High voltage switch suitable for non-volatile memories

[0002] 1. Field of the Invention

[0003] This invention relates generally to integrated circuit semiconductor devices, and, more specifically, to high voltage switches.

[0004] 2. Background Information

[0005] In an integrated circuit, it is common to need a circuit to provide a voltage from a voltage source to an output in response to an input signal. An example is a wordline select circuit of in a non-volatile memory. In such a circuit, a relatively high programming voltage is supplied to a wordline in response to an input signal at the device to device logic level. For example, in fairly typical values for a NOR type FLASH memory, 8-10V is provided on a wordline in response to an input going from ground to “high” value of 3-5V. To improve the operation of the circuit, it is important that the voltage on the wordline reaches its full value quickly in response to the input going high.

[0006] Many designs exist for such switches. A number of common designs use an NMOS transistors and a local charge pump to raise the gate voltage values used to turn on the transistor and pass the high voltage from the source to the output. Due to the body bias of the NMOS transistors and charge pump ramping speed, these switches generally take a relatively long time to reach the passing voltage level need to pass the full high voltage. These problems are aggravated by both higher programming voltage level needed and lower device supply voltages as these combine to make it harder to pump efficiently and timely due to body effects of NMOS transistors in the charge pump.

[0007] The present invention utilizes a boost-strap method to improve switch operation in a design that is particularly advantageous for supplying high voltages within a low voltage design. The invention utilizes a native NMOS transistor, a PMOS transistor, and a capacitor connected in series between the high voltage source and the output. In a first embodiment, a native NMOS transistor is connected between the voltage source and a first node, a PMOS device being connected between this first node and a second node, and a capacitor being connected between the second node and the output. The input signal is supplied through an intrinsic NMOS to the gate of the native NMOS device and output side of the capacitor, with the delayed input supplied to the node between the capacitor and the PMOS and, in inverted form, to the gate of the PMOS.

[0008] In response to the input signal going high, the delay allows for an initialization phase to precharge the capacitor and partially turn on the native NMOS device. After the delay, the PMOS is turned on and the native NMOS is further turned on by its gate being boosted up by the capacitor to allow the voltage from the source to boost up the output. The use of the delay for precharging the capacitor allows the output to be pre-charged to close to the full high logic level. During the boost phase, due to the predefined boosting ratio, the output is raised above the value of the high voltage supply. This higher than high voltage supply voltage will allow better over drive to overcome the threshold voltage of body-biased native NMOS transistors.

[0009] The non-delayed input is supplied to the gate of the native NMOS and the delayed input is supplied to the node between the PMOS in the first exemplary embodiments. In one variation of these first embodiments, the input is supplied to the gate of the native NMOS device and the output side of the capacitor through a second native NMOS device whose gate is connected to receive the input in an inverted, delayed form. The use of the second native NMOS allows for a better overdrive as the threshold voltage of the second native NMOS will provide a higher voltage to the first NMOS's gate in the initialization phase.

[0010] A further exemplary embodiment adds an additional path from the node between the native NMOS and the PMOS to the output and uses an enable signal in addition to the input signal. The enable signal is a delayed version of the input signal when the input signal is being asserted and the same as the input when it is de-asserted. In the initialization phase, when the input signal has gone high but the enable signal is still low, the additional path is used to equalize between the gate and drain of the native NMOS in the initialization phase and is cut off in other cases. In a further aspect of this embodiment, instead of supplying the input to the gate of the PMOS transistor, the gate is diode-connected to the chip voltage supply, and it level can be altered through the coupling between the source-drain junction and its channel. In both this and the other embodiments, the PMOS is designed to be tolerant of high voltage levels across its oxide.

[0011] Additional aspects, features and advantages of the present invention are included in the following description of specific representative embodiments, which description should be taken in conjunction with the accompanying drawings.

[0012] FIG. 1 shows a switch using a native NMOS and PMOS to supply a high voltage to the gate of a transistor.

[0013] FIG. 2 presents the elements of the present invention between the voltage source and the output.

[0014] FIG. 3 is a first exemplary embodiment of the present invention.

