Abstract:
A pull-down circuit for pulling a high-impedance node to ground when a pull-down (PD) signal driving the pull-down circuit is Logic 1. The pull-down circuit comprises: 1) a first pull-down N-channel transistor having a drain coupled to the high-impedance node, a gate coupled to the PD signal, and a source coupled to a common node; 2) a second pull-down N-channel transistor having a drain coupled to the common node, a gate coupled to the PD signal, and a source coupled to a ground rail;, wherein the first and second pull-down N-channel transistors are off when the PD signal is Logic 0 and are on when the PD signal is Logic 1; and 3) a gate-biasing circuit driven by the PD signal. The gate-biasing circuit is off when the PD signal is Logic 1 and the gate-biasing circuit applies a Logic 1 bias voltage to the common node when the PD signal is Logic 0. The Logic 1 bias voltage creates a negative Vgs bias on the first pull-down N-channel transistor when the PD signal is Logic 0. An analogous pull-up circuit also is disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention is related to those disclosed in: 
   1) U.S. patent application Ser. No. 10/630,311, filed concurrently herewith, entitled “CIRCUITRY FOR REDUCING LEAKAGE CURRENTS IN A PRE-CHARGE CIRCUIT USING VERY SMALL MOSFET DEVICES;” and 
   2) U.S. patent application Ser. No. 10/630,504, filed concurrently herewith, entitled “CIRCUITRY FOR REDUCING LEAKAGE CURRENTS IN A TRANSMISSION GATE SWITCH USING VERY SMALL MOSFET DEVICES.” 
   U.S. patent application Ser. Nos. 10/630,311 and 10/630,504 are commonly assigned to the assignee of the present invention. The disclosures of the related patent applications are hereby incorporated by reference for all purposes as if fully set forth herein. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention is generally directed to analog circuits that are fabricated using small feature-sized MOSFET processes and, in particular, to a circuit that reduces sub-threshold leakage currents in small MOSFET devices connected to sensitive analog circuit nodes. 
   BACKGROUND OF THE INVENTION 
   As the feature size of MOSFET processes shrink, the MOSFET sub-threshold drain-to-source leakage current when the transistor is supposedly turned off becomes increasingly large. In analog circuits where it is critical for a node to stay at high impedance, this increased leakage current may no longer be ignored. When the devices connected to the high impedance node draw large enough leakage currents, the performance of the circuit may suffer significantly. For instance, in a phase-locked loop (PLL), the devices connected to the high-impedance node of the loop filter may draw enough current when the devices are supposedly off to cause jitter in the PLL output. 
   Therefore, there is a need in the art for improved analog circuits that are fabricated using small feature-sized MOSFET processes. In particular, there is a need for circuits that reduce the sub-threshold leakage currents in small MOSFET devices connected to sensitive analog circuit nodes. 
   SUMMARY OF THE INVENTION 
   Low leakage current versions of three commonly used analog switches are shown to demonstrate techniques of reducing MOSFET sub-threshold leakage currents which can be significant in modern small-feature-sized CMOS processes. These circuits may be coupled to the high-impedance node of a phase-locked loop (PLL), for example. The three circuits include 1) pull-up/pull-down devices, 2) a pre-charge circuit, and 3) a transmission switch (T-switch) for analog testing. It should be noted that the low leakage current designs disclosed herein are general purpose and are not necessarily limited to PLL designs. 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use with an operational circuit comprising at least one high-impedance node, a pull-down circuit capable of pulling the high-impedance node down to ground when a pull-down (PD) signal driving the pull-down circuit is Logic 1. According to an advantageous embodiment of the present invention, the pull-down circuit comprises: 1) a first pull-down N-channel transistor having a drain coupled to the high-impedance node, a gate coupled to the PD signal, and a source coupled to a common node; 2) a second pull-down N-channel transistor having a drain coupled to the common node, a gate coupled to the PD signal, and a source coupled to a ground rail;, wherein the first and second pull-down N-channel transistors are off when the PD signal is Logic 0 and are on when the PD signal is Logic 1; and 3) a gate-biasing circuit driven by the PD signal, wherein the gate-biasing circuit is off when the PD signal is Logic 1 and the gate-biasing circuit applies a Logic 1 bias voltage to the common node when the PD signal is Logic 0, the Logic 1 bias voltage creating a negative Vgs bias on the first pull-down N-channel transistor when the PD signal is Logic 0. 
