Abstract:
Content addressable memory device utilizing phase change devices. An aspect of the content addressable memory device is the use of a comparatively lower power search-line access element and a comparatively higher power word-line access element. The word-line access element is only utilized during write operations and the search-line access element is only utilized during search operations. The word-line access element being electrically coupled to a second end of a phase change memory element and a word-line. The search-line access element also being electrically coupled to the second end of the phase change memory element and a search-line. The search-line being electrically coupled to a match-line. A bit-line is electrically coupled to a first end of the phase change memory element. Additionally, a complementary set of access elements, a complementary phase change memory element, a complementary search-line, and a complementary bit-line are also included in the content addressable memory device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to phase change memory, and more specifically to content addressable memory utilizing phase change memory and separate word-line and search-line access elements. 
         [0003]    2. Description of Background 
         [0004]    Content addressable memory (CAM) is a type of computer memory utilized in high speed searching applications. Most CAM devices utilize static random access memory (SRAM) as data storage devices (utilizing transistors to store data) and access transistors for match operations. Often in these CAM devices the access transistors large and power-intensive field effect transistors (FET). 
         [0005]    Phase change material can also be utilized to store data for CAM devices. Information is stored in materials that can be manipulated into different phases. Each of these phases exhibits different electrical properties that can be used for storing information. The amorphous and crystalline phases of the phase change material are typically two phases used for bit storage (1&#39;s and 0&#39;s) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase. 
         [0006]    Chalcogenides are a group of materials commonly utilized as phase change material. They typically contain a chalcogen (Periodic Table Group 16/VIA) and a more electropositive element. Selenium (Se) and tellurium (Te) are the two most common semiconductors in the group used to produce a chalcogenide when creating a phase change memory cell. An example of this would be Ge 2 Sb 2 Te 5  (GST), SbTe, and In 2 Se 3 . However, some phase change materials do not utilize chalcogen, such as GeSb. Thus, a variety of materials can be used in a phase change material cell as long as they can retain separate amorphous and crystalline states. 
       SUMMARY OF THE INVENTION 
       [0007]    One aspect of the invention is a content addressable memory cell. The content addressable memory cell includes a bit-line, a complementary bit-line, a phase change memory element, a complementary phase change memory element, a search-line access element, a word-line access element, a search-line, a word-line, a complementary search-line access element, a complementary word-line access element, a complementary search-line, and a match-line. The search-line access element and the complementary search-line access element include a field effect transistors (FET). The word-line access element and complementary word-line access element include, for example, a PN diode or a bipolar junction transistor (BJT). The word-line access element and complementary word-line access element require more power to operate than the search-line access element and complementary search-line access element. Additionally, the word-line access element and the complementary word-line access element are used only for writing operations. The search-line access element and the complementary search-line access element are used only for searching operations. 
         [0008]    Another aspect of the invention is a content addressable memory device. The content addressable memory device comprise a plurality of content addressable memory cells arranged in an array, a plurality of bit-lines and a plurality of word-lines arranged in columns and rows, a plurality of word-line access elements, a plurality of complementary bit-lines, a plurality of complementary word-line access elements, a plurality of search-lines, a plurality of complementary search-lines, and a plurality of match-lines. 
         [0009]    Each content addressable memory cell includes a phase change memory element and a complementary phase change memory element. The phase change memory element includes a first end and a second end. Similarly, the complementary phase change memory element includes a first end and a second end. The phase change memory element is configured to store a data bit and the complementary phase change memory element is configured to store a complementary data bit that is the inverse bit of the data bit. 
         [0010]    Each bit-line is electrically coupled to the first end of at least one phase change memory element. At least one word-line access element is electrically coupled to each word-line. Each word-line access element is electrically coupled to the second end of one phase change memory element. Each complementary bit-line is electrically coupled to the first end of at least one complementary phase change memory element. At least one complementary word-line access element is electrically coupled to each word-line, with each complementary word-line access element electrically coupled to the second end of one complementary phase change memory element. 
         [0011]    Each search-line is electrically coupled to at least one search-line access element. The search-line access element is electrically coupled to the second end of one phase change memory element. Each complementary search-line electrically is coupled to at least one complementary search-line access element. The complementary search-line access element is electrically coupled to the second end of one complementary phase change memory element. The plurality of match-lines are electrically coupled to at least one search-line and to the complementary search-line of the at least one search-line. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0013]      FIG. 1  illustrates a circuit diagram of a content addressable memory cell contemplated by the present invention. 
           [0014]      FIG. 2   a  illustrates information exchange between a content addressable memory device and a central processing unit. 
           [0015]      FIG. 2   b  illustrates a table of values for match-line charge or discharge. 
           [0016]      FIG. 3  illustrates a content addressable memory array contemplated by the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The present invention is described with reference to embodiments of the invention. Throughout the description of the invention reference is made to  FIGS. 1-3 . 