[0015] FIG. 4 is a second exemplary embodiment of the present invention.

[0016] FIG. 5 shows the select response of the second exemplary embodiment of the present invention.

[0017] FIG. 6 is an enlarged detail of FIG. 5.

[0018] FIG. 7 shows the deselect response of the second exemplary embodiment of the present invention.

[0019] FIG. 8 is an enlarged detail of FIG. 7.

[0020] FIG. 9 is a third exemplary embodiment of the present invention.

[0021] FIG. 10 shows the response of the third exemplary embodiment of the present invention.

[0022] FIG. 1 shows another prior design that overcomes many of the problems described in the Background section. The switch 100 supplies a voltage derived from the voltage source V_pin response to an input signal V_into, in this case, the gate of a transistor 141 that supplies the voltage V_pto a wordline WL 143. The switch supplies the voltage V_pto the output through a native NMOS transistor 103 and PMOS transistor 105 connected in series. The gate of the PMOS transistor 105 is connected to the input V_inthrough inverter 107. A transistor 109, whose gate is connected to V_dd, is connected between the input voltage V_inand output node Z, that is also connected to the gate of NMOS 103.

[0023] In an application as a wordline select circuit, V_pis the passing voltage and may have a value of 20V or more in the case of a NAND memory structure. In the examples below, V_p=24V is used. In a low voltage circuit, the “low” value of V_inwill taken to be ground and the “high” value V_ddas 2V or less, for example 1.6V. Examples of non-volatile memories where a high voltage switch such as described here can be utilized are given in U.S. patent application Ser. No. 09/893,277, entitled “Operating Techniques for Reducing Effects of Coupling Between Storage Elements of a Non-Volatile Memory Operated in Multiple Data States”, filed on Jun. 27, 2001, and references incorporated therein, all of which are hereby incorporated herein by this application.

[0024] To see the function of the circuit in FIG. 1, consider its operation starting from the input being low, V_in=0V, such as when the wordline 143 is deselected and switch 100 is in standby mode. The threshold voltage of PMOS can be taken as −1V. Due to inverter 107, the gate of PMOS 105 is held high V_ddand the PMOS is consequently turned off. The gate of the NMOS 103 is at 0V and is also consequently turned off. As NMOS 103 is a native device, it has a negative threshold value, say −0.8V, and node X would be at about 0.8V. Node Z is at ground and transistor 141 will be off, so that word line 143 is disconnected from the voltage supply.

[0025] When V_inincrease from ground to V_dd=1.6V, if transistor 109 is taken to have a threshold voltage of 0.8V, V_Zwill be charged at 0.8V and NMOS 103 will begin to turn on. The voltage on the gate of the PMOS 105 will drop to 0V and it will turn on. Consequently, the voltage at node X will go from 0.8V to 1.6V. The voltage at node Z, and at the gate of NMOS 103, will consequently rise until it reaches V_p. The native NMOS 103 is selected so that its threshold voltage is below 0V even with the effects of the body bias resulting from the high value of V_p. When V_in=V_dd, as the gate of transistor 109 is held at V_dd, transistor 109 is diode connected and no charge will leak away from node Z through NMOS 109.

[0026] The voltage V_Z=V_pis then applied to the control gate of the transistor 141 and the high voltage is applied to the wordline WL 143. The full value of the program voltage will not be passed, however. The threshold voltage of the pass gate V_th, that may be, say, 0.8V, may increase to something like 2V due to body bias when a voltage such as V_p≈24V is placed on the gate. Consequently, the wordline will receive a voltage of V_p−V_th≈22V, or somewhat less than the full programming voltage. Thus, if V_WLis the desired voltage on wordline WL 143, the passing voltage V_pmust be raised to V_p=V_WL+V_th(V_WL).

[0027] The circuit of FIG. 1 improves over the prior art as described in the Background section by using a smaller area on the device. As noted above, in order to supply a voltage of V_WLto the wordline WL 143, a higher voltage of V_p=V_WL+V_th(V_WL) needs to be supplied to the pass gate 141 and this additional higher voltage places extra stress on the system. Additionally, it is difficult to determine exactly the exact voltage that needs to be supplied by the charge pump as the dependence of the transistor's threshold voltage on the voltage on the wordline due to body bias, V_th=f(V_WL), is non-linear.

