Abstract:
A gate-coupled MOSFET ESD protection circuit. The circuit has a gate-node potential controlled by an inverter and a timing control circuit. Unlike current-shunting ESD clamping devices that turn the MOSFET fully on during an ESD event, a pull-down element is included to form a voltage divider like circuit, such that the gate-node potential is limited to around 1 to 2 volts during a positive ESD transient event. Unlike GCNMOS (Gate-Coupled NMOS), the invention has better control of the transient gate potential for more effective triggering of the NMOS into snapback during an ESD event.

Description:
Pursuant to 35 U.S.C. § 119(a)-(d), this application claims priority from Taiwanese application no. 91109035, filed on April 30,2002. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an ESD protection circuit and particularly to a gate-coupled MOSFET ESD protection circuit. 
   2. Description of the Prior Art 
   In a Human-Body-Model ESD transient, a 100PF capacitor is first charged up to an ESD pulse voltage, and then discharged through a 1.5-kohm resistor onto an IC pin. Typically, a pulse voltage level of 2 KV is used to qualify an IC package. The initial peak current is roughly 1.2 A with a rise time of approximately 10 nsec. For integrated circuit packages, the VDD-to-VSS capacitance is typically larger than 1 NF. If the ESD energy is directly absorbed by the power bus (for ESD stress of VDD pin to VSS pin), or indirectly absorbed by the power bus (for example, positive ESD stress on an input or I/O pin that has a pull up device such as p+/n well or PMOSFET), then the voltage-rising rate inside an IC can reach 1 to 2 volt per nano-second for a Human-Body-Model ESD pulse at 2 to 3 KV level. 
   Transistors, such as grounded-gate NMOS (GGNMOS), field-oxide device, or output buffer transistors, have been commonly used as primary ESD protection elements for integrated circuits. 
   For ESD protection of an IC pin or a power bus, GGNMOS can be used as the primary ESD protection. The drain of the NMOS transistor is connected to VDD or the IC pin, while the source of the NMOS transistor is connected to VSS. The gate is either grounded (GGNMOS), or coupled to VDD by a capacitor and to VSS by a resistor (GCNMOS or Gate-Coupled NMOS). 
   ESD Voltage Clamping Device 
   One well known ESD protection circuit involves the use of a transistor controlled by a resistance-capacitance (RC) circuit for shunting the flow of ESD current between the protected bond pad and a power supply pad (e.g., VSS). 
     FIG. 1  shows a conventional RC-triggered active MOSFET ESD clamp circuit. The clamp circuit provides a current shunt to protect internal circuit for a VDD-to-VSS positive voltage ESD event. The inverter  11  composed of the transistors N 1  and P 1  inverts a voltage on a node E to an output voltage on a node G, which keeps the transistor N 1  conductive for a period of time as determined by the RC time constant (R 1 C 1 ). It is critical that this RC time constant is long enough to exceed the maximum expected duration of an ESD event, typically in the range of 50 nanoseconds to a few hundred nanoseconds, while short enough to avoid false triggering of the clamp circuit during normal ramp-up of the VDD power bus, typically a few milliseconds. During normal operation of the IC, with a constant VDD power supply level, the transistor N 1  is biased in a nonconductive state due to the resistor R 1  pulling node E at High and node G at Low. 
   The described voltage-clamping ESD protection device can be used to protect between VDD and VSS power supply rails. However, certain concerns are that (i) the device size is typically very large, e.g., with a number of power-bus-clamp NMOSFETs having a total channel width of 4000 to 10,000 m; and (ii) the inverter  11  amplifies the power-bus noise through the node E, causing undesirable leakage current at N 2  during circuit operation. 
