Abstract:
A voltage pump comprising a charging transistor responsive to a first control signal, the charging transistor operable to connect a node to a first voltage, a pumping capacitor responsive to a second control signal, the pumping capacitor operable to pump additional charge the node, and a pumping transistor responsive to a third control signal, the pumping transistor operable to connect the node to an output, wherein the charging transistor, the pumping capacitor, and the pumping transistor are thin-gate transistors. A method comprising charging a node to a first voltage, boosting the node to a second voltage, and connecting the node to an output, wherein the absolute value of the gate-to-source, gate-to-drain, and drain-to-source voltages of the plurality of thin-gate transistors does not exceed the absolute value of a supply voltage. Because of the rules governing abstracts, this abstract should not be used to construe the claims.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to a voltage charge pump and more particularly to a voltage charge pump for use in integrated circuits, among others.  
         [0002]     Integrated circuits, such as a dynamic random access memory device (DRAM), typically employ one or more voltage pumps which are used to create voltages that are more positive or more negative than the available supply voltages. A DRAM may use two types of voltage pumps. The first type is a V ccp  pump which generates a positive voltage that is used by the DRAM, for example, as a boosted wordline voltage. The second type is a V bb  pump (or back-bias voltage pump) which generates a negative voltage that is used, for example, to negatively bias the DRAM&#39;s substrate.  
         [0003]     Charge pumps typically use voltages that exceed |Vcc| and thus require specialized transistors. For example, V bb  pumps typically use high voltages (e.g., greater than |Vcc|) to pump the substrate to a negative voltage. Thus, V bb  pumps require the use of specialized transistors having a thick-gate oxide (e.g., approximately 50-60 Å) and/or a unique doping profile. The thick-gate oxide and unique doping profile help to prevent breakdown and punch-through (among others) when the transistors are subjected to the high voltages. These specialized transistors may be referred to as “thick-gate oxide transistors” and/or “thick-gate transistors”.  FIG. 16  illustrates a prior art V bb  pump utilizing thick-gate transistors. The thick-gate transistors are required because, in certain instances, the absolute value of the drain-to-source voltage (|V DS |) may exceed V CC  (i.e., which causes the breakdown and punch-through as discussed above).  
         [0004]     Thick-gate transistors, however, have certain performance shortcomings as compared to thin-gate transistors (the use herein of the terms “thin-gate oxide transistor” and/or “thin-gate transistors” refers to common transistors, for example, transistors having an gate oxide thickness of approximately 25-30 Å and/or having a common doping profile). For example, thick-gate transistors typically pass less current than a thin-gate transistor having a similarly-sized channel width. Thus to charge a capacitor in an equivalent amount of time as the thin-gate transistor, the channel width of the thick-gate transistor must be increased to allow more current to flow. Additionally, the threshold voltage (i.e., Vt) of a thick-gate transistor is higher than that of the thin-gate transistor. Thus, thick-gate transistors require more die space and consume more power during normal operation than an equivalent performing thin-gate transistor.  
         [0005]     Several attempts have been made to use thin-gate transistors for pump circuits.  FIGS. 17 and 18  illustrate a single stage prior art V ccp  pump and a single stage prior art V bb  pump, respectively. Referring to the V CCP  pump in  FIG. 17 , signals PH 1  and PH 2  are non-overlapping active-low clock signals which are operated such that the gate-to-source voltage (V GS ), the gate-to-drain (V GD ), and the drain-to-source voltage (V DS ) of transistors M 2  and M 4  do not exceed V CC . Additionally, signals PH 1  and PH 2  also prevent the absolute value of the gate-to-source voltage (|V GS |) and the absolute value of the gate-to-drain (|V GD |) of transistors M 1 , M 3 , M 5 , and M 6  from exceeding V CC , and further prevent the absolute value of the drain-to-source voltage (|V DS |) of transistors M 1  and M 3  from exceeding V CC . Thus, thin-gate transistors may be used in the single stage V ccp  pump illustrated in  FIG. 17 . However, when PH 1  (PH 2 ) goes low, PMPN (PMPN 2 ) may briefly transition below V CC  thereby causing charge injection within transistors M 2  and M 4 .  
         [0006]     Referring to the single stage V bb  pump in  FIG. 18 , signals PH 1  and PH 2  are non-overlapping active-high clock signals which are operated such that the gate-to-source voltage (V GS ), the gate-to-drain (V GD ), and the drain-to-source voltage (V DS ) of transistors M 2  and M 4  do not exceed V CC . Additionally, signals PH 1  and PH 2  also prevent the absolute value of the gate-to-source voltage (|V GS |) and the absolute value of the gate-to-drain (|V GD |) of transistors M 1 , M 3 , M 5 , and M 6  from exceeding V CC , and further prevent the absolute value of the drain-to-source voltage (|V DS |) of transistors M 1  and M 3  from exceeding V CC . Thus, thin-gate transistors may be used in the single stage V bb  pump illustrated in  FIG. 18 . However, when PH 1  (PH 2 ) goes high, PMPN (PMPN 2 ) may briefly transition above V SS  thereby causing charge injection within transistors M 1 , M 3 , M 5 , and M 6 .  