[0028] Another area where the circuit of FIG. 1 can be improved is in its speed to raise the output from ground to a high V_pin response to the input. As the voltage on node Z raises and the voltage on the gate of the native device 103 increases and the voltage on the gate increases further. Again, due to body bias, the threshold voltage of transistor 103 begins to increase so that the rate of increase of V_Zbegins to roll off significantly, producing a long tail region above, say, 15V as it asymptotically approaches a V_pof ˜24V. For typical values component values in the circuit of FIG. 1, this can result in several microseconds to get the full passing voltage to the gate of word line select transistor. A discussed below, the exemplary embodiments of FIGS. 3, 4 and 9 allow a higher speed to reach the passing voltage.

[0029] The present invention improves the speed of the device in FIG. 1 by adding a capacitor in series with the transistors 103 and 105 between the output and voltage source and precharging the output side of the capacitor. The gate voltage on the pass gate 141 is also increased. These changes will be described with respect to FIG. 2 that shows the circuit elements between the voltage source and output along with various inputs and nodes.

[0030] FIG. 2 shows a native NMOS transistor 203 connected between a voltage source V_pand a node X and with a gate connected to node Z. Between node X and the output at node Z the PMOS transistor 205 is connected and the input to the gate is indicated by b. Capacitor 211 has been added between the output at node Z and node Y below the PMOS transistor 205. The output is again shown connected to the gate of a transistor 241 for supplying a programming voltage to a wordline WL 243.

[0031] The present invention differs from the embodiment of FIG. 1 by initializing the circuit by precharging the lower plate of the capacitor. This is done by delaying the input to the gate of PMOS 205 and node Y, which is now differentiated form node Z by the introduction of capacitor 211. The initialization is performed by charging node Z to V_ddwhile keeping node Y near 0V. By additionally precharging the capacitor, an extra voltage above the supply voltage can be supplied to help overcome the body bias of pass gate.

[0032] In FIG. 2, the initialization circuits to nodes Z and Y are not shown, although specific examples are shown in the embodiments of FIGS. 3, 4, and 9. These circuits initialize the capacitor 211 before boosting by pre-charging node Z to V_ddwhile node Y is kept at 0V. During the boosting, node Z is left floating and node Y is charged from PMOS 205.

[0033] In more detail, in the standby mode when V_in=low (0V) and wordline WL 243 is deselected, the voltage on the output node Z and the gate of transistor 203 is also low, V_Z=0V. The input is supplied to PMOS 205 in inverted form so that V_b≈V_dd. The voltage at node X is set by the threshold voltage of, say −0.8V, the native NMOS 203, V_X≈−V_th,203≈0.8V. With all the given conditions, the native NMOS transistor 203 and PMOS 205 are both off.

[0034] When the input signal goes high, V_in=V_ddand the transition/initialization phase begins. The input is supplied to node Z, so that V_Z≈V_dd. (As described below with respect to FIGS. 3 and 4, these values will differ from V_dddue to being supplied through a transistor, NMOS transistor 321 in FIG. 3 and native NMOS 421 in FIG. 4.) Consequently, NMOS 203 begins to turn on some. This places the node X at V_X≈V_dd+(−V_th,203)≈V_dd+0.8V. As the delay to the gate of PMOS 205 and node Y is delayed, these stay at V_b=·V_ddand V_Y=0V respectively.

[0035] Once the system is initialized, it is fully enabled by providing the delayed input in inverted form to b so that V_b=0V and the PMOS will turn on. As with FIG. 1, since both transistors are on nodes X and Y will then go to the value of the voltage source, V_X=V_Y=V_p, as will the output at node Z. However, due to the initialization, this transition will occur more quickly by charging up node Z in transition for a boost-strap effect. As described below with respect to FIG. 4, by precharging node Z with V_ddthrough transistor 421, the output these voltage can be raised above V_p, V_dd<V_Z<V_p+V_dd, to offset body bias effects, allowing node X to reach V_pfaster and also allowing the full programming voltage to reach the wordline quicker. To keep the charge on nodes Y and Z after initialization, the precharging devices are diode connected to avoid charge leaking away.