   ESD Protection by Avalanche Breakdown 
   Another well-known ESD protection method is based on the avalanche breakdown and snapback of a MOSFET device. At the beginning, the high electric field at the drain junction causes impact ionization with generation of both minority and majority carriers. The minority carriers are collected at the drain (anode), while the majority carriers flow toward the substrate or p-well contact (cathode) causing a local potential buildup in the p-well. When the local substrate potential is 0.8V higher than the adjacent n+ source potential, the source junction becomes forward biased. The forward biased source junction injects minority carriers into the p-well. Some of those injected minority carriers are recombined in the substrate while the rest of them reach the drain junction to further enhance the impact ionization. As a continuous loop, the MOSFET enters a low impedance (snapback) state to conduct large amounts of ESD current. 
   It is of great advantage to reduce the triggering voltage of a MOSFET during an ESD event. The ESD protection can occur sooner, and the transient voltage imposed on the I/O and internal circuit can be lower, for better overall ESD protection. 
     FIG. 2  shows a conventional gate-coupled ESD protection circuit. The RC time constant is chosen such that the node G is at about 1 to 2 volts (or around 0.7V to 2V) during an ESD transient for reducing ESD triggering voltage for avalanche breakdown and snapback. 
   For GGNMOS or GCNMOS, because the conduction of ESD current is through the drain/substrate/source (npn) bipolar junction, it can conduct a large ESD current with a smaller MOSFET when compared to a voltage clamping ESD protection device, and for power bus ESD protection, typically one with a total channel width of, for example, 600 to 1200 m may provide sufficient ESD protection. 
   However, for a GCNMOS, the selection of the RC time constant for optimized ESD triggering duration and the transient voltage on the node G for different ESD pulse levels may sometimes impose some difficulty. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide an ESD protection circuit using an improved gate-coupled MOSFET having a stable transient gate-node voltage when the ESD event occurs. 
   The present invention provides a gate-coupled MOSFET ESD protection circuit providing an ESD path from a first to a second node when an ESD voltage applied to the first node. The circuit comprises a timing-control circuit outputting a first voltage when the ESD voltage is applied to the first node, a voltage divider outputting a second voltage divided from the ESD voltage when activated by the first voltage output from the timing-control circuit, and a shunt transistor having a drain coupled to the first node, a source coupled to the second node and a gate-coupled to the voltage divider, and entering into a snapback to provide the ESD path by the drain and gate receiving the ESD and second voltage when the ESD voltage is applied to the first node. 
   The present invention further provides a gate-coupled MOSFET ESD protection circuit providing an ESD path from a first to a second power bus when an ESD voltage applied to the first power bus. The circuit comprises a timing-control circuit outputting a first voltage when the ESD voltage is applied to the first power bus, a voltage divider outputting a second voltage divided from the ESD voltage when activated by the first voltage output from the timing-control circuit, and a shunt transistor having a drain coupled to the first power bus, a source coupled to the second power bus and a gate-coupled to the voltage divider, and entering into a snapback to provide the ESD path by the drain and gate receiving the ESD and the second voltage when the ESD voltage is applied to the first power bus. 
   The present invention also provides a gate-coupled MOSFET ESD protection circuit providing an ESD path from a pad to a second power bus when an ESD voltage is applied to the pad, the ESD voltage is coupled through a PN junction to a first power bus. The circuit comprises a timing-control circuit outputting a first voltage when the ESD voltage is applied to the pad, a voltage divider outputting a second voltage divided from the ESD voltage when activated by the first voltage output from the timing-control circuit, and a shunt transistor having a drain coupled to the pad, a source coupled to the second power bus and a gate-coupled to the voltage divider, and entering into a snapback to provide the ESD path by the drain and gate receiving the ESD and second voltage when the ESD voltage is applied to the pad. 
   The present invention provides a gate-coupled MOSFET ESD protection circuit providing an ESD path to a second node when an ESD voltage applied to one of first nodes. The circuit comprises a timing-control circuit outputting a first voltage when the ESD voltage is applied to one of the first nodes; a voltage divider outputting a second voltage divided from the ESD voltage when activated by the first voltage output from the timing-control circuit, and a shunt transistor having a drain coupled to the first nodes, a source coupled to the second power bus and a gate-coupled to the voltage divider, and entering into a snapback to provide the ESD path by the drain and gate receiving the ESD and second voltage when the ESD voltage is applied to one of the first nodes. 
