Abstract:
In an integrated circuit device requiring electrostatic discharge (ESD) protection, a circuit is added between a power supply bus and a ground supply bus to shunt the ESD event current and thereby avoiding damage to the device. Specifically, the circuit uses bipolar junction transistors of the PNP type to shunt the supply buses. The emitter junctions are connected to the positive supply bus. The collector junctions are connected to the ground bus. The PNP transistors conduct when a control circuit senses an ESD event and increases the base current in the PNP transistor.

Description:
CONTINUING DATA 
     This application claims benefit of No. 60/124,106 filed on Mar. 12, 1999. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of electrical safety systems using on-chip protection elements to prevent damage to the device. More particularly, the present invention relates to circuits and devices for providing electrostatic discharge protection between power supply buses and the ground bus in a CMOS integrated circuit. 
     The accumulation of static electricity in the vicinity of an integrated circuit (IC) exposes the circuit to a potential failure in the form of an electrostatic discharge (ESD) event. This event refers to the phenomena in which the high static potential (ranging from hundred to thousand of volts) causes a discharge of current in excess of an ampere to flow between at least two external terminals of an integrated circuit. The ESD event current, if not properly handled within the integrated circuit, has the potential to disable or destroy the entire integrated circuit. 
     IC designs contain additional devices and circuits to handle the ESD event. These additional devices and circuits operate during an ESD event. When in the normal operating mode, the circuit terminals function at the normal design potential while either sending and receiving electrical signals from circuitry external to the IC or receiving power from circuitry external to the IC. The circuit network that conducts the ESD current, the ESD network, is required to conduct the ESD current and thereby reducing any potential damage to itself or to the remainder of the IC. The ESD damage to the IC must remain below detectable limits. It is highly desirable that the same ESD network causes negligible performance impact on the normal mode circuit function. As the ESD event is a fast transient event, the peak ESD event current flowing in the first few nanoseconds, the ESD network must conduct this fast transient current. During the normal mode of IC operation, the ESD network must not conduct any transient current. 
     To reduce the cost of implementing the ESD network, it is desirable to minimize the area of the IC used just for said network. As a result of this, where possible, the ESD network uses some IC devices and circuits that are also used during the normal operating mode of the IC. In CMOS, all the NMOS and PMOS transistors contain diodes between their sources and drains; and the particular well in which they are physically located. In the normal mode of operation, the proper external power is supplied to the IC and these diodes are biases between zero and some reverse bias resulting in a minimal current flow though the diodes. During an ESD event, power is not applied and these transistor diodes may be forward biased by the ESD event itself and conduct current accordingly. 
     In particular, CMOS output circuits or combination input-and-output circuits use a combination of large NMOS and large PMOS transistors. These large transistors, the output transistors, are connected to the IC external terminals. The drains of the output transistors form the cathode or the anode of large diodes wherein the opposite diode terminal is connected to the respective well of each output transistor. The n type well for the PMOS transistor is connected to VDD, the positive or power supply potential in the IC. The p type well for the NMOS transistor is connected to the ground potential in the IC. In the most commercially prevalent CMOS, all p type wells are connected together through additional p type material which results in all n type wells individually forming diode connections with the one p type material at ground potential. Some ICs use ESD networks which add additional diodes in parallel to the diodes that are an integral part of the output transistors. This practice improves the diode connectivity between the IC external terminals and the power buses internal to the IC. 
     During testing of an IC, the ESD event is caused to occur between pairs of the external terminals with the polarity of current applied one way and then a similar ESD event is applied with the external terminals reversed. As there are diodes between the external terminal pins and both the power and ground buses, the result of the applied ESD potential is that the ESD network routes the ESD current into the power and ground buses through the appropriate forward biased diodes. To complete the network and safely pass the ESD current through the network, the ESD current must pass between the power and ground bus. There are two possible polarities for the ESD current, both of which the ESD network must handle as a result of the reversal of the external terminals during ESD testing. 