[0036] FIG. 3 shows a first exemplary embodiment of the present invention that implements the process described with respect to FIG. 2. This circuit is similar to that of FIG. 1, but has a number of added elements that implement the process described with respect to FIG. 2. A native NMOS transistor 303 is connected in series through node X with the PMOS 305 between the voltage source at V_pand the output at node Z. The capacitor 311 is added between node Y below PMOS and node Z. To initialize the capacitor for bootstrap effect, the input is now driving node W and the lower plate of capacitor 311, as well as node Y. (Node W is shown distinct from node Z to simplify the discussion.) The gate of transistor 321 is connected to V_ddso that when the input goes high, this will act as a diode to keep the high voltage from flowing back to the input. The delay for V_into node Y and the gate of PMOS 305 is respectively provided by the pairs of inverters 325a,325band 323a,323b.Inverter 307 and transistor 309 provide the same function as the corresponding elements of FIG. 1.

[0037] FIG. 4 shows a second exemplary embodiment. The embodiment of FIG. 4 differs from that of FIG. 3 in that transistor 421, which is now a native NMOS device instead of a regular NMOS, is now connected with its gate to receiving a delayed, inverted input. In the circuit of FIG. 2, the higher the voltage on the node Z and during the initialization, the quicker the full programming voltage will be supplied to the wordline WL 243. This is due to the higher voltage on the output side of the capacitor 211, the higher voltage on the gate of transistor 203, and the extra voltage on the gates of transistors 203 and 241 that helps offset the body bias as the voltage through these transistors approach V_p. In the embodiment of FIG. 4, a native NMOS device 421, with a V_th≈−0.8V, replaces the non-native transistor 321 of FIG. 3, with V_th≈+0.8V. The negative threshold value of the native device 421 produces the extra voltage on node W during the initialization phase. Consequently the embodiment of FIG. 4 provides improved performance at the cost of some extra complexity over that of FIG. 3.

[0038] The gate of transistor 421 is connected to receive the delayed, inverted output. During standby when V_in=0V, the gate of NMOS 421 is at V_ddand transistor 421 is on. When V_infirst goes high, transistor 421 stays on due to the delay from the inverters 431a,431b,and 431cand node W goes to near V_dd. After the delay, the gate of NMOS 421 will go low and turn the transistor off at the same time that PMOS 405 turns on, thereby trapping the charge on nodes W and Z and preventing it from flowing back out the input.

[0039] In both of FIGS. 3 and 4, the delays are implemented through the use of inverters. More generally, other circuit elements could be utilized to implement this function. Also, in both of these figures a pair of transistors (309 and 321, 409 and 421) are used to allow the precharging of nodes Y and Z during the initialization phase and prevent the charge leaking away during boosting. More generally, other circuit elements could also be utilized to implement these functions.

[0040] FIGS. 5-8 are a simulation demonstrating the operation of the exemplary embodiment of FIG. 4 using the values V_dd=1.6V and V_p=24V. FIG. 5 shows the select process where the input goes from a low value V_in=0V to a high value of V_in=V_dd. During the initialization phase, V_Zincreases from ground to near V_ddas the input is supplied to the output side of the capacitor, while the level between the capacitor and the PMOS stays low, V_Y=0V. After the delay, V_Yand V_Zrapidly increases to near V_pand V_p+V_dd−Δ, respectively, where Δ is design dependant and depends on the capacitance ratio between the boosting capacitor and the output node loading capacitance. The voltage on the wordline WL 443, V_WL, follows the voltage V_Zon the select gate 441, rising to near V_ddduring initialization and to V_pafter the delay. Note that during the enable phase, V_Zis above V_Yproviding the extra voltage on the gate to offset the body bias of transistor 441 that allows V_WLto approach V_p. The tail of the V_WLcurve is also shortened compared t embodiment of FIG. 1, which typically would require 3-4 μs to pass 24V to the wordline.