         [0007]     Accordingly, a need exists for a voltage pump that utilizes thin-gate transistors, increases the pumping capacity/efficiency of the voltage pump, and overcomes the limitations inherent in prior art.  
       SUMMARY  
       [0008]     One aspect of the invention relates to a method for operating a voltage pump having a plurality of transistors comprising charging a node to a first voltage, boosting the node to a second voltage and connecting the node to an output, wherein the absolute value of the gate-to-source, gate-to-drain, and drain-to-source voltages of the plurality of transistors does not exceed the absolute value of a supply voltage during the charging, boosting, and connecting.  
         [0009]     Another aspect of the invention relates to a method for operating a voltage pump having a plurality of transistors, comprising charging a node to a first voltage, boosting the node to a second voltage, connecting the node to an output, wherein the absolute value of the gate-to-source, gate-to-drain, and drain-to-source voltages of the plurality of transistors does not exceed the absolute value of a supply voltage during the charging, boosting, and connecting, and preventing a second node from exceeding the voltage of a start-up transistor by more than a threshold voltage.  
         [0010]     Another aspect of the invention relates to a voltage pump comprising a charging transistor responsive to a first control signal, the charging transistor operable to connect a node to a first voltage, a pumping capacitor responsive to a second control signal, the pumping capacitor operable to pump additional charge the node, and a pumping transistor responsive to a third control signal, the pumping transistor operable to connect the node to an output, wherein the charging transistor, the pumping capacitor, and the pumping transistor are thin-gate transistors.  
         [0011]     Another aspect of the invention relates to a voltage pump comprising a charging transistor responsive to a first control signal, the charging transistor operable to connect a node to a first voltage, a pumping capacitor responsive to a second control signal, the pumping capacitor operable to pump additional charge the node, and a pumping transistor responsive to a third control signal, the pumping transistor operable to connect the node to an output, wherein the absolute value of the gate-to-source, gate-to-drain, and drain-to-source voltages of the charging transistor, the pumping capacitor, and the pumping transistor do not exceed the absolute value of a supply voltage during the charging, boosting, and connecting.  
         [0012]     Another aspect of the invention relates to a memory device comprising a memory array having a plurality of memory cells, a plurality of peripheral devices for reading data out of and writing data into the memory array, and a voltage pump, the voltage pump comprising, a charging transistor responsive to a first control signal, the charging transistor operable to connect a node to a first voltage, a pumping capacitor responsive to a second control signal, the pumping capacitor operable to pump additional charge the node, and a pumping transistor responsive to a third control signal, the pumping transistor operable to connect the node to an output, wherein the charging transistor, the pumping capacitor, and the pumping transistor are thin-gate transistors.  
         [0013]     Another aspect of the invention relates to a method for operating a voltage pump, comprising activating a charging transistor to drive a node to a first voltage, wherein the charging transistor is a thin-gate transistor, deactivating the charging transistor, activating a pumping capacitor to drive the node to a second voltage, wherein the charging transistor includes a thin-gate transistor, deactivating the pumping capacitor, activating a pumping transistor to connect an output to the node, wherein the pumping transistor is a thin-gate transistor, and deactivating the pumping transistor.  
         [0014]     Another aspect of the invention relates to a method for operating a voltage pump, comprising, activating a charging transistor to drive a node to a first voltage, wherein the charging transistor is a thin-gate transistor, deactivating the charging transistor, activating a pumping capacitor to drive the node to a second voltage, wherein the charging transistor includes a thin-gate transistor, deactivating the pumping capacitor, activating a pumping transistor to connect an output to the node, wherein the pumping transistor is a thin-gate transistor, deactivating the pumping transistor, and preventing a second node from exceeding the voltage of a start-up transistor by more than a threshold voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     To enable the present invention to be easily understood and readily practiced, the present invention will now be described for purposes of illustration and not limitation, in connection with the following figures wherein:  
         [0016]      FIG. 1  is a simplified diagram of a V bb  pump according to one embodiment.  
         [0017]      FIG. 2  is a detailed diagram of the V bb  pump of  FIG. 1  according to one embodiment.  
         [0018]      FIG. 3  is a detailed diagram of the V bb  pump of  FIG. 1  according to an alternative embodiment.  
         [0019]      FIG. 4  is a detailed diagram of the V bb  pump of  FIG. 1  according to another alternative embodiment.  
         [0020]      FIG. 5  illustrates a portion from each of the V bb  pumps of  FIG. 2-4  according to one embodiment.  
         [0021]      FIG. 6  illustrates timing waveforms for the clock signals and nodes of the V bb  pumps of  FIGS. 2-4  according to one embodiment.  