     The most strenuous ESD event for the ESD network to handle is the situation in which the both external terminals used for the ESD testing are connected to output or input-and-output circuits. In other ESD test combinations, a least one external terminal is a power or ground terminal. These test conditions are less strenuous for the ESD network. 
     If in the ESD event the polarity of the event causes current to flow into the first terminal, that is the first terminal is at a more positive potential than the second terminal, the ESD event current is conducted readily from the first terminal to the power bus by means of the forward biased PMOS transistor drain inside the n type well connected to the power bus. Similarly, the ESD event current will flow out of the second terminal through the forward biased NMOS transistor drain inside the p type well connected to the ground bus. The ESD event current must also flow between the power bus and the ground bus to complete the current conduction loop from the first terminal to the second terminal thereby passing the ESD current safely through the IC and avoiding damage to the IC. 
     In normal operation the power supply potential is greater than the ground potential and a minimal current flows between the n type wells and the p type wells. Without additional devices and circuits in the ESD network, the ESD event current only passes from the power bus to ground bus by means of AC current, that current which is proportional to the product of the rate of change of the difference of the power and ground bus potentials; and the capacitance between buses. In some IC designs, the capacitance between the power and ground that exists between the p type wells and n type wells together with the rapid change in the relative bus potentials caused by the ESD event provides sufficient ESD current flow to protect the IC from damage. If insufficient capacitance is unavailable or cannot be feasibly added to the IC, the ESD network conducts the current from the power bus to the ground bus by an appropriate collection of devices and circuits, the ESD power to ground clamp, also called an ESD power to ground shunt. The ESD clamp must conduct the ESD event current while not being damaged by the event, not conducting current during the normal operation of the IC, and not being physically large as to adversely affect the cost of the IC. 
     A variety of clamp circuits have been used with ICs. These clamps consist of a primary device to carry the current and a control circuit to condition the primary conduction device to conduct during an ESD event, but not conduct under normal IC operation. The primary conduction devices that have previously been used in CMOS ICs are the NMOS transistor, the PMOS transistor, and a special device called a silicon controlled rectifier (SCR). Puar in U.S. Pat. No. 5,287,241 describes an ESD network using a PMOS clamping circuit. Dabral in the 1994 EOS/ESD Symposium Proceedings describes and NMOS clamping circuit. Ker in U.S. Pat. No. 6,011,681 used an SCR clamp. Each of these primary conduction devices has their respect advantages and disadvantages. The NMOS transistor has a high conductivity, but is itself susceptible to damage by the ESD event. The PMOS transistor is more rugged than the NMOS type, but the PMOS is less than half the conductivity per unit area when compared to the NMOS type. The SCR is both highly conductive and rugged, but difficult to appropriately control. Both Voldman in 1994 EOS/ESD Symposium Proceedings and Maloney in U.S. Pat. No. 5,530,612 discuss diodes that function as clamp circuits that result in parasitic PNP transistors for use between isolated power buses. 
     The clamping circuit requires that the control circuitry be relatively simple, spatially compact, electrically rugged, and also reliable, triggering the conduction of the primary conduction device only during the ESD event. The primary feature of most ESD control circuits is their use of the fast transient nature of the ESD event to trigger the conduction device. The control circuits switch the conducting device to the conducting state when the power bus to ground bus potential increases faster than a certain rate and the increase is greater than a certain value. In some cases, the clamp circuit may become conductive simply when a certain power bus to ground bus potential is exceeded. Dugan in U.S. Pat. No. 5,311,391 describes improvements to the control circuitry and thereby minimize triggering the ESD conducting device when the IC is in normal operation. Ker in the 1998 EOS/ESD Symposium Proceedings reported techniques for improving the SCRs used as conduction devices and their control circuitry, but at the expense of additional area and circuit complexity. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, several objects and advantages of this invention are gained by the use of a PNP transistor as the conduction device to shunt the ESD current from the power bus to the ground bus. The PNP transistor is more robust than the NMOS transistor, can conduct more current than the PMOS transistor, and is more easily controlled than the SCR. The PNP transistor base current can be supplied by an NMOS or PMOS transistor, or directly by a diode string or diode-connected-FET string, if the leakage current from said string is sufficiently low. 
