Abstract:
A semiconductor memory device includes a plurality of phase change memory cells connected to the same bitline and different respective word lines. A read operation is performed on one of the memory cells by selecting the bitline and a corresponding wordline. While the read operation is performed, leakage current produced by non-selected memory cells is detected by a leakage detecting circuit and compensated by a leakage current supply circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to a semiconductor memory device. More particularly, the invention relates to a phase change memory device adapted to compensate for leakage current in a read operation.  
         [0003]     A claim of priority is made to Korean Patent Application No. 2005-0012746 filed on Feb. 16, 2005, the disclosure of which is hereby incorporated by reference in its entirety.  
         [0004]     2. Description of Related Art  
         [0005]     Phase change memory devices store data using phase change materials, such as chalcogenide, which are capable of stably transitioning between amorphous and crystalline phases. The amorphous and crystalline phases (or states) exhibit different resistance values, which are used to distinguish different logic states of memory cells in the memory devices. In particular, the amorphous phase exhibits a relatively high resistance, and the crystalline phase exhibits a relatively low resistance.  
         [0006]     At least one type of phase change memory device—phase change random access memory (PRAM)—uses the amorphous state to represent a logical ‘1’ and the crystalline state to represent a logical ‘0’. In a PRAM device, the crystalline state is referred to as a “set state”, and the amorphous state is referred to as a “reset state”. Accordingly, a memory cell in a PRAM stores a logical ‘0’ by “setting” a phase change material in the memory cell to the crystalline state, and the memory cell stores a logical ‘1’ by “resetting” the phase change material to the amorphous state. Various PRAM devices are disclosed, for example, U.S. Pat. Nos. 6,487,113 and 6,480,438.  
         [0007]     The phase change material in a PRAM is converted to the amorphous state by heating the material to above a predetermined melting temperature and then quickly cooling the material. The phase change material is converted to the crystalline state by heating the material at another predetermined temperature below the melting temperature for a set period of time. Accordingly, data is written to memory cells in a PRAM by converting the phase change material in memory cells of the PRAM between the amorphous and crystalline states using heating and cooling as described.  
         [0008]     The phase change material in a PRAM typically comprises a compound including germanium (Ge), antimony (Sb), and tellurium (Te), i.e., a “GST” compound. The GST compound is well suited for a PRAM because it can quickly transition between the amorphous and crystalline states by heating and cooling.  
         [0009]     The memory cells in a PRAM are called “phase change memory cells”. A phase change memory cell typically comprises a top electrode, a chalcogenide layer, a bottom electrode contact, a bottom electrode, and an access transistor. In the phase change memory cell, the chalcogenide layer is the phase change material. Accordingly, a read operation is performed on the phase change memory cell by measuring the resistance of the chalcogenide layer, and a write operation is performed on the phase change memory cell by heating and cooling the chalcogenide layer as described above.  
         [0010]      FIGS. 1A and 1B  illustrate a conventional phase change memory cell  100  in two different states. In particular,  FIG. 1A  shows phase change memory cell  100  with a phase change layer (e.g., a chalcogenide layer) in the crystalline state, and  FIG. 1B  shows phase change memory cell  100  with the phase change layer is in the amorphous state.  
         [0011]     Referring to  FIGS. 1A and 1B , phase change memory cell  100  comprises a top electrode  12  formed on a phase change layer  14 , and a bottom electrode contact (BEC)  16  connecting top electrode  12  to a bottom electrode  18  through phase change layer  14 .  
         [0012]     In  FIG. 1A , memory cell  100  is in the “set state”, and therefore stores a logical ‘0’, and in  FIG. 1B , memory cell  100  is in the “reset state”, and therefore stores a logical ‘1’.  
         [0013]     Memory cell  100  further comprises an access transistor N 20  to control the flow of current through phase change layer  14 . When current flows through phase change layer  14 , bottom electrode contact  16  acts as a heater to heat phase change layer  14  and change its state. Access transistor N 20  typically comprises a negative metal-oxide semiconductor (NMOS) transistor.  
         [0014]      FIG. 2  is a circuit diagram of memory cell  100  shown in  FIG. 1 . In  FIG. 2 , top electrode  12 , phase change layer  14 , BEC  16 , and bottom electrode  18  are represented as a phase change resistance element “R”.  
         [0015]     Referring to  FIG. 2 , memory cell  100  is controlled by a word line WL and a bit line BL. Wordline WL controls whether access transistor N 20  is turned on and bitline BL provides a voltage for a current “ICELL” flowing through access transistor N 20 . Current “ICELL” flows through memory cell  100  when wordline WL and bitline BL are both activated. Wordline WL and bitline BL are used for both programming and reading memory cell  100 .  
         [0016]      FIG. 3  is a timing diagram illustrating a programming operation of memory cell  100 . In particular,  FIG. 3  shows how time and a temperature applied to phase change layer  14  are used to program memory cell  100 .  
         [0017]     Referring to  FIG. 3 , a first curve  35  shows a time/temperature combination used to place memory cell  100  in the “reset state”, and a second curve  36  shows a time/temperature combination used to place memory cell  100  in the “set state”.  