         [0022]      FIG. 7  is a more detailed circuit diagram of the V bb  pump of  FIG. 4  according to one embodiment.  
         [0023]      FIG. 8  is a detailed diagram of a V CCP  pump according to one embodiment.  
         [0024]      FIG. 9  is a simplified block diagram of a memory system according to one embodiment.  
         [0025]      FIG. 10  is a simplified block diagram of the memory device of  FIG. 9  according to one embodiment.  
         [0026]      FIGS. 11A and 11B  are simplified and cross-sectional diagrams, respectively, illustrating the electrical connections for a typical p-channel transistor.  
         [0027]      FIGS. 12A and 12B  are simplified diagram and cross-sectional diagrams, respectively, illustrating the electrical connections for a p-channel transistor according to one embodiment.  
         [0028]      FIGS. 13A and 13B  are simplified diagram and cross-sectional diagrams, respectively, illustrating the electrical connections for a p-channel transistor according to an alternative embodiment.  
         [0029]      FIG. 14  is a cross-sectional diagram illustrating the electrical connections for a typical n-channel transistor.  
         [0030]      FIG. 15  is a cross-sectional diagram illustrating the electrical connections for an n-channel transistor according to one embodiment.  
         [0031]      FIG. 16  illustrates a V ccp  pump which utilizes thick-gate transistors according to the prior art.  
         [0032]      FIG. 17  illustrates a single stage V ccp  pump according to the prior art.  
         [0033]      FIG. 18  illustrates a single stage V bb  pump according to the prior art. 
     
    
     DETAILED DESCRIPTION  
       [0034]     The detailed description sets forth specific embodiments which are described in sufficient detail to enable those skilled in the art to practice the present invention. It should be apparent to those skilled in the art that other embodiments may be utilized, and that logical, mechanical, and electrical changes may be made, while remaining within the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the invention is defined only by the appended claims.  
         [0035]      FIG. 1  is a simplified diagram of a back bias voltage pump (V bb  pump) according to one embodiment. The V bb  pump includes a p-MOS transistor MP 1 , an n-MOS transistor MN 1 , and a second p-MOS transistor MC 1  which functions as a capacitor. In the current embodiment, each transistor used by the V bb  pump is a thin-gate transistor.  
         [0036]     The source, drain, and gate terminals of p-MOS transistor MP 1  are connected to V SS  (i.e., ground), node PMPN, and control signal CS-P, respectively. The source, drain, and gate terminals of n-MOS transistor MN 1  are connected to node PMPN, to an output line carrying V bb , and to the control signal CS-N, respectively. The source and drain terminals of p-MOS transistor MC 1  are connected to the control signal BN and the gate terminal of p-MOS transistor MC 1  is connected to node PMPN. In the current embodiment, the control signals CS-P, CS-N, and BN are non-overlapping, active low signals.  
         [0037]     A pumping operation for the V bb  pump may be sub-divided into two-steps: a charging step and a boost step. During the charging step, MN 1  and MC 1  are rendered non-conductive by control signals CS-N and BN, respectively. Control signal CS-P is then driven to −V CC , thus rendering transistor MP 1  conductive and charging node PMPN to V SS  (i.e., ground). After node PMPN is driven to V SS , control signal CS-P is driven to 0V, thus rendering transistor MP 1  non-conductive.  
         [0038]     During the boosting step, control signal BN is driven to −V CC , thus rendering capacitor MC 1  conductive and driving node PMPN to −(V CC −V t ), where V t  is the threshold voltage of MC 1 . Control signal CS-N is then driven from 0V to −V CC  volts, thus rendering MN 1  conductive and allowing the voltage stored on node PMPN to be transferred to the output line. In the current embodiment, the voltage transferred to the output line is equal to −(V CC −2Vt) and may be referred to as the back bias voltage, V bb .  
         [0039]     In the current embodiment, thin-gate transistors may be used for MP 1 , MN 1 , and MC 1 . Accordingly, efficiencies of 80% or more have been simulated for the V bb  pump with V CC =1.2V, V T =0.3V, and V bb =−0.6V. To avoid breakdown or punch-though of the thin-gate transistors, the gates of MP 1  and MN 1  are driven to 0V when node PMPN is driven to voltage −V CC , thus guarantying that absolute value of the gate-to-source voltage (|V GS |), the absolute value of the gate-to-drain (|V GD |), and absolute value of the drain-to-source voltage (|V DS |) do not exceed V CC . Additionally, when node PMPN is driven to 0V, the gates of MP 1  and MN 1  are driven to −V CC , thus again guarantying that |V GS |, |V GD |, and |V DS | do not exceed V CC .  