         [0018]    As described in detail below, an aspect of the current invention is a content addressable memory device utilizing phase change memory elements. The content addressable memory device is comprised of a plurality of content addressable memory cells. Each individual content addressable memory cell includes a search-line access element and a word-line access element. The search-line access element is activated during searching operations, while the word-line access element is activated during programming operations. 
         [0019]    Also included in each individual content addressable memory cell are a phase change memory element and a complementary phase change memory element. Both the phase change memory element and the complementary phase change memory element are comprised of a phase change material, such as Germanium-Antimony-Tellurium (GST). Typically, the phase change memory element may be programmed to one of two states: a crystalline state or an amorphous state. For example, the crystalline state may represent a stored “0” value and the amorphous state may represent a stored “1” value. In the crystalline state, the phase change memory element exhibits a relatively low resistance. On the other hand, in the amorphous state, the phase change memory element has a relatively high resistance. 
         [0020]    Changing a phase change memory element&#39;s state requires heating the material to a melting point and then cooling the material to one of the two states. A current is passed through the phase change memory element inducing ohmic heating and causing the phase change material to melt. Melting and gradually cooling down the phase change material in the phase change memory element allows time for the phase change material to form the crystalline state. Melting and abruptly cooling the phase change material in the phase change memory element quenches the phase change material into the amorphous state. 
         [0021]    The complementary phase change memory element is programmed to the inverse state of the phase change memory element. For example, if the phase change memory element is in the crystalline state then the complementary phase change memory element is programmed to the amorphous state. 
         [0022]    In  FIG. 1 , an embodiment of the invention is illustrated. The content addressable memory cell  102  is comprised of a bit-line  104  a complementary bit-line  106 , a phase change memory element  108 , a complementary phase change memory element  110 , a search-line access element  112 , a complementary search-line access element  114 , a search-line  116 , a complementary search-line  118 , a word-line access element  120 , a complementary word-line access element  122 , a word-line  128 , a ground  124 , and a match-line  126 . 
         [0023]    As shown, the bit-line  104  is electrically coupled to a first end of the phase change memory element  108 . The complementary bit-line is electrically coupled to a first end of the complementary phase change memory element  110 . The search-line access element  112  is electrically coupled to a second end of the phase change memory element  108  and the search-line  116 . The complementary search-line access element  114  is electrically coupled to a second end of the complementary phase change memory element  110  and the complementary search-line  118 . The word-line access element  120  is electrically coupled to the second end of the phase change memory element  108 , the word-line  128 , and the ground  124 . The complementary word-line access element  122  is electrically coupled to the second end of the complementary phase change memory element  110 , the word-line  128 , and the ground  124 . The search-line  116  and the complementary search-line  118  are electrically coupled to the match-line  126 . 
         [0024]    In  FIG. 2   a,  an operation diagram of the content addressable memory device  210  is shown. A central computing source  208  such as a central processing unit (CPU), an encoder  204 , a memory array  202 , and a match circuit  206  are shown. In one particular embodiment of the invention, the encoder  204  and the match circuit  208  are integrated into the content addressable memory device  210 . In an alternate embodiment of the invention, the encoder  204  and the match circuit  208  are separate from the content addressable memory device  210 . 
         [0025]    Operation of the content addressable memory cell can be broken down into two processes: programming and searching. During the programming process, the encoder  204  receives desired storage data bits from the CPU  208  and programs the data bits into the memory array  202 . During the searching process, the match circuit  206  receives desired search data bits from the CPU  208  and searches for the data bits in the memory array  202 . 
         [0026]    Referring back to  FIG. 1 , programming the content addressable memory cells involves storing the desired data bit in the phase change memory element  108  and the complementary data bit (the inverse of the data bit) in the complementary phase change memory element  110 . During this operation, the encoder first sets the word-line  128  of the desired content addressable memory cell  102  to a relatively high voltage, thereby activating the word-line access element  120  and the complementary word-line access element  122 . 
         [0027]    In order to store a “ 1 ” (amorphous state in this particular embodiment of the invention) in the content addressable memory cell  102 , the encoder sets the bit-line  104  to a relatively high voltage and the complementary bit-line  106  to a relatively low voltage. Current flows through and melts the phase change memory element  108  and the complementary phase change memory element  110 . To store a data bit value of “1”, the bit-line  104  is then turned off with an abrupt trailing edge and the phase change memory element  108  cools to the amorphous state. Meanwhile, the complementary bit-line  106  is turned off with a gradual trailing edge and the complementary phase change memory element  110  cools to the crystalline state. This causes the complementary data bit to store a representation of “0”. 
         [0028]    In order to store a “0” (crystalline state in this particular embodiment of the invention) in the content addressable memory cell  102 , the inverse operations are performed. Specifically, the encoder sets the bit-line  104  to a relatively low voltage and the complementary bit-line  106  to a relatively high voltage. Charge flows through and melts the phase change memory element  108  and the complementary phase change memory element  110 . The bit-line  104  is then turned off with a gradual trailing edge and the phase change memory element  108  cools to the “0” state. Meanwhile, the complementary bit-line  106  is turned off with an abrupt trailing edge and the complementary phase change memory element  110  cools to the “1” state. 