[0041] FIG. 6 is a detail of FIG. 5. The standby state begins at about 2 ns once node X is charged up. In this phase, the gate of PMOS 405 (V_b) and the gate of transistor 421 (V_g,T2) are both at V_dd. The value of V_Xis due to the negative threshold of NMOS 403. At about 9 ns, the input goes high, V_in=V_dd, and the output V_Zbegins to rise to near V_dd, followed by V_WL. V_Xalso rises as V_W=V_Z, turning transistor 403 partially on. After the delay, the voltage on the gate of transistor 421 (V_g,T2) goes low, trapping the voltage on node Z, and transistor 409 is diode connected so there is no more pull down on node Y and it begins to rise also, and V_bgoes low, turning on PMOS 403, at which time V_Xand V_Yall go towards V_p, with V_Zgoing to V_p+Δ, followed by V_WL.

[0042] The corresponding de-select process is shown in FIG. 7, with the details of the delay portion of this figure shown in FIG. 8. The internal nodes are discharged through transistors 421 and 409.

[0043] The PMOS transistor (405 in FIG. 4) in the present invention will pass the voltage V_p, which may be 24V or higher. The voltage on the source is that of node X, V_s=V_X, the voltage on the drain is that of node Y, V_d=V_Y, the well (or body) voltage, V_w, is tied to the higher of these, and the voltage at the gate is V_g=V_b. During standby, using the exemplary values, V_g=V_dd, V_s=V_b≈0.8V, V_d=0V; during initialization, V_g=V_dd, V_s=V_b≈V_dd+0.8V, V_d=0V; and during enable V_g=0V, V_s=V_b=V_d=V_p. Thus, for a low voltage device with, say, V_dd=1.6V, the voltage between the source and drain will not exceed about 2.4V, as can be seen be by comparing the V_Xand V_Ycurves in FIG. 6.

[0044] The voltage difference between the gate and the channel is, however, the full programming voltage V_pduring the enable phase. Consequently, the present invention employs a thick gate oxide for the PMOS transistors 305 and 405 of FIGS. 3 and 4, respectively. As the final output voltage depends on the size of the capacitor, it needs to have a sufficient size to maintain the desired output. This is discussed further below with respect to the embodiment of FIG. 9, but here the term in the parasitic capacitance C₂due to transistor 852 in FIG. 9 is lacking.

[0045] FIG. 9 presents a third exemplary embodiment. This embodiment differs from that of FIG. 4 in several respects. The first of these is that a path has been added between node X and the output at node Z. The second is that the gate of the PMOS transistor between nodes X and Y and the left hand connection of the transistor connected to node Y are no longer connected to receive V_in.

[0046] The input signal is now applied only to node W through transistor 821, which is again preferable a native NMOS device, and in inverted, delayed form to the gate of transistor 821 and the added PMOS transistor 851. The delay and inversion of the input signal is again implemented here by the set of inverters 831a-c.Node W is again connected to the native NMOS 803. In this embodiment, the node X at the lower end of transistor 803 has the new connection to node Z through the PMOS 851 and the diode connected NMOS 852, which is also preferably a native device or low threshold voltage NMOS.

[0047] PMOS 805 has its gate connected to V_ddthrough the diode connected transistor 853. Rather than being turned on by the delayed, inverted version of the input signal, PMOS 853 operation is now determined by the relative levels at nodes X and Y and its gate, as will be described below.

[0048] NMOS transistor 809 is connected as before, except that it now receives a delayed version of the signal V_enablethrough the pair of inverters instead of V_in. The enable signal is a delayed version of the input signal on a rising waveform and the same as the input signal on a falling waveform, as described more fully below. Capacitor 811 is connected as in the other embodiments, although its value may differ here. The output node Z is again shown connected to the gate of a select transistor 841 for supplying the programming voltage to a word line WL 843, although it can easily be employed in other applications.

[0049] Relative to the other embodiments, the current path from node X to output node through transistors 851 and 852 has been added. The PMOS transistor 851 will be turned on as transistor 821 is turned off. NMOS transistor 852 is diode connected and is preferable a native device. During the precharge phase prior to the enable signal being asserted, this allows a voltage higher than V_ddto be applied to node Z from node X.