         [0018]     As shown in curve  35 , phase change layer  14  is heated above a melting point “Tm” and then quickly cooled to change it to the amorphous state. As shown in curve  36 , phase change layer  14  is heated to an intermediate temperature between melting point “Tm” and a crystalline temperature “Tx” for a predetermined amount of time, and then cooled to change it to the crystalline state. In  FIG. 3 , melting point “Tm” is set to 610° C. and crystalline temperature “Tx” is set to 450° C. However, these temperatures can be varied within reasonable ranges and still perform their desired function.  
         [0019]      FIG. 4  illustrates a relationship between the voltage across phase change layer  14  and the amount of current flowing through phase change layer  14 . Typically, the voltage across phase change layer  14  is varied by changing the voltage on bitline BL.  
         [0020]     Referring to  FIG. 4 , phase change layer  14  performs very differently in the set state and the reset state. In  FIG. 4 , a symbol □ labels a curve showing the amount of current passing through phase change layer  14  in the reset state, and a symbol □ labels a curve showing the amount of current flowing through phase change layer  14  in the set state. A symbol □ labels a curve showing the amount of current flowing through phase change layer  14  when it is being programmed.  
         [0021]     As seen in  FIG. 4 , a program voltage above a predetermined threshold voltage V th  is applied to bitline BL to program memory cell  100 , and a read voltage below threshold voltage V th  is applied to bitline BL to read memory cell  100 .  
         [0022]      FIG. 4  shows an exemplary voltage level “Vread” used to read memory cell  100 . When the voltage across phase change layer  14  has voltage level Vread, the current passing through phase change layer  14  has a level “Iread”. A typical value for Vread is 0.4 to 0.6 times Vth.  
         [0023]     Unfortunately, whenever a read voltage is applied to memory cell  100 , leakage current escapes through non-selected memory cells that share bitline BL. As a result, the resistance of phase change layer  14  may be incorrectly read. Leakage currents become increasingly problematic as the size and power consumption of phase change memory devices becomes smaller, because as they do, their margin of error also decreases.  
       SUMMARY OF THE INVENTION  
       [0024]     According to one embodiment of the invention, a semiconductor memory device comprises a plurality of phase change memory cells, each connected to a corresponding bitline and wordline, a read current supply circuit adapted to supply a read current to a sensing node connected to the bitline during a read operation of the semiconductor memory device, and a leakage compensating circuit comprising a capacitor adapted to store a leakage current volume and generate a leakage compensating current based on the stored leakage current volume, and apply the leakage compensating current to the sensing node during the read operation. The semiconductor memory device further comprises a sense amplifier circuit adapted to compare a voltage level of the sensing node with a sensing reference voltage.  
         [0025]     According to another embodiment of the present invention, a method of performing a read operation in a phase change memory device comprising a plurality of phase change memory cells including a selected phase change memory cell connected to a bitline and at least one non-selected phase change memory cell also connected to the bitline, wherein the selected and non-selected phase change memory cells are connected to different respective word lines is provided. The method comprises applying a read current to the bit line of the selected phase change memory cell, sensing a leakage current generated by the at least one non-selected phase change memory cell, and storing a leakage current volume corresponding to the leakage current in a capacitor, generating a leakage compensating current corresponding to the leakage current volume, and applying the leakage compensating current to a sensing node electrically connected to the bitline, and comparing a voltage apparent at the sensing node to a reference voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The invention is described below in relation to several embodiments illustrated in the accompanying drawings. Throughout the drawings like reference numbers indicate like exemplary elements, components, or steps. In the drawings:  
         [0027]      FIGS. 1A and 1B  illustrate a conventional phase change memory cell  100  in two different states;  
         [0028]      FIG. 2  is a circuit diagram of the memory cell shown in  FIG. 1 ;  
         [0029]      FIG. 3  is a timing diagram illustrating a programming operation of the memory cell shown in  FIG. 1 ;  
         [0030]      FIG. 4  illustrates a relationship between a voltage across phase change layer and the amount of current flowing through the phase change layer;  
         [0031]      FIG. 5  illustrates a structure of a semiconductor memory device adapted to compensate for leakage current;  
         [0032]      FIG. 6  illustrates the timing of a read operation performed on the semiconductor memory device shown in  FIG. 5 ; and,  
         [0033]      FIG. 7  shows resistance distributions for phase change layers in phase change memory cells in respective set and reset states. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     Exemplary embodiments of the invention are described below with reference to the corresponding drawings. These embodiments are presented as teaching examples. The actual scope of the invention is defined by the claims that follow.  
         [0035]      FIG. 5  illustrates a structure of a semiconductor memory device adapted to compensate for leakage current, and  FIG. 6  illustrates the timing of a read operation performed in the semiconductor memory device shown in  FIG. 5 .  
         [0036]     The semiconductor memory device described in relation to  FIGS. 5 and 6  is a phase change memory device comprising a plurality of phase change memory cells formed with a phase change material, such as chalcogenide.  