         [0040]      FIG. 2  is a detailed diagram of the V bb  pump of  FIG. 1  according to one embodiment. The V bb  pump  15  is comprised of first and second stages. The first stage is comprised of transistors M 51 , M 5 , M 7 , M 1 , M 8 , and M 2 . The second stage is comprised of transistors M 61 , M 6 , M 9 , M 3 , M 10 , and M 4 . The stages are responsive to control signals PH 1 , PH 1 P, PH 2 , and PH 2 P. More specifically, control signals PH 1 , PH 1 P, PH 2 , and PH 2 P are applied in a manner which guarantees that the voltages present at nodes N 1  and N 2  are never higher than |V CC |. Accordingly, charge injection into the n-well of the p-transistors (e.g., M 1 , M 5 , M 3 , M 6 , etc.) is eliminated. In the current embodiment, transistors M 1 -M 9 , M 51 , and M 61  may be thin-gate transistors.  
         [0041]      FIG. 6  illustrates the timing waveforms for the clock signals PH 1 , PH 1 P, PH 2 , and PH 2 P for the V bb  pump  15  of  FIG. 2  according to one embodiment. Signals PH 1 , PH 2 , PH 1 P, and PH 2 P are active-low clock signals. The control signals may be generated by a timing control logic (not shown). The control signals alternately charge and fire each stage to generate the output voltage, V bb . Additionally,  FIG. 6  illustrates timing waveforms for signals present at nodes N 1  and N 2  of the V bb  pump  15  of  FIG. 2  according to one embodiment.  
         [0042]     Returning to  FIG. 2 , the first and second stages are symmetrical to each other in the current embodiment. The operation of the first stage and second stage are essentially the same with the exception of the complimentary timing required to alternate the firing of each stage. As discussed above in conjunction with  FIG. 1 , the timing and values of the control signals are managed to prevent |V GS |, |V GD |, and |V DS | for the transistors from exceeding V CC . Thus, the thin-gate transistors used by V bb  pump  15  are protected from punch-through, latch-up, gate oxide degradation (for example, from hot electron effects such as time dependent dielectric breakdown (TDDB), voltage threshold (|V t |) shifts from hot electron trapping, and excess substrate current (for example, due to avalanche multiplication), among others. Other benefits, such as the reduction and/or elimination of charge injection, may also be achieved.  
         [0043]     Referring to the first stage, during the charging step, M 2  and M 8  are rendered non-conductive by control signals PH 1  (which connects the gate of M 2  to V SS  via M 3 ) and PH 1 P, respectively. Control signal PH 2  is then driven to −V CC , thus rendering transistor M 1  conductive and charging node N 1  to V SS  (i.e., ground). After node N 1  is driven to V SS , control signal PH 2  is driven to 0V, thus rendering transistor M 1  non-conductive. Because it is used to charge node N 1  to V SS , transistor M 1  may be referred to as a “charging transistor.” During the boosting step, control signal PH 1 P is driven to −V CC , thus rendering capacitor M 8  conductive and driving node N 1  to −V CC −V t ), where V t  is the threshold voltage of M 8 . Because it is used to boost node N 1  from V SS  to −V CC −V t ), capacitor M 8  may be referred to as a “pumping capacitor.” Control signal PH 1  is then driven from 0V to −V CC  volts, thus rendering M 2  conductive and allowing the voltage stored on node N 1  to be transferred to the output line. In the current embodiment, the voltage transferred to the output line is equal to −(V CC −2Vt) and may be referred to as V bb . Because it is used to pump the charge stored on node N 1  to the output line, transistor M 2  may be referred to as a “pumping transistor.” It should be apparent to one of ordinary skill in the art that the second stage operates in a similar manner.  
         [0044]     Typically, p-channel transistors have wells connected to V CC  to prevent forward biasing and charge injection. Referring briefly to  FIGS. 11A and 11B , a simplified view and a cross-sectional view, respectively, of the electrical connections for a typical p-channel transistor are shown, where V G  represents the gate voltage, V S  represents the source voltage, V D  represents the drain voltage, and V W  represents the well voltage. If electrically connected as shown, the transistor will conduct when the absolute value of the gate-to-source voltage (|V GS |) is greater than the absolute value of the p-channel transistor&#39;s threshold voltage (|V T |). The typical p-channel transistor has a threshold voltage (V T ) of approximately −0.3V. Thus the p-channel transistor will conduct if, for example, V G =0, V S =V CC =V W =1V, and V T =−0.3V (i.e., V GS =0V−1V=−1V and 1|V GS |&gt;|V T |).  
         [0045]     As best seen in  FIG. 11B , the p-channel transistor includes a p −  substrate  100 , a n −  well  101 , n +  doped well connection region  102 , p +  doped source connection  103 , p +  doped drain connection  104 , and a polysilicon gate connection  105 . The voltage V CC  is applied to both the well and source connections.  