[0050] The design of FIG. 9 uses the boost-trap method to achieve a higher voltage at the output node Z than the maximum supply voltage V_p. When this boosted voltage is applied to the gate of the passgate 841, it produces a cancellation of the threshold voltage of transistor 841 when passing high voltage V_pto WL 843.

[0051] To consider the operation of the circuit of FIG. 9, assume the maximum voltage supply is again, say, V_p=24 volts at all time, that the high logic level is V_ddand the low level is 0 volts. There are two input controls: V_inputand V_enable, which are turned on sequentially. The output of this local pump is V_Z, which will go the gate of transistor 841.

[0052] During the standby phase, V_p=24 volts, V_input=V_enable=0V. For node V, connected to the gate of transistors 821 and 851, V_V=V_dd. Similarly, V_Z=0V, V_X=abs(V_th,803), V_a=V_dd, and V_Y=0V, where abs(V_th,803) is the absolute value of the threshold voltage of the native NMOS 803. Boosted node Z, which goes to the gate of the high voltage passgate transistors such as 841, is kept at 0V so there is no passing of high voltage. During the standby condition, PMOS 805 is in cut-off region so there is no current flow of the high voltage V_pto V_Y.

[0053] When V_inputis enabled and switches from 0V to V_dd, V_Wis charged up to V_ddand V_Xis charged up to V_dd+abs(V_th,803). The signal on V_Vis delayed and inverted relative to V_inputand goes to ground after passing through inverters 831a-c.Transistors 851 and 852 are then enabled thereby allowing V_Xand V_Zto be equalized to same potential. As long as the threshold voltage of transistor 803 is less than V_Z−V_X, the voltage source at V_pwill continuously charge up V_Xand, through 851 and 852, V_Zuntil an equilibrium state is reached. The equilibrium state is determined by the pull-up of transistor 803 and the combined pull-down of transistor 805, transistor 809 and inverter 825b.For typical values of these elements, this voltage level could be around 10 volts.

[0054] During all the preceding operations, V_Ymust be kept close to the low voltage level, here ground, to allow the future boost from node X to be transferred better to node Z. During this phase, PMOS 805 could be slightly turned on, but V_Yis kept near 0V strongly by transistor 809.

[0055] The purpose of introducing the elements PMOS 851 diode connected NMOS 852 is to allow node Z to be precharged to a higher level than the chip Power Supply V_dd. During this precharge phase, the voltage on the gate of PMOS 805 is coupled up by V_Xto reduce the leakage between nodes X and Y.

[0056] At certain determined time, determined for example by a global clock, the boost-trapping process is enabled. V_enableis switched from low to high, and raises the input to transistor 809. Node Y is no longer being kept low and will by charged up quickly by X. In turn, V_Zis boosted up by some percentage of V_X. 805, 853, and 809 are ratioed to allow 805 to conduct little current prior to the point V_enablegoes high. During the boost-trap phase, transistor 852 is diode connected between PMOS 851 and node Z. Since the node between PMOS 851 and NMOS 852 is at V_X, the path between PMOS 851 and V_Zis cut off so that no charge on Z will be lost and no back leakage occurs.

[0057] The size of boost-trap capacitor 811 can be adjusted depending on the precharge level on V_Zduring the first phase of precharge (V_input=high, V_enable=low) and the capacitive loading on the output node Z. The higher V_Zcan be precharged, the smaller the capacitance of 811 can be.

[0058] During the discharge phase, V_inputand V_enableare switched together from V_ddto 0V and the circuit will return to the standby condition.

[0059] Unlike the embodiments of FIGS. 3 and 4, the embodiment of FIG. 9 uses two separate but related input signals, V_inputand V_enable. V_enableis a delayed version of V_inputfor the low to high edge and the same as V_inputfor the high to low edge. This delay may be controlled, for example, by a global clock (not shown) external to circuit 800 or other mechanism after V_inputis enabled and switches high. When V_inputswitches from 0v to V_ddat time 0 ns, V_enablewill switch from 0v to V_ddat a time of, say, 50 ns when the global clock switches from low to high. V_enableis supplied to transistor 809 through the inverters 825a-bto reduce the loading as the node V_Yis kept near 0V before V_enableis asserted.