         [0037]     Referring to  FIG. 5 , a phase change memory device  200  comprises a memory array comprising a plurality of phase change memory cells  100 , a column selection circuit  110 , a clamping circuit  120 , a precharge circuit  130 , a read current supply circuit  140 , a leakage compensating circuit  170  and a sense amplifier circuit  180 . Leakage compensating circuit  170  comprises a leakage current detecting circuit  150  and a leakage current supply circuit  160 .  
         [0038]     Phase change memory cell  100  has the same structure in  FIG. 5  that it does in  FIG. 2 . However, in various alternative embodiments, phase change resistance element “R” and access NMOS transistor N 20  can be formed in different positions.  
         [0039]     Column selection circuit  110  selects a bit line BL in response to a column selection signal “Y” input from a column decoder (not shown).  
         [0040]     Clamping circuit  120  clamps the voltage level of bitline B/L at a level below threshold voltage V th  of phase change layer  14  in response to a clamping signal VCMP.  
         [0041]     Precharge circuit  130  precharges a sensing node (node A) through a positive metal-oxide semiconductor (PMOS) transistor P 30  in response to a precharge signal PREB.  
         [0042]     Read current supply circuit  140  supplies a read current “i(READ)” for a read operation to bit line BL and the sensing node through a PMOS transistor P 40  in response to a read current supply signal VBIAS.  
         [0043]     Leakage compensating circuit  170  stores a detected leakage current volume in a capacitor C 30  through a NMOS transistor N 50  controlled by a leakage detection signal PDET, and supplies a leakage current compensating current to the sensing node based on the stored leakage current volume. The term “leakage current volume” here denotes electrical charges accumulated in capacitor C 30  due to leakage current flowing through non-selected phase change memory cells.  
         [0044]     Once the leakage current compensating current is supplied to the sensing node, sense amplifier circuit  180  compares a voltage level of the sensing node with a sensing reference voltage VREF, and performs a read operation for a selected memory cell.  
         [0045]     Referring to  FIG. 6 , leakage current in semiconductor memory device  200  is compensated in three steps: a precharge step □, a leakage detection step □, and a leakage compensating step □.  
         [0046]     Precharge step □ is executed in response to a transition of a read signal REb. In precharge step □, column selection signal “Y” is activated to connect bit line B/L to a data line D/L. At the same time, clamping signal VCMP is activated to clamp the voltage levels of data line D/L and bitline B/L at a level below threshold voltage V th .  
         [0047]     Precharge signal PREB is deactivated to turn on PMOS transistor P 30  so that a power supply voltage VDD is supplied to the sensing node through PMOS transistor P 30 . In addition, read current supply signal VBIAS is deactivated to turn on PMOS transistor P 40  so that read current i(READ) is supplied to the sensing node through PMOS transistor P 40 . At the same time, a discharge signal PDIS is activated to turn on a NMOS transistor N 40  N 60  so that charges stored in capacitor C 30  are discharged through NMOS transistor N 40 N 60 .  
         [0048]     In leakage detection step □, precharge signal PREB is deactivated to turn off PMOS transistor P 30 . Discharge signal PDIS is also deactivated to turn off NMOS transistor N 60 . Leakage detection signal PDET is activated to turn on NMOS transistor N 50 , and a leakage current volume is stored at capacitor C 30 .  
         [0049]     In leakage compensating step □, leakage detection signal PDET is deactivated to turn off NMOS transistor N 50 , thereby disabling leakage current detecting circuit  150 . A word line selection signal W/L(N) is activated, and a leakage compensating enable signal PLCE controlling a PMOS transistor P 50  is deactivated to turn on PMOS transistor P 50 . As a result, a leakage compensating current corresponding to the leakage current volume stored at capacitor C 30  is supplied to the sensing node through PMOS transistor P 50 .  
         [0050]      FIG. 7  illustrates how the leakage compensating current changes the resistance distribution of phase change layer  14  in phase change memory cell  100  in the set state and the reset state, respectively.  
         [0051]     Referring to  FIG. 7 , the distribution of phase change layer  14  stays the same for the set state even when the leakage compensating current is applied to the sensing node. However, the distribution of phase change layer  14  changes from a distribution shown by a dotted line to a distribution shown by a solid line when the leakage compensating current is applied to the sensing node. Accordingly, a margin ΔM, defined as a difference between a minimum value of the resistance of phase change layer  14  in the reset state and a maximum value of the resistance of phase change layer  14  in the set state, decreases increases by applying the leakage compensating current to the sensing node.  
         [0052]     Although the phase change memory cell described above comprises an access transistor coupled to a phase change resistance element, the invention could also be applied to phase change memory cells constructed of a diode and a resistor, or to a phase change memory cell comprising a phase change layer having a first side connected to ground and an access transistor having a gate connected to a word line, a first terminal connected to a second side of the phase change material, and a second terminal connected to a bitline.  
         [0053]     As described above, a semiconductor memory device according to an exemplary embodiment of the invention compensates for a leakage current when necessary for a read operation by applying a leakage compensating current to a selected bit line.  
         [0054]     The foregoing preferred embodiments are teaching examples. Those of ordinary skill in the art will understand that various changes in form and details may be made to the exemplary embodiments without departing from the scope of the present invention as defined by the following claims.