         [0046]     In the embodiment illustrated in  FIG. 2 , however, p-channel transistors M 1 , M 3 , M 5  and M 6  have their n-wells connected to V SS  (i.e., ground). Referring briefly to  FIGS. 12A and 12B , a simplified view and a cross-sectional view, respectively, of the electrical connections for a p-channel transistor according to one embodiment are shown. As seen in  FIGS. 12A and 12B , V SS  (which is equal to 0V) is applied to the well and source connections (as opposed to V CC  in  FIGS. 11A and 11B ). By applying V SS  to the well and source, any n-well bias is removed and the threshold voltage of the transistor is reduced. A p-channel transistor connected in this manner may be turned on by coupling the gate low, for example, as best seen in  FIG. 12A , the gate is coupled low (i.e., V GS =−V CC −0=−V CC ) using transistor M 9  as a capacitor.  
         [0047]     By connecting the transistor wells to V SS  as in the current embodiment, the threshold voltage (|V t |) is reduced because the bulk voltage (V sb ) is equal to zero volts. A lower |V t | allows the transistor to operate at a lower voltage and to pass a larger drive current (I ON ). The clocking of the p-channel transistors (e.g., M 1 , M 3 , M 5 , and M 6 ) is controlled to prevent forward biasing of junctions while utilizing the grounded n-well connection.  
         [0048]      FIGS. 13A and 13B  are simplified diagram and cross-sectional diagrams, respectively, illustrating the electrical connections for a p-channel transistor according to an alternative embodiment. As seen in  FIGS. 13A and 13B , V CC  is applied to the well connection and V SS  is applied to the source connection. This type of connection, however, increases the threshold voltage of the transistor in comparison to the connection illustrated in  FIGS. 12A and 12B .  
         [0049]     Typically n-channel transistors have their p-well connected to the lowest potential voltage which is usually V SS  (i.e., ground). Referring briefly to  FIG. 14 , a cross-sectional diagram illustrating the electrical connections for a typical n-channel transistor is shown. As best seen in  FIG. 14 , the n-channel transistor includes a p −  substrate  100 , a p +  doped well connection  114 , n +  doped source connection  113 , n +  doped drain connection  112 , and a polysilicon gate connection  115 .  
         [0050]     In the embodiment illustrated in  FIG. 2 , however, n-channel transistors M 2  and M 4  have their p-wells connected to V bb  to prevent forward biasing and charge injection, for example, when the voltages at nodes N 1  and N 2  are more negative than V SS  (e.g., at V bb , as seen in  FIG. 6 ).  
         [0051]     In an alternative embodiment, the n-channel transistors (e.g., M 2  and M 4 ) may utilize deep n-wells.  FIG. 15  is a cross-sectional diagram illustrating the electrical connections for an n-channel transistor utilizing a deep n-well according to one embodiment. The n-channel transistor includes a p −  substrate  110  having a n +  doped deep well  111  therein, an n +  doped drain connection  114 , an n +  doped source connection  113 , a p +  doped well connection  112 , and a polysilicon gate connection  115 . As seen in  FIG. 15 , the p +  doped well connection  112  is switched and the deep n-well  111  is connected to V CC .  
         [0052]     In the current embodiment, M 2  is used as the main pumping transistor for the first stage. Accordingly, M 2  is sized large enough to pass a desired amount of output current. To turn on transistor M 2 , the gate of M 2  is driven to V SS  (as compared to V CC  in prior art pumps). More specifically, PH 1  goes low thereby driving node N 2  to V SS  through transistor M 3 . M 2  turns on because N 1  couples negative (i.e., is drawn to −V CC  by control signal PH 1 P) and thus, V GS | M2 =V N2 −V N1 =0−(−V CC )=V CC , which guarantees that the gate-to-source voltage (|V GS |) does not exceed V CC .  
         [0053]     As discussed above, the first stage includes transistor M 51 . Transistor M 51  is a p-channel transistor that is used for start-up purposes. Transistor M 51  insures that node N 6  is never more positive than a V t  of transistor M 51 . In the current embodiment, the well of transistor M 51  is connected to V SS  to allow a lower startup voltage, however, the well of M 51  may be connected to another voltage (e.g., V CC ) if desired.  
         [0054]     Transistor M 5  is a p-channel transistor that precharges (to V SS ) the gates of transistors M 3  and M 6 . Transistors M 1  and M 5  are gated by control signal PH 2  through transistor M 9 . Transistor M 1  is a p-channel transistor for precharging node N 1  (to V SS ). Transistor M 9  is a p-channel transistor, connected as a capacitor, which is responsive to control signal PH 2 .  
         [0055]     Transistor M 8  is a p-channel transistor, connected as a capacitor, that pumps node N 1  to V bb . As discussed above, transistor M 2  is a large n-channel transistor for pumping the negative charge from node N 1  to V bb .  
         [0056]     Likewise, M 4  is used as the main pumping transistor for the second stage. Accordingly, M 4  is sized large enough to pass a desired amount of output current. To turn on transistor M 4 , the gate of M 4  is driven to V SS  (as compared to V CC  in prior art pumps), which guarantees that the gate-to-source voltage (|V GS |) does not exceed V CC . More specifically, PH 2  goes low thereby driving N 1  to V SS  through transistor M 1 .  
