 
   On the other hand, when the memory device  300  determines that the slope is located between a second preset value and a third preset value, for example, the slope is located between 1 and 2, the memory cell  301  has only one storage area in the program state. Furthermore, when the memory device  300  determines that the slope is located between a fourth preset value and a fifth preset value, for example, the slope is located between 2 and 3, the memory cell  301  has two storage areas in the program state. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.