   According to another embodiment of the present invention, the gate-biasing circuit comprises a P-channel transistor having a gate coupled to the PD signal, a drain coupled to the common node, and a source coupled to a VDD power supply rail. 
   According to still another embodiment of the present invention, the gate-biasing circuit comprises: 1) an inverter having an input coupled to the PD signal; and 2) a biasing N-channel transistor having a gate coupled to an output of the inverter, a source coupled to the common node, and a drain coupled to a VDD power supply rail. 
   It is another primary object of the present invention to provide, for use with an operational circuit comprising at least one high-impedance node, a pull-up circuit capable of pulling the high-impedance node up to a high voltage when a pull-up (PU*) signal driving the pull-up circuit is Logic 0. According to an advantageous embodiment of the present invention, the pull-up circuit comprises: 1) a first pull-up P-channel transistor having a drain coupled to the high-impedance node, a gate coupled to the PU* signal, and a source coupled to a common node; a second pull-up P-channel transistor having a drain coupled to the common node, a gate coupled to the PU* signal, and a source coupled to a VDD power supply rail, wherein the first and second pull-up P-channel transistors are off when the PU* signal is Logic 1 and are on when the PU* signal is Logic 0; and a gate-biasing circuit driven by the PU* signal, wherein the gate-biasing circuit is off when the PU* signal is Logic 0 and the gate-biasing circuit applies a Logic 0 bias voltage to the common node when the PU* signal is Logic 1, the Logic 0 bias voltage creating a positive Vgs bias on the first pull-up P-channel transistor when the PU* signal is Logic 1. 
   In another embodiment of the present invention, the gate-biasing circuit comprises a biasing N-channel transistor having a gate coupled to the PU* signal, a drain coupled to the common node, and a source coupled to a ground power rail. 
   In still another embodiment of the present invention, the gate-biasing circuit comprises: 1) an inverter having an input coupled to the PU* signal; and 2) a biasing P-channel transistor having a gate coupled to an output of the inverter, a source coupled to the common node, and a drain coupled to a ground power rail. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation. A controller may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with a controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
       FIG. 1  illustrates an exemplary phase-locked loop (PLL) that incorporates commonly used analog switches in which MOSFET sub-threshold leakage currents are reduced according to the principles of the present invention; 
       FIG. 2A  illustrates a conventional pull-down circuit according to an exemplary embodiment of the prior art; 
       FIG. 2B  illustrates a conventional pull-up circuit according to an exemplary embodiment of the prior art; 
       FIG. 3A  illustrates a pull-down circuit according to an exemplary embodiment of the present invention; 
       FIG. 3B  illustrates a pull-up circuit according to an exemplary embodiment of the present invention 
       FIG. 4  illustrates a conventional pre-charge circuit according to an exemplary embodiment of the prior art; 
       FIG. 5  illustrates a pre-charge circuit according to an exemplary embodiment of the present invention; 
       FIG. 6  illustrates a conventional test circuit according to an exemplary embodiment of the prior art; and 
       FIG. 7  illustrates a test circuit according to an exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 7 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged small feature-sized MOSFET device. 
     FIG. 1  illustrates exemplary phase-locked loop (PLL)  100 , which incorporates commonly used analog switches in which MOSFET sub-threshold leakage currents are reduced according to the principles of the present invention. PLL  100  comprises frequency divider  110 , phase-frequency detector  120 , charge pump and loop filter circuit  130 , voltage controlled oscillator  140  and frequency divider  160 . Frequency divider  110  divides the frequency of the input signal, VIN, by R, where R may be an integer of a fractional value. Frequency divider  150  divides the frequency of the output signal, VOUT, by N, where N may be an integer or a fractional value. 
   PFD  120  receives and compares the frequency-divided reference signal from frequency divider  110  and the frequency-divided feedback signal from frequency divider  150 . Depending on whether the frequency of the feedback signal is greater than or less than the frequency of the reference signal, PFD  130  generates either a Pump Up signal or a Pump Down signal that is applied to charge pump and loop filter  130 . If a Pump Up signal is received, charge pump and loop filter  130  adds charge to the loop filter, which is typically a large storage capacitor. If a Pump Down signal is received, charge pump and loop filter  130  discharges the loop filter. The voltage on the loop filter is the control voltage, VC, at the output of charge pump and loop filter  130 . 
   Voltage-controlled oscillator  140  produces the output signal, VOUT, which has a frequency that is controlled by the control voltage, CV. As the CV voltage increases, the frequency of the VOUT output signal increases. As the CV voltage decreases, the frequency of the VOUT output signal decreases. Thus, through the operation of the negative feedback path in PLL  150 , the frequency of the VOUT output signal is held at some multiple of the frequency of the VIN input signal, where the multiple is determined by the values of R and N of frequency dividers  110  and  150 , respectively. 