   Thus, in the present invention, the device size is smaller than that of the conventional voltage-clamping ESD protection circuit and the gate-node voltage is more easily controlled than that of the conventional gate-coupled ESD protection circuit. 
   Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention. 
       FIG. 1  shows a conventional RC-controlled active MOSFET ESD clamp circuit. 
       FIG. 2  shows a conventional gate-coupled ESD protection circuit. 
       FIG. 3  is a diagram showing an ESD protection circuit according to a first embodiment of the invention. 
       FIG. 4A  is a diagram showing an ESD protection circuit according to a second embodiment of the invention. 
       FIG. 4B  is a diagram showing an ESD protection circuit according to a third embodiment of the invention. 
       FIG. 5  is a diagram showing an ESD protection circuit according to a fourth embodiment of the invention. 
       FIG. 6  is a diagram showing an ESD protection circuit according to a fifth embodiment of the invention. 
       FIG. 7  is a diagram showing an ESD protection circuit according to a sixth embodiment of the invention. 
       FIG. 8  is a diagram showing an ESD protection circuit according to a seventh embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  is a diagram showing an ESD protection circuit according to a first embodiment of the invention. A gate-coupled MOSFET ESD protection circuit provides an ESD path from a node A to node B when an ESD pulse voltage applied to the node A. The circuit comprises a timing-control circuit  33  composed of a resistor R 1  and a capacitor C 1 , a voltage divider  35  composed of transistors P 1  and P 2  and resistors R 2  and R 3 , and a N type shunt transistor N 2 . The timing-control circuit  33  outputs a low voltage at the node E when the ESD pulse voltage is applied to the node A. The voltage divider  35  outputs a voltage divided from the ESD voltage at the node G when activated by the low voltage output from the timing-control circuit  33 . The shunt transistor N 2  has a drain coupled to the node A, a source coupled to the node B and a gate coupled to the voltage divider  35 , and enters into a snapback to provide the ESD path by the drain and gate receiving the ESD and the voltage at node G when the ESD voltage is applied to the node A. A triggering voltage for the shunt transistor N 2  to enter into the snapback is reduced since the shunt transistor N 2  is weakly turned on by the gate receiving a voltage at the node G from the voltage divider  35  when the ESD voltage is applied to the node A. In the voltage divider  35 , the P type transistor P 1  has a gate receiving the low voltage output from the timing-control circuit  33 , a source coupled to the node A, and a drain coupled and outputting the voltage at the node G to the gate of the shunt transistor N 2 . The P type transistor P 2  has a gate and drain commonly coupled to the node B, and a source coupled to the gate of the shunt transistor N 2 . The N type transistor N 1  and P type transistor P 1  also form an inverter  31 . 
   During the initial phase of a positive ESD event, as the voltage at the node E increases, the voltage at the node E stays at low, both of the transistors P 1  and P 2  turn on, and the voltage at the node G is determined by the on-resistance ratio of the transistors P 1  and P 2 . By adjusting the W/L size ratio, considering the body effect of the transistors P 1  and P 2  and the approximate triggering voltage of the transistor N 2 , one can keep the voltage at the node G in the desirable range of 1-to-2 volt when the voltage at node E increases to approximately the triggering voltage level of the transistor N 2 . 
   The n-well-node W of the transistor P 2  can be coupled to the node G, or to the node A, with different body effect for the transistor P 1  and P 2  W/L size considerations. 
   The p-well-node K of the transistor N 2  can be coupled to the node B, or to the node G. If The p-well-node K of the transistor N 2  is coupled to the node G, the source junction of the transistor N 2  is forward biased to facilitate the triggering of the transistor N 2  into the snapback during an ESD event. 
   The R 1 C 1  time constant provides sufficient time for NMOS triggering. R 1 C 1  can be, as an example, 15 to 50 nanoseconds. 