[0060] As already noted, the embodiment of FIG. 9 also introduces the new path between nodes X and Z. The diode connected transistor 852 is a preferably a native device to get the minimum voltage drop on node V_Zfrom node V_Xdue to the threshold value of the native device during the precharge phase. Alternately, 852 could be any other NMOS type device, but the lower its threshold, the higher the value of V_Zclose to V_X. The different device type, and its corresponding threshold voltage, will determine the initial precharge level on node Z, and consequently determine the size of boosting capacitor of 811.

[0061] The more the threshold voltage drop through transistor 852 due to body effect can be reduced, the higher the value of V_Zvoltage during the precharge phase and so the smaller size needed for capacitor 811. In the embodiments of FIGS. 3 and 4, node Z is precharged to only the V_ddlevel in the best case with the subsequent boosting starting only from this level. This lower level requires a much larger boosting capacitor 311 or 411. The embodiment of FIG. 9 allows node Z to be precharged to an equilibrium point where V_Zis much higher than V_dd, say 8 volts. At the equilibrium point, NMOS 852 should be cut off, or close to being cut off, by the body effect of its threshold voltage due to the equilibrium state between V_Xand V_Zduring the initialization phase.

[0062] Transistor 851 is a PMOS device. During standby, V_X=V_th,803and V_Z=0V. PMOS 851 prevents current flow between nodes X and Z as the gate of 851 is at V_dd. When the circuit is enabled, 851 and 852 are used only in precharge phase. The more voltage drop between nodes X and Z is reduced, the higher the precharge on node Z. The equilibrium point will be the determined by the voltages V_Zand V_Xand transistors 805 and 809.

[0063] During the boost phase when V_Zis boosted high, the path between nodes Z and X should be disconnected. NMOS 852 supplies this function since the node between 851 and 852 is high enough during precharge to cut off 852 and preserve the charge on node Z from leaking onto node X.

[0064] The node Z is capacitively coupled on one side to node Y through capacitor 811. Node Z also has a number of parasitic capacitances on the other side due to the junction capacitances of transistors 852 and 821, the gate capacitances of transistor 803, and any load capacitances, here the gate capacitance of transistor 841. If the capacitance of capacitor 811 is called C₂and the combined parasitic capacitance is called C₁=C_j,852+C_j,821+C_gate,803+C_gate,841, then the final voltage on node Z due to charge conservation, V_finalon node Z is given by

[0065] V_final=V_precharge+(C₂/(C₁+C₂))V_p,

[0066] where V_prechargeis the level on node Z in the initialization phase. Thus V_finaldepends on V_precharge, C₁, C₂, and V_pp. As C1 and V_ppare more or less fixed, the value of V_finalis most easily changed by either increasing V_prechargeor making C₂much larger than C₁. As increasing capacitance is generally expensive, to obtain a given V_finalvalue it is generally preferable to increase V_precharge. As described, the path between nodes Z and X through PMOS 851 and NMOS 852 of the embodiment of FIG. 9 allows this initialization value to be set higher and, consequently, reduce the size of capacitor 811.

[0067] Referring back to FIG. 2, as in the embodiment of FIG. 4, the embodiment of FIG. 9 again applies the input signal to node a and node Z when V_inputis first asserted. Unlike the earlier embodiments, after a delay the path between node X and Y is then opened during the initialization phase to obtain the higher value for V_precharge. This path is then closed again in the enable phase after V_enableis asserted. Also unlike the earlier embodiments, the voltage level at node b is not supplied directly by V_input.