     FIG. 2A  illustrates conventional pull-down circuit  210  according to an exemplary embodiment of the prior art. Pull-down circuit  210  comprises. N-channel transistor  210 , which has a gate coupled to the pull-down signal, PD, a drain coupled to the VC node at the output of charge pump and loop filter  130 , and a source coupled to the VSS power rail (e.g., ground rail). According to the exemplary embodiment, N-channel transistor  210  is a metal-oxide-silicon field effect transistor (MOSFET). 
   The VC node at the output of charge pump and loop filter  130  is a high impedance node. When the pull-down signal, PD, is at Logic 1, N-channel transistor  210  is turned on, thereby pulling the node VC to ground. This discharges the loop filter capacitor. When PD is Logic 0, N-channel transistor  210  is off and should not have any measurable effect on the PLL operation. If reality, however, if N-channel transistor  210  is made from a small-feature-sized CMOS process, the sub-threshold drain-to-source leakage current (Ids) when N-channel transistor  210  is off is no longer negligible. As a result, even if Vgs of N-channel transistor  210  is zero volts (0 V), Ids of N-channel transistor  210  could be on the order of hundreds of nano-amperes. In the case of PLL  100 , is this non-zero leakage current drains significant charge from the loop filter capacitor even when the PD signal is Logic 0, thereby causing unacceptably large amounts of jitter at the output of PLL  100 . 
     FIG. 2B  illustrates conventional pull-up circuit  250  according to an exemplary embodiment of the prior art. Pull-up circuit  250  comprises P-channel transistor  250 , which has a gate coupled to the pull-up signal, PU*, a drain coupled to the VC node at the output of charge pump and loop filter  130 , and a source coupled to the VDD power supply rail. According to the exemplary embodiment, P-channel transistor  250  is a metal-oxide-silicon field effect transistor (MOSFET). The pull-up signal, PU* is an active low signal. 
   The VC node at the output of charge pump and loop filter  130  is a high impedance node. When the pull-up signal, PU*, is at Logic 0, P-channel transistor  250  is turned on, thereby pulling the node VC up to the. VDD rail voltage. This charges the loop filter capacitor. When PU* is Logic 1, P-channel transistor  250  is off and should not have any measurable effect on the PLL operation. If reality, however, if P-channel transistor  250  is made from a small-feature-sized CMOS process, the sub-threshold drain-to-source leakage current (Ids) when P-channel transistor  250  is off is no longer negligible. As a result, even if Vgs of P-channel transistor  250  is zero volts (0 V), Ids of P-channel transistor  250  could be on the order of hundreds of nano-amperes. In the case of PLL  100 , this non-zero leakage current adds significant charge to the loop filter capacitor even when the PU* signal is Logic 1, thereby causing unacceptably large amounts of jitter at the output of PLL  100 . 
     FIG. 3A  illustrates pull-down circuit  300  according to an exemplary embodiment of the present invention. Pull-down circuit  300  comprises N-channel transistors  310 ,  320  and  330 , and inverter  340 . The gates of N-channel transistors  310  and  320  are coupled to the pull-down signal, PD. The drain of N-channel transistor  310  is coupled to the VC node at the output of charge pump and loop filter  130 . The source of N-channel transistor  310  is coupled to the drain of N-channel transistor  320 . The source of N-channel transistor  320  is coupled to the VSS power rail (e.g., ground rail). 
   The input of inverter  340  is coupled to the pull-down signal, PD. The output of inverter  340  drives the gate of N-channel transistor  330 . The drain of N-channel transistor  330  is coupled to the VDD power supply rail. The source of N-channel transistor  330  is coupled to the drain of N-channel transistor  320 . 
   Pull-down circuit  300  performs the same function as the circuit in  FIG. 2A , without the leakage problem. When the pull-down signal, PD, is Logic 1, N-channel transistors  310  and  320  are turned on, thereby pulling the VC node at the output of charge pump and loop filter  130  to ground. Also, when PD is Logic 1, N-channel transistor  330  is turned off and does nothing. It is noted the widths of N-channel transistors  310  and  320  are twice the width of N-channel transistor  210  in order to maintain the same pull-down impedance. 