   The resistances R 2  and R 3  can be simply the metal wiring resistance. Alternatively, the resistances R 2  and R 3  can be resistor elements (such as those formed by polysilicon or n-well) for limiting ESD current flow through the transistors P 1  and P 2  during an ESD event. The voltage divider circuit formed by the transistors P 1  and P 2 , and resistors R 2  and R 3 , thus have a voltage dividing ratio of
 
 V   G   /V   A   =[R   ON ( P   2 )+ R   3 ]/[ R   ON ( P   1 )+ R   2 + R   ON ( P   2 )+ R   3 ]
 
in a steady state. Of course, during ESD transient, the electrical charging of the gate capacitance of the transistor N 2  will play a role in the transient response. Those skilled in the art will appreciate that the desirable resistances R 2  and R 3 , and sizes of the transistors P 1  and P 2 , can be obtained by circuit simulation based on the ESD transient waveform.
 
   For node A as a VDD power bus node, the transistor N 1  turns on(pulled high through the resistor R 1 ) during powering on and normal circuit operation to keep the transistor N 2  in an off state. 
   During circuit operation, even if there is noise at the node E(through ground bounce or power supply noise), the voltage at the node G is limited for suppressing leakage current of the transistor N 2 . 
     FIG. 4A  is a diagram showing an ESD protection circuit according to a second embodiment of the invention. This is an alternative circuit modified from the one shown in FIG.  3 . The transistor P 2  is replaced by the resistor R 3  and the transistor N 1  is removed. The voltage dividing ratio is
   V   G   /V   A   =R   3 /[ R   ON ( P   1 )+ R   2 + R   3 ] 
which is preferably from {fraction (1/15)} to ⅗ so that the gate voltage of the transistor N 2  can range from 1 to 2 volts, 0.5 to 2.5 volts or 0.5V to half of the voltage at the node A.
 
     FIG. 4B  is a diagram showing an ESD protection circuit according to a third embodiment of the invention. This is an ESD protection circuit providing ESD paths from more than one node (IC pins or power bus) to the VSS power bus. When the ESD event occurs, the transistor N 2  provides an ESD path from the pad to the VSS power bus by entering into the snapback while the transistor N 4  provides an ESD path from the VDD to the VSS power bus by being turned on. 
   When a positive ESD event occurs on the VDD power bus, the ESD pulse voltage is divided by the voltage divider with a voltage-dividing ratio of V G1 /V A =[R 2 +R 3 ]/[R ON (P 1 )+R 2 +R 3 ], received by the gate of the transistor N 4  and directly turns on the transistor N 4 . When a positive ESD event occurs on the pad, the ESD pulse voltage is coupled to the VDD power bus, which turns on the transistor N 4  and applies a voltage divided from the ESDF pulse voltage with a ratio of V G2 /V A =[R 3 ]/[R ON (P 1 )+R 2 +R 3 ]. This reduces the triggering voltage of the transistor N 2 . The transistor N 2  enters into the snapback earlier with the lower triggering voltage. The ratio of the resistance R 3  to R 2  is preferably the range of {fraction (1/12)} to 2.5/5, or 1.5/7 to 2.5/5. 
   Alternatively, the resistance R 2  can be relatively low such that the node G 1  and G 2  are substantially shorted. This results in the voltages on the gates of the transistors N 2  and N 4  being the same. In this case, both of the transistors N 2  and N 4  provide the ESD paths by entering into the snapback. 
     FIG. 5  is a diagram showing an ESD protection circuit according to a fourth embodiment of the invention. This is an alternative circuit modified from the one shown in FIG.  4 B. By comparing the circuits shown in  FIGS. 3 and 5 , it is noted that the resistor R 1  and capacitor C 1  are interchanged, the P type transistor P 2  is replaced by the N type transistor N 3 , the gate of the transistor N 3  is coupled to the node G, and the inverter  51  is added between the inverter  31  and the node G. 