     The PNP transistor may be implemented as a lateral PNP device, a vertical PNP device or as a combination of the two within the same CMOS technology. The p type well that forms the PNP collectors are all connected together. This common collector connection further serves to improve the conduction of the PNP transistor. 
     The technique of using a PNP can be extended by physically implementing a Darlington type connection of two PNP transistors, in place of the single PNP conduction device thereby increasing the equivalent PNP current gain. As the current gain of the PNP increases, the PNP sensitivity to leakage into its base from the control circuit increases accordingly. For the purposes of this invention, the single PNP can be replace by a pair of Darlington connected PNPs without otherwise changing the clamp circuit connections or altering the control circuitry operation. 
     Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWING 
     FIG. 1 is a perspective view of a circuit diagram of clamp circuit which the invention is applied. 
     FIG. 2 is a circuit diagram of an RC delay circuit indicated in FIG.  1 . 
     FIG. 3 is a voltage waveform of VDD, node  25  of FIG.  1  and node  45  of FIG.  6 . 
     FIG. 4 is a circuit diagram of generic CMOS output circuit with the addition of the present invention. 
     FIG. 5 is a circuit diagram of a generic CMOS output circuit with additions of alternative approach of this invention to be used as output ESD protection circuit. 
     FIG. 6 is an alternative circuit diagram of the clamp circuit in which the present invention is applied. 
     FIG. 7 is a circuit diagram of RC delay circuit  1  indicated in FIG.  6 . 
     FIG. 8 is a circuit diagram of a generic CMOS output circuit with additions of a clamp circuit shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The circuit diagram of the present invention is shown in FIG.  1 . The clamp circuit consists of two circuit blocks as shown in FIG. 1 which are (1) the current discharge circuit  200  which provides a current discharge path triggered by the potential at node  25  and (2) a delay circuit  300  (which is termed an RC delay even though the delay may be the result of factors other than R &amp; C) which provides a power up delay signal at node  25 . As shown in FIG. 1, the current discharge circuit  200  consists of a resistor R 2 , a PNP bipolar transistor B 3 , and a P-channel MOS transistor P 1 . This invention is applicable to a conventional P-type substrate complementary metal-oxide silicon (CMOS) technology. The PNP bipolar transistor B 3  can be a lateral PNP bipolar transistor or a vertical PNP bipolar transistor or can use both lateral and vertical PNP bipolar transistors as a pair. Alternately, a Darlington connections of two PNP transistors can function in an equivalent manner to a single PNP transistor throughout this description. 
     The detailed circuit diagram of RC delay circuit  300  is shown in FIG.  2 . Node  26  is connected to ground (zero volts) in the chip. Node  101  is connected to a positive supply voltage VDD. The RC delay circuit  300  generates a signal at node  25  which follows the increase in power supply VDD with a delay period  91  shown in FIG.  3 . The duration of a delay time  91  depends on the resistance of P 8  and the capacitance of N 9 . 