   When the PD pull-down signal is Logic 0, N-channel transistors  310  and  320  are both off. At the same time, N-channel transistor  330  is turned on, thereby pulling the source of N-channel transistor  310  and the drain of N-channel transistor  320  up to the VDD rail (i.e., Logic 1). As a result, the Vgs voltage of N-channel transistor  310  is negative, rather than merely 0 volts. This is a “hard” shut-off that effectively reduces the sub-threshold leakage current of N-channel transistor  310  to a negligible amount, thereby avoiding leakage problems. 
   Other circuit designs may be used to create a negative Vgs voltage bias on N-channel transistor  310 . For example, in an alternate embodiment of the present invention, N-channel transistor  330  and inverter  340  may be replaced by a single P-channel transistor that has a gate coupled to the PD input signal, a source coupled to the VDD power supply rail, and a drain coupled to the source of N-channel transistor  310 . 
     FIG. 3B  illustrates pull-up circuit  350  according to an exemplary embodiment of the present invention. Pull-up circuit  350  comprises P-channel transistors  360  and  370 , and N-channel transistor  380 . The gates of P-channel transistors  360  and  370  are coupled to the pull-up signal, PU*. The drain of P-channel transistor  370  is coupled to the VC node at the output of charge pump and loop filter  130 . The source of P-channel transistor  370  is coupled to the drain of P-channel transistor  360 . The source of P-channel transistor  360  is coupled to the VDD power supply rail. 
   The pull-up signal, PU* also drives the gate of N-channel transistor  380 . The source of N-channel transistor  380  is coupled to the VSS supply rail (i.e., ground). The drain of N-channel transistor  380  is coupled to the common node at the drain of P-channel transistor  360  and the source of P-channel transistor  370 . 
   Pull-up circuit  350  performs the same function as the circuit in  FIG. 2B , without the leakage problem. When the pull-up signal, PU*, is Logic 0, P-channel transistors  360  and  370  are turned on, thereby pulling the VC node at the output of charge pump and loop filter  130  up to the VDD supply voltage. Also, when PU* is Logic 0, N-channel transistor  380  is turned off and does nothing. It is noted the widths of P-channel transistors  360  and  370  are twice the width of P-channel transistor  250  in order to maintain the same pull-up impedance. 
   When the pull-up signal, PU*, is Logic 1, P-channel transistors  360  and  370  are both off. At the same time, N-channel transistor  380  is turned on, thereby pulling the source of P-channel transistor  370  and the drain of P-channel transistor  360  down to ground (i.e., Logic 1). As a result, the Vgs voltage of P-channel transistor  370  is positive, rather than merely 0 volts. This is a “hard” shut-off that effectively reduces the sub-threshold leakage current of P-channel transistor  370  to a negligible amount, thereby avoiding leakage problems. 
   Other circuit designs may be used to create a positive Vgs voltage bias on P-channel transistor  310 . For example, in an alternate embodiment of the present invention, N-channel transistor  380  may be replaced by an inverter that is driven by the PU* pull-down signal and a single P-channel transistor that has a gate coupled to the output of the inverter. The P-channel transistor would also have a drain coupled to the VSS power supply rail, and a source coupled to the source of P-channel transistor  370 . 
     FIG. 4  illustrates conventional pre-charge circuit  400  in exemplary charge pump and loop filter  130  according to an exemplary embodiment of the prior art. Pre-charge circuit  400  comprises P-channel transistors  421 - 425 , N-channel transistor  431 , and inverter  410 . P-channel transistor  425  and N-channel transistor  431  form a transmission gate switch. When the Pre-Charge input signal is at Logic 1, pre-charge circuit  400  is enabled and P-channel transistor  425  and N-channel transistor  431  are both on. When the Pre-Charge input signal is at Logic 0, pre-charge circuit  400  is disabled and P-channel transistor  425  and N-channel transistor  431  are both off. 
   When Pre-Charge=1, P-channel transistor  421  is off and P-channel transistor  422  is on. When Pre-Charge=0, P-channel transistor  421  is on and P-channel transistor  422  is off. P-channel transistor  423  and P-channel transistor  424  are connected as diodes (i.e., Vgd=0). It is noted that the gate and drain of P-channel transistor  424  are directly connected together (i.e., Vgd=0 always) and the gate and drain of P-channel transistor  423  are shorted together when P-channel transistor  422  is on (i.e., Vgd=0 when Pre-Charge=1). Because P-channel transistor  423  and P-channel transistor  424  are the same type and size devices and are connected in series between the VDD rail and the VSS rail (i.e., ground), the voltage, VMID, at the drain of P-channel transistor  422  is VDD/2. 