[0068] The gate of PMOS 805 is connected to V_ddthrough the diode connected transistor 853. During standby, the voltage on the gate of PMOS 805 is given by V_b=(V_dd−V_th,853), where the threshold voltage V_th,853of NMOS 853 is preferably close to 0V. Consequently, the path from node X to node Y is cut off and V_X=(V_dd−V_th,803) while V_Y=0V. During the initialization of the precharge phase, V_Xand V_Zwill rise to the equilibrium level of, say, 8V. As the gate of 805 is coupled up through the gate-source overlap capacitance, C_jo, the level on the gate is dependent on the coupling ratio, having a value of, say, 4V in an exemplary embodiment. This will turn on PMOS 805 weakly and reduce the pull up strength than with its gate fixed at V_dd. However, V_Yis kept close to 0v with by maintaining V_enablelow. The strong pulldown of transistor 809 and inverter 825bis used to keep V_Y≈0v and V_Z≈8v. This is the pre-condition to set up the boost trapping process. The combination of the strong pulldown on node Y and strong pullup on node Z is used to set up this equilibrium in the initialization phase. When the enable signal is asserted and V_enable=V_dd, the equilibrium state between transistors 803, 805, 809 and inverter 825B is broken and the node Z is boosted to, say, V_Z=26V.

[0069] More generally, the arrange of FIG. 9 for the operation of PMOS 805 can also be used in the corresponding element (305, 405) of the first and second exemplary embodiments of FIGS. 3 and 4. Conversely, in circuit 800 of FIG. 9, the gate of PMOS could alternately be controlled by inverting the enable or input signal with proper delay. Boosting the voltage on the gate of PMOS 805 as in FIG. 9 reduces the power consumption from the voltage source at V_p.

[0070] The operation of circuit 800 is shown in FIG. 10. This shows the values of V_Zand V_Yfor two different load levels on the output and the voltage level on the gate of PMOS 805. The level of the load corresponds to the C_gate,841term of C₂in the expression for V_finalabove. Lines 901 and 902 respectively correspond to V_Zand V_Yfor a higher load, lines 911 and 912 respectively correspond to the same nodes for a lesser load, and line 920 is the voltage on the gate of PMOS 805. As in the earlier figures, FIG. 10 again uses the exemplary values of V_dd=1.6V and V_p=24V. On the time scale along the bottom, the initialization phase begins at time 0 and runs to 400 ns, the enabled phase from 400 ns to 3 μs, and the system returns to standby after 3 μs.

[0071] During standby mode, as shown at time 0, V_Zfor both loads (901 and 911) and V_Yfor both loads (902 and 912) are at the lower logic level of 0V while the gate of PMOS 805 (920) is at V_ddless any voltage drop across 853. At time 0, the initialization phase begins when V_ingoes high. Node Z quickly rises, first due to the input signal passing through transistor 821 to node W, followed by the path from node X to node Z opening. For the exemplary values, this take V_Yto around 9V, with the value for the higher load (901) being slightly higher than for the lower load (911). Through the coupling by its overlap capacitance, the gate of PMOS 805 is taken to near 8V (920), while V_Y(902 and 912) are held near ground by the pulldown.

[0072] At 400 ns, V_enablegoes high and the circuit passes into the enabled phase. The gate voltage of PMOS 805 jumps to near 20V and then decays back towards around 6V (920). V_Yrises to just under V_p=24V, with the value for the higher load (902) lying slightly above the value for the loser load (912). The output voltage on node Z rises to over the value of the voltage source: for the higher load (901) V_Z≈26.5V and for the lower load (911) V_Z≈25V for the exemplary values. In both cases, V_Zis boosted over the value of V_ppto offset body effects in the pass gate 841.

[0073] At 3 μs, both the input and enable signals are de-asserted. The voltage on the gate of PMOS 805 (920) bounces down below ground and returns back to around V_dd. V_Y(902, 912) go quickly to ground, closely followed by V_Z(901, 911), to return the circuit to the standby state.

[0074] The exemplary embodiments have been discussed in the context of a wordline select circuit, although the uses of the present invention extend to broader application. The V_pand V_invalues need not be, respectively, as high and as low as those described, but the present invention is able to operate into these more difficult ranges or whenever there a need to speed up the transition time of a high voltage switch.

[0075] Although the various aspects of the present invention have been described with respect to specific exemplary embodiments, it will be understood that the invention is entitled to protection within the full scope of the appended claims.