         [0029]    To search for the content addressable memory cells in accordance with one embodiment of the invention, the bit-line  104  and the complimentary bit-line  106  are set to a “low voltage”, the match-line  126  of the desired content addressable memory cell is precharged to a “high” voltage by the match circuit, while the search-line access elements  112  and the complementary search-line access element  114  are turned off. Also note that the word-line access element  120  and the complementary word-line access element  122  are not turned on. The search-line  116  is set to the data bit searched and the complementary search-line  118  is set to the complementary data bit searched. For example, if a “1” needs to be found, the search-line  116  is set to “1”. If the data bit stored in the phase change memory element  108  is “0” and the search-line  116  is set to “1” then the match-line  126  is discharged through search-line access element  112 . If the data bit stored in the phase change memory element  108  is “1” and the search-line  116  is set to “0” then the match-line  126  is discharged through complementary search-line access element  114 . However, if the search-line  116  and the phase change memory element  108  possess the same setting, the match-line  126  remains pre-charged at high voltage indicating a match. 
         [0030]    As stated above, the word-line  128  is only activated during the programming operations (requiring larger transistors), and the search-line  116  and the complementary search-line  118  are only activated during the searching operations (requiring smaller transistors). The word-line access element  120  and the complementary word-line access element  122  are required to be larger transistors able to allow sufficient current to flow in order to melt the phase change elements  108 ,  110 . In one particular embodiment of the invention, the word-line access element  120  and the complementary word-line access element  122  are bipolar junction transistors (BJT). In an alternate embodiment of the invention, the word-line access element  120  and the complementary word-line access element  122  are PN diodes. 
         [0031]    Large transistors are not required for searching operations. Therefore, the search-line access element  112  and the complementary search-line access element  114  can be small field effect transistors. By eliminating the use of high power transistors during the searching operations, power usage is reduced in the overall operation of the content addressable memory device. 
         [0032]      FIG. 2   b  illustrates a table of values  212  relating to the search-line, the data bit stored in the phase change memory element, and the match-line. As stated above, when the search-line value  214  and the phase change memory element value  216  are not equal the match-line is discharged to low voltage by a discharging current  218  (No match). When the search-line value  214  and the phase change memory element value  216  are equal the match-line remain pre-charged at high voltage (Match). 
         [0033]    Now turning to  FIG. 3 , the memory array  202  of the phase change memory device is shown. It is more common, in data bit searches, to perform a “word” (a sequence of data bits) search by searching collections of content addressable memory cells in the memory array  202 . As illustrated, each match-line  126  runs through the same content addressable memory cells as a paired word-line  128 . When searching for a specific word, the word is broadcasted to every search-line  116 . The match-lines  126  then determine where the word match occurs. 
         [0034]    An example of a 4-bit word search is as follows. A specific memory array  202  has 16 content addressable memory cells, with each of the four memory cells in the word-lines  128   a,    128   b,    128   c  and  128   d  storing one bit of information. The four memory cells in the first word-line  128   a  contains the word “1000”, the four memory cells in the second word-line  128   b  contains the word “0000”, the four memory cells in the third word-line  128   c  contains the word “1101”, and the four memory cells in the fourth word-line  128  contains the word “1001”. (This means that the PCM element  108   aa  is in amorphous state ‘1’, the PCM element  110   aa  is in crystalline state ‘0’, the PCM element  108   ab  is in crystalline state ‘0’, the PCM element  110   ab  is in amorphous state ‘1’ and so on.) 
         [0035]    The CPU requires the address for the word “1001”. First, all the match lines  126   a,    126   b,    126   c  and  126   d  are pre-charged to a high voltage. Then the voltage at search line  116   a  is set to level ‘1’, at search line  116   b  to level ‘0’, at search line  116   c  to level ‘0’ and at search line  116   d  to level ‘1’. At the same time, the voltage at the complementary search line  118   a  is set to level ‘0’, at the complementary search line  118   b  to level ‘1’, at the complementary search line  118   c  to level ‘1’ and at the complementary search line  118   d  to level ‘0’. The voltage level ‘1’ turns on the search FETs (for instance, FET  112   aa  and FET  114   ab ), while the voltage level ‘0’ turns off the search FETs(for instance, FET  114   aa  and FET  112   ab ). 
         [0036]    The match-line  126   a  paired to the first word-line  128   a  discharges at the fourth content addressable memory cell. The match-line  126   b  paired to the second word-line  128   b  discharges at the first and the fourth content addressable memory cell. The match-line  126   c  paired to the third word-line  128   c  discharges at the second content addressable memory cell. Meanwhile, the match-line  126   d  paired to the fourth word-line  128   d  remains pre-charged. This indicates that the search word is located on the fourth word-line  128   d.    
         [0037]    Having described preferred embodiments for the content addressable memory device (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.