   When Pre-Charge=1, the transmission gate switch formed by P-channel transistor  425  and N-channel transistor  431  is on (i.e., closed), thereby shorting the VMID node to the VC node. This drives the high-impedance VC node to approximately VDD/2. When Pre-Charge=0, the transmission gate switch is off, thereby isolating the VMID node from the VC node. Also, when Pre-Charge=0, P-channel transistor  422  is off and P-channel transistor  421  is on, thereby shorting the gate of P-channel transistor  423  to the VDD rail. Since the source of P-channel transistor  421  also is connected to the VDD rail, the Vgs for P-channel transistor  423  is zero and P-channel transistor  423  is off. This cuts off current flow through P-channel transistor  423  and P-channel transistor  424 . 
   Unfortunately, pre-charge circuit  400  experiences high leakage current when pre-charge circuit  400  is disabled. When Pre-Charge=0, P-channel transistor  423  is off, but P-channel transistor  424  is still on Thus, the VMID node sits at approximately 0 volts. Since Pre-charge=0 is coupled to the gate of N-channel transistor  431  and VMID=0 is coupled to the source of N-channel transistor  431 , the Vgs of N-channel transistor  431  is approximately 0 volts. This permits sub-threshold leakage currents in small-feature-sized processes. Therefore, a leakage current path forms between the high impedance node, VC, and the VSS rail (i.e., ground) through N-channel transistor  431  and P-channel transistor  424 . 
     FIG. 5  illustrates pre-charge circuit  500  in exemplary charge pump and loop filter  130  according to an exemplary embodiment of the present invention. Pre-charge circuit  500  comprises P-channel transistors  521 - 525 , N-channel transistors  531 - 534 , and inverter  510 . P-channel transistor  525  and N-channel transistor  534  form a transmission gate switch. When the Pre-Charge input signal is at Logic 1, pre-charge circuit  500  is enabled and P-channel transistor  525  and N-channel transistor  534  are both on. When the Pre-Charge input signal is at Logic 0, pre-charge circuit  500  is disabled and P-channel transistor  525  and N-channel transistor  534  are both off. 
   When Pre-Charge=1, P-channel transistors  521  and  523  are off and N-channel transistors  531  and  532  are on. When Pre-Charge=0, P-channel transistors  521  and  523  are on and N-channel transistors  531  and  532  are off. When Pre-Charge=1, P-channel transistor  522  and P-channel transistor  524  are connected as diodes (i.e., Vgd=0). The gate and drain of P-channel transistor  522  are shorted together when N-channel transistor  531  is on (i.e., Vgd=0 when Pre-charge=1). Similarly, the gate and drain of P-channel transistor  524  are shorted together when N-channel transistor  532  is on (i.e., Vgd=0 when Pre-Charge=1). Because P-channel transistor  522  and P-channel transistor  524  are the same type and size devices and are connected in series between the VDD rail and the VSS rail (i.e., ground), the voltage, VMID, at the drain of P-channel transistor  522  is VDD/2. 
   The gate and source of N-channel transistor  533  are connected together, so that N-channel transistor  533  is off all the time. N-channel transistor  533  has negligible effect when P-channel transistors  522  and  524  are on. However, when Pre-Charge=0, P-channel transistors  521  and  523  are on and N-channel transistors  531  and  532  are off. Since P-channel transistors  521  and  523  are both on, the gate-to-source voltages (Vgs) of P-channel transistors  522  and  524  are both 0 volts. Therefore, P-channel transistors  522  and  524  are off. 
   Because P-channel transistors  522  and  524  are the same type and size devices, the impedances of P-channel transistors  522  and  524  are approximately the same when P-channel transistors  522  and  524  are off. When pre-charge circuit  500  is in this state, N-channel transistor  533  is off, but has a Vgs of zero volts and therefore has a sub-threshold leakage current. It is noted that when Pre-Charge=0, P-channel transistor  523  is on and shorts the VMID node to the drain of N-channel transistor  532 , which is off. However, N-channel transistor  532  still has a sub-threshold leakage current that can discharge the VMID node through P-channel transistor  523 . Therefore, N-channel transistor  533  is introduced to cancel the leakage current of N-channel transistor  532 . In this way, the VMID node sits at approximately VDD/2. Note the size of N-channel transistor  533  is larger than the size of N-channel transistor  532  in order to compensate for the body effect of N-channel transistor  533  when an n-well process is used. 