         [0057]     As discussed above, the second stage includes a transistor M 61 . Transistor M 61  is a p-channel transistor that is used for start-up purposes. Transistor M 61  insures that node N 5  is never more positive than a V t  of transistor M 61 . In the current embodiment, the well of transistor M 61  is connected to V SS  to allow a lower startup voltage, however, the well of M 61  may be connected to another voltage (e.g., V CC ) if desired. In the current embodiment, each stage includes a start-up transistor (e.g., M 51 , M 61 ), however, it should be apparent to one skilled in the art that the number of start-up transistors used may be varied while remaining within the scope of the present invention. For example, start-up transistors may be eliminated from the V bb  pump, a single start-up transistor may be utilized, etc.  
         [0058]     Transistor M 6  is a p-channel transistor that precharges (to V SS ) the gates of transistors M 1  and M 5 . Transistor M 3  is a p-channel transistor for precharging node N 2  (to V SS ). Transistors M 3  and M 6  are gated by control signal PH 1  through transistor M 7 . Transistor M 7  is a p-channel transistor, connected as a capacitor, which is responsive to control signal PH 1 .  
         [0059]     Transistor M 10  is a p-channel transistor, connected as a capacitor, that pumps node N 2  to V bb . As discussed above, transistor M 4  is a n-channel transistor for pumping the negative charge from node N 2  to V bb .  
         [0060]      FIG. 3  is a detailed diagram of the V bb  pump of  FIG. 1  according to an alternative embodiment. The construction and operation of the V bb  pump  20  illustrated in  FIG. 3  is similar to V bb  pump  15  illustrated in  FIG. 2 . As discussed above in conjunction with  FIG. 2 , the first and second stages of V bb  pump  20  are responsive to control signals PH 1 , PH 1 P, PH 2 , and PH 2 P which are applied in a manner to guarantee that voltages present at nodes N 1  and N 2  are never higher than |V CC |. Accordingly, charge injection into the n-well of the p-transistors (e.g., M 1 , M 5 , M 3 , M 6 , etc.) is eliminated.  
         [0061]     However, the first stage of V bb  pump  20  further includes transistors M 21  and M 22  and the second stage of V bb  pump  20  further includes transistors M 41  and M 42 . The transistors M 21  and M 22  and transistors M 41  and M 42  are used to form a switched floating p-well for transistors M 2  and M 4 , respectively. More specifically, the wells of transistors M 2 , M 21 , and M 22  are switched to node WN 1  and the wells of transistors M 4 , M 41 , and M 42  are switched to node WN 2 . Typically n-channel transistors have their p-well connected to the lowest potential voltage which is usually V SS  (i.e., ground). However, because nodes N 1  and N 2  can be more negative than V SS  (e.g., at V bb , as seen in  FIG. 3 ) in the current embodiment, the p-well connection is switched to either node WN 1  or to node WN 2 , respectively. In the current embodiment, transistors M 1 -M 9 , M 21 , M 22 , M 41 , M 42 , M 51 , and M 61  may be thin-gate transistors.  
         [0062]     In the current embodiment, M 2  is used as the main pumping transistor for the first stage. Accordingly, M 2  is sized larger than M 21  and M 22 . M 21  and M 22  are thus used to switch the well connection. For example, when N 1  is more negative than V bb  (e.g., when N 1  is at −V cc  as seen in  FIG. 6 ), M 22  is conductive and N 1  is connected to node WN 1 . When V bb  is more negative than N 1  (e.g., when N 1  is at V SS  as seen in  FIG. 6 ), M 21  is conductive and V bb  is connected to node WN 1 . To facilitate well switching, the n-channel transistors of the current embodiment are formed in a p-well inside of a deep well. Accordingly, either V bb  or the voltage at node N 1  (whichever is more negative) may be used to drive the p-well and deep n-well of the n-channel transistors M 2 , M 21 , and M 22 .  
         [0063]     Likewise, M 4  is used as the main pumping transistor for the second stage. Accordingly, M 4  is sized larger than M 41  and M 42 . M 41  and M 42  are used to switch the well connection. For example, when N 2  is more negative than V bb  (e.g., when N 2  is at −V cc  as seen in  FIG. 6 ), M 42  is on and N 2  is connected to node WN 2  and when V bb  is more negative than N 2  (e.g., when N 2  is at 0V as seen in  FIG. 6 ), M 41  is one and V bb  is connected to node WN 2 . To facilitate well switching, the n-channel transistors of the current embodiment are formed in a p-well inside of a deep well. Accordingly, either V bb  or the voltage at node N 2  (whichever is more negative) may be used to drive the p-well and deep n-well of the n-channel transistors M 4 , M 41 , and M 42 .  