   The voltage divider here is composed of two inverters  31  and  51  connected in series, and the N type transistor N 3 . The inverter  31  has an input receiving the voltage output from the timing-control circuit  53  at node E and an output coupled to the gate of the shunt transistor N 2 . The transistor N 3  has a source coupled to the node B, and a drain and a gate commonly coupled and outputting the divided ESD voltage to the gate of the shunt transistor N 2 . During the initial phase of a positive ESD event, as the voltage at the node A increases, the voltage at the node E follows the node A voltage due to the capacitor C 1 . Coupling to the node E by inverters  31  and  51 , the voltage at the node G also increases by the pull-up element of the inverter  51 . But the voltage at the node G cannot be pulled high close to the voltage at the node A because the transistor N 3  starts to conduct when the voltage at the node G is higher than a threshold voltage of the transistor N 3 . As a result, by adjusting the size (W/L) ratio of the transistor N 3  and the pull-up element of the inverter  51 , the voltage at the node G can be around 1-to-2 volts when the voltage at the node A increases to about the triggering voltage (e.g. 8V to 12V) of the transistor N 2 . The actual device sizes can be selected based on circuit simulation of a Human-Body-ESD event. 
   The p-well (the node K) of the transistor N 2  can be coupled to the node B (as a typical case for p-substrate). Alternatively, the node K can be coupled to the node G, or as a floating node, for n-substrate/p-well process technology. 
     FIG. 6  is a diagram showing an ESD protection circuit according to a fifth embodiment of the invention. This is an alternative circuit modified from the one shown in FIG.  5 . By comparing the circuits shown in  FIGS. 5 and 6 , it is noted that the resistor R 1  and capacitor C 1  are interchanged, the N type transistor N 3  is replaced by a diode D 1  and the inverter  51  is removed. 
   The diode D 1  functions similarly to the transistor N 3  in FIG.  5 . The voltage divider here is composed of the inverter  31  and the diode D 1 . The inverter  31  has an input receiving the voltage output from the timing-control circuit  63  at the node E and an output coupled to the gate of the shunt transistor N 2 . The diode D 1  has a cathode coupled to the node B and an anode coupled and outputting the divided ESD voltage to the gate of the shunt transistor N 2 . During the initial phase of a positive ESD event, as the voltage at the node A increases, the voltage at the node E stays close to VSS due to the capacitor C 1 . Through the inverter  31 , the voltage at the node G increases by the pull-up element of the inverter  31 . But the voltage at the node G cannot be pulled high close to the voltage at the node A because the diode D 1  starts to conduct when the voltage at the node G is higher than the turn-on voltage of the diode D 1 . As a result, by adjusting the size (W/L) of the diode D 1  and the pull-up element of the inverter  31 , the voltage at the node G can be around 1-to-2 volts when the voltage at the node A increases to about the triggering voltage of the transistor N 2 . The actual device sizes can be selected based on circuit simulation of a Human-Body-ESD event. 
   Additionally, the diode D 1  may be replaced by a resistor. 
     FIG. 7  is a diagram showing an ESD protection circuit according to a sixth embodiment of the invention. This is an alternative circuit modified from the one shown in  FIG. 6 , which provides an ESD path from the pad to the VSS power bus. 
   The resistor R 1  and the transistor P 1  are coupled to the VDD power bus, and the transistor N 2  is coupled to the pad. A P type MOSFET P 3  is coupled between the pad and the VDD power bus. 
   During a positive ESD event occurring between the IC pad and the VSS power bus, the positive ESD voltage is coupled to the VDD power bus through forward biasing a parasitic p+/n-well junction diode of the MOSFET P 3 . 
   The MOSFET P 3  cab be replaced by a diode, as shown in FIG.  8  and optimally, a resistor R 3  can be instead connected in series with the PMOS transistor P 2 , and become part of the voltage dividing circuit during an ESD event. 
   In conclusion, the present invention provides an ESD protection circuit using an improved gate-coupled MOSFET having a stable transient gate-node voltage when the ESD event occurs. The device size is smaller than that of the conventional voltage-clamping ESD protection circuit and the gate-node voltage is more easily controlled than that of the conventional gate-coupled ESD protection circuit. 
   The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.