   The source of N-channel transistor  534  is coupled to the VMID node and the drain of N-channel transistor  534  is coupled to the VC node. The source of P-channel transistor  525  is coupled to the VMID node and the drain of P-channel transistor  525  is coupled to the VC node. When the VMID node is at VDD/2, the sub-threshold leakage currents of both N-channel transistor  534  and P-channel transistor  525  are negligible because N-channel transistor  534  and P-channel transistor  525  are both “hard” off. That is, the Vgs bias of N-channel transistor  534  is negative (i.e., −VDD/2) and the Vsg bias of P-channel transistor  525  is positive (i.e., +VDD/2). 
     FIG. 6  illustrates conventional test circuit  600  according to an exemplary embodiment of the prior art. For measurement purposes, test circuit  600  transmits the voltage at an internal node (the VC voltage in this case) to an externally accessible test point, namely the input/output (I/O) pad VEXT. Test circuit  600  comprises N-channel transistors  611 - 613 , P-channel transistors  621  and  622 , and inverter  630 . N-channel transistor  611  and P-channel transistor  621  form a first transmission gate switch. N-channel transistor  612  and P-channel transistor  622  form a second transmission gate switch. N-channel transistor  613  operates as a pull-down device. 
   When the ON signal is Logic 1, N-channel transistors  611  and  612  are on, P-channel transistors  621  and  622  are on, and N-channel transistor  613  is off. Since both transmission gates are on, the VC node is shorted to the VEXT node. This allows the user to either monitor or drive the internal analog node, VC. When the ON is Logic 0, both transmission switches are off and N-channel transistor  613  is on and pulls the V 1  node between the transmission switches to ground. This is done to minimize potential interferences from the VEXT external node to internal node VC via capacitive couplings. As in the cases of pull-down circuit  210  and pre-charge circuit  400 , a sub-threshold leakage current path exists from the VC to ground through N-channel transistor  611  and N-channel transistor  613  when test circuit  600  is off. 
     FIG. 7  illustrates test circuit  700  according to an exemplary embodiment of the present invention. For measurement purposes, test circuit  700  transmits the voltage at an internal node (the VC voltage in this case) to an externally accessible test point, namely the input/output (I/O) pad VEXT. Test circuit  700  comprises N-channel transistors  711 - 715 , P-channel transistors  721 - 723 , and inverter  730 . N-channel transistor  711  and P-channel transistor  721  form a first transmission gate switch. N-channel transistor  712  and P-channel transistor  722  form a second transmission gate switch. N-channel transistor  713  and P-channel transistor  723  form a third transmission gate switch. N-channel transistor  715  operates as a pull-down device. The gate and source of N-channel transistor  714  are coupled together (i.e., Vgs=0), so that N-channel transistor  714  is always off. However, N-channel transistor  714  has a sub-threshold leakage current when Vgs=0. 
   When the ON signal is Logic 1, all three transmission gate switches are on, allowing test circuit  700  to function in a manner similar to test circuit  600 . However, the switch sizes in test circuit  700  are 50% larger than those in test circuit  600  to maintain the same on-resistance. When the ON signal is Logic 0, all three transmission gate switches are off. The V 1  node is pulled down to ground by N-channel transistor  715 , keeping interference low. 
   However, the sub-threshold leakage current path is eliminated in test circuit  700 . N-channel transistor  712  is still leaky because its Vgs is 0 volts. However, N-channel transistor  714  is also leaky and has approximately the same impedance as N-channel transistor  712 . So the voltage at the V 2  node is approximately VDD/2 when the V 1  node is pulled down to ground. It is noted that the size of N-channel transistor  714  is bigger than the size of N-channel transistor  712  to compensate for the body effect. Because the V 2  node is at VDD/2 when the V 1  node is at ground and the ON signal is Logic 0, N-channel transistor  711  and P-channel transistor are “hard” off (i.e., Vgs&lt;0 for N-channel transistor  711  and Vgs&gt;0 for P-channel transistor  721 ). Hence, there is a negligible amount of leakage current and no leaky path is connected to the VC node. 
   The above-described circuits can be used to reduce sub-threshold leakage currents in small-feature-sized CMOS processes. All three circuits involve leaky switches when the Vgs values of the MOSFET devices are 0 volts (i.e., when the switches are off). The new circuit designs modify the prior art circuits such that the leakage paths are eliminated by making Vgs&lt;0 for the N-channel devices and Vgs&gt;0 for the P-channel devices. This is accomplished without impacting circuit performances or affecting power consumption. 
   Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.