         [0064]      FIG. 4 s  a detailed diagram of the V bb  pump of  FIG. 1  according to another alternative embodiment. The construction and operation of the V bb  pump  25  illustrated in  FIG. 4  is similar to V bb  pump  15  and V bb  pump  20  as illustrated in  FIGS. 2 and 3 , respectively. As discussed above in conjunction with  FIG. 2 , the first and second stages of V bb  pump  25  are responsive to control signals PH 1 , PH 1 P, PH 2 , and PH 2 P which are applied in a manner which guarantee that voltages present at nodes N  1  and N 2  are never higher than |V CC |. Accordingly, charge injection into the n-well of the p-transistors (e.g., M 1 , M 5 , M 3 , M 6 , etc.) is eliminated.  
         [0065]     Additionally as discussed in conjunction with  FIG. 3 , the first stage of V bb  pump  25  includes transistors M 21  and M 22  and the second stage of V bb  pump  25  includes transistors M 41  and M 42 , which are used to form a switched floating p-well for transistors M 2  and M 4 , respectively. Accordingly, either V bb  or the voltage at nodes N 1 , N 2  (whichever is more negative) may be used to drive the p-well and deep n-well of the n-channel transistors. In the current embodiment, transistors M 1 -M 9 , M 21 , M 22 , M 41 , M 42 , M 51 , and M 61  may be thin-gate transistors.  
         [0066]     The first stage of V bb  pump  25 , however, includes a thick-gate transistor M 11  and the second stage of V bb  pump  25  includes a thick-gate transistor M 12 . The gates of transistors M 11  and M 12  are connected to the output of inverter  30 . The input of inverter  30  receives the control signal FLOAT V bb . In the current embodiment, thick-gate transistors M 11  and M 12  remain on unless the control signal FLOAT V bb  is driven high. Accordingly, transistors M 11  and M 12  protect the thin-gate transistors of V bb  pump  25  when V bb  is forced to a voltage that is more negative than −V CC  (for example, during device testing or burn-in).  
         [0067]      FIG. 5  illustrates a portion from each of the V bb  pumps described in conjunction with  FIGS. 2-4 . More specifically, the layout of a portion from each of the V bb  pumps is redrawn to more clearly illustrate the operation of the cross-coupled transistors M 5  and M 6 . As discussed above, control signals PH 1  and PH 2  are non-overlapping, active-low clock signals (ranging from 0V to −V CC ). When PH 2  goes low (i.e., is driven to −V CC ), node N 5  couples low (i.e., to −V CC ) through transistor M 9 , and transistors M 1  and M 5  are turned on. Transistor M 1  couples node N 1  to V SS  and transistor M 5  couples node N 6  to V SS . At this time, transistor M 3  is off. After node N 1  is driven to V SS , control signal PH 2  goes high (i.e., is driven to 0V) coupling node N 5  to 0V and thus transistors M 1  and M 5  are turned off.  
         [0068]     Control signal PH 1  then goes low (i.e., is driven to −V CC ) and couples node N 6  low (i.e., to −V CC ) through transistor M 7 , and transistors M 3  and M 6  are turned on. Transistor M 3  couples node N 2  to V SS  and transistor M 6  couples node N 5  to V SS . At this time, transistor M 1  is off. After node N 2  is driven to V SS , control signal PH 1  goes high (i.e., is driven to 0V) coupling node N 6  to 0V and thus transistors M 3  and M 6  are turned off.  
         [0069]      FIG. 7  is a more detailed circuit diagram of the V bb  pump  25  of  FIG. 4  according to one embodiment. More specifically, the functions of transistors M 11  and M 12  are implemented using circuits  11  and  12 , respectively. Circuits  11  and  12  are used to protect the thin-gate transistors of V bb  pump  25  when V bb  is forced to a voltage that is more negative than −V CC  (for example, during device testing or burn-in). During testing or burn-in, the signal FLOATV bb  is driven high which causes, in circuit  11 , the gates of transistors M 50  and M 51  to be driven to V bb  through a series of inverters (e.g., M 58 -M 61 ) and which causes the gate of transistor M 52  to be driven to V CC  through a series of inverters (e.g., M 54 -M 57 ). A V bb  voltage on the gates of NMOS transistors M 50  and M 51  forces these transistors to be in the off state, thereby protecting transistors M 1  and M 2  from receiving a high negative voltage.  
         [0070]     The V CC  voltage on the gate of M 52  causes the p-well (WN 2 ) to be connected to V bb  for all three transistors (i.e., M 50 , M 51 , and M 52 ) hence protecting the p-well (WN 2 ) of these transistors from forward biasing. It should be apparent to one skilled in the art that transistors M 70 , M 71 , and M 72  operate in a similar manner in circuit  12 . Transistors M 50 , M 51 , M 52 , M 70 , M 71 , and M 72  are thick oxide transistors and can accept or tolerate high negative voltages used during burn-in.  
         [0071]     Although the current discussion has thus far been focused on V bb  pumps, it should be apparent to one having ordinary skill in the art that the use of thin-gate transistors in V CCP  pumps is also within the scope of the present invention. For example,  FIG. 8  illustrates a V CCP  pump  35  according to one embodiment. One having ordinary skill in the art will appreciate the general operational characteristics of a V CCP  pump (e.g., charging a node to a first voltage, boosting the node to a second voltage, and connecting the node to an output). Thus, a detailed description of the general operation of the V CCP  pump  35  is omitted.  
         [0072]     In the current embodiment, V CCP  pump  35  is comprised of symmetrical first and second stages. The operation of the first stage and second stage are essentially the same with the exception of the complimentary timing required to alternate the firing of each stage. It may be noted that, in the current embodiment, the timing and values of the control signals are managed to prevent |V GS |, |V GD |, and |V DS | for transistors, most notably the thin-gate transistors, from exceeding V CC . Accordingly, the thin-gate transistors used by V CCP  pump  35  are protected from punch-through, latch-up, gate oxide degradation (for example, from hot electron effects such as time dependent dielectric breakdown (TDDB), voltage threshold (|V t |) shifts from hot electron trapping, and excess substrate current (for example, due to avalanche multiplication), among others. Other benefits, such as the reduction and/or elimination of charge injection, may also be achieved.  
         [0073]      FIG. 9  is a simplified block diagram of a memory system  58  according to one embodiment. The memory system  58  includes a memory controller  60  and a memory device  50 . It should be apparent to those skilled in the art that the memory system  58  may include other components while remaining within the scope of the present invention. For example, memory system  58  may include a microprocessor, micro-controller, ASIC, etc. which is in communication with the memory controller  60  and the memory device  50 .  
         [0074]     The memory controller  60  and the memory device  50  communicate via a system bus  57 . In the current embodiment, the system bus  57  carries command signals, address signals, and data signals, among others. The system bus  57  may be sub-divided into two or more buses, for example a command bus (not shown), an address bus (not shown), and a data bus (not shown). The command bus may carry row address strobe (RAS#), column address strobe (CAS#), and write enable (WE#) command signals, among others. The address bus may carry bank address (BA 0 , BA 1 ) and address input (A 0 -A 12 ) signals, among others. The data bus may carry data input/output signals (DQ 0 -DQ 15 ), data strobe signals (LDQS, LDQS#, UDQS, UDQS#), and data mask signals (LDM, UDM), among others. Additionally, some command signals, such as the chip select (CS#), clock enable (CKE), and on-die termination (ODT) signals may be carried by another portion of the system bus  57 . It should be apparent to one skilled in the art that the topology of the system bus  57  (and its component parts) may be varied while remaining within the scope of the present invention. It should further be apparent to one skilled in the art that the illustrated signals are for exemplary purposes only and not intended to limit the present invention in any manner.  
         [0075]      FIG. 10  is a simplified block diagram of the memory device  50  of  FIG. 9  according to one embodiment. Memory device  50  is comprised of a read/write control unit  51 , a row decoder  52 , a column decoder  53 , a sense amplifier  54 , a memory array  55 , and a voltage pump which may incorporate one or more features of the present invention.  
         [0076]     The memory array  55  includes a plurality of memory cells organized in rows and columns. Each memory cell can hold one bit of data representing a binary zero and binary one. To access a particular memory cell within the memory array  55 , for example during a read operation, the read/write control circuit  51  issues the row address of the cell to the row decoder  52  and the column address of the cell to the column decoder  53 . The row and column decoders  52 ,  53  activate the cell and the sense amplifier  54  senses the information stored within the cell. The sense amplifier  54  may also be used to refresh the selected memory cell during the read operation. The voltage pump may be used to provide a voltage (for example, the voltage V bb ) to the memory device  50 . The voltage pump may incorporate one or more features of the present invention. More specifically, V bb  pump  15 , V bb  pump  20 , V bb  pump  25 , and/or V CCP  pump  35  may be incorporated into memory device  50  and supply the voltage V bb  and/or V CC  to one or more components of the memory device  50 .  
         [0077]     The read/write control unit  51  may be responsive to a plurality of command signals issued by the memory controller  60  via the a system bus  57  as discussed above in  FIG. 9 . The read/write control unit  51  may include a command decode circuit and mode register circuits, among others to decode and store the command signals. As discussed above, the read/write control unit  51  issues commands to the row decoder  52  and column decoder  53  and receives information from sense amplifier  54 , among others. It should be apparent to those skilled in the art that the memory device  50  may include other components while remaining within the scope of the present invention.  
         [0078]     It should be apparent to those of ordinary skill in the art that equivalent logic or physical circuits may be constructed using alternate logic elements while remaining within the scope of the present invention. For example, complementary logic (e.g., substituting p-MOS transistors for n-MOS transistors and vice versa) may be used with the appropriate logic changes while remaining with the scope of the present invention.  
         [0079]     It should be recognized that the above-described embodiments of the invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.