     When the power supply is not connected to the IC, the voltage level of  101  is near zero volts and all the internal nodes in FIGS. 2,  25 ,  26 ,  27  and  28  are also near zero volts. As the voltage of VDD  101  rises, which is driven by the ESD transient voltage, transistor P 8 , P 6 , and P 4  begin to turn on and to conduct current from VDD (node  101 ) to charge up node  28 ,  27  and  25 . With a combination of the high transistor on-resistance value of P 8  and a high gate capacitance value of N 9 , node  28  rises slowly in comparison to the rise time of VDD. For the time period  91  shown in FIG. 3, node  28  stays at a relatively low voltage to keep transistor P 6  and N 5  conducting. The voltage level of node  25  is low which keeps P 1  of FIG. 1 conducting. A conducting P 1  provides current for current-discharge circuit  200  from node  22  to ground  24 . After a delay period  91 , shown in FIG. 3, from the rising edge of VDD, nodes  28  and  25  have charged up to a higher voltage level which in turn makes transistor P 1  non-conducting and thus stops the current flow through R 2  and B 3  in FIG.  1 . The delay time  91  shown in FIG. 3 is determined by Ron of P 8 , and Cgate of N 9  that can be adjusted by varying the channel widths and lengths of P 8  and N 9 . In the normal IC operation mode, the voltage level of node  101  is nearly constant at VDD. The voltage level of nodes  28  and  25  are also near VDD level and node  27  is near zero volts (ground level). 
     As shown in FIG. 1, node  24  and  23  are connected to the IC ground level (zero volts). Resistor R 2  can be P diffusion resistor, N-well resistor, N diffusion resistor, polysilicon resistor or MOS transistor. PNP bipolar transistor B 3  can be lateral PNP transistor or vertical PNP transistor or a combination of lateral and vertical transistors in parallel. PNP bipolar transistor B 3  can also be a combination of two lateral or vertical PNP transistors in the Darlington configuration. The VDD (node  100 ) of the RC delay circuit  300  rises from zero due to the ESD transient current. During a time period  91  shown in FIG. 3 the voltage level of node  25  generated by RC delay circuit  300  is low or closed to zero. Transistor P 1  of current discharge circuit  200  is conducting and conducts current from node  22  to ground node  24 . The total current passing through P 1  consists of the current through resistor R 2  from node  21  and base current of transistor B 3  through the emitter of B 3  connected to node  21 . The resistance value of R 2  will limit the current through R 2  from node  21  since the base-to-emitter voltage, Vbe ( emitter to base voltage when the emitter-base junction is forward biased ), of B 3  is nearly constant. Bipolar transistor B 3  turns on and discharges current from node  21  to ground  23  directly. Current from node  21  to  23  of B 3  is called Ice, which is the collector-to-emitter current. Ice approximately equals Beta times Ib. Beta is defined as the current gain of PNP transistor B 3 . Ib is the base current of B 3 . Therefore; there are two low resistance paths to discharge current from node  21  to nodes  23  and  24  during the period of time node  25  stays low and P 1  stays conducting (which is controlled by RC delay circuit  300 ). These current paths are as follows: 
     (1) Current conducts from emitter to collector of bipolar transistor B 3 , and 
     (2) Current conducts from drain to source of PMOS transistor P 1 . 
     R 2  resistor in block  200  functions (1) to isolate the high voltage node  21  during the ESD transient current and to limit the drain current into PMOS transistor P 1  to avoid any device damage to P 1 , and (2) to charge up node  22  to the same voltage potential of node  21  in the normal mode to which removes any base current from PNP transistor B 3 . 
     In the event that VDD (node  100 ) of RC delay circuit  300  does not rise when there is ESD transient current applied to the IC, node  25  will stay low to keep P 1  and B 3  on to discharge current from node  21  to the grounded nodes. 
     FIG. 4 shows one example circuit application which uses the present invention to clamp the power supply to ground to prevent ESD damage to the IC. A pull-up PMOS transistor P 11  and a pull-down NMOS transistor N 12  are the driver transistors of the output buffer. The gate  32  of P 11  and gate  31  of N 12  are driven by the output buffer control circuitry which is not shown in the figures, but provided by other on-chip circuitry. Diode D 13  is a diode connected from output pad  34  to VDD and D 14  is diode connected from output pad  34  to ground. The nodes  23 ,  24 ,  26 ,  33 , and  35  are connected to ground (zero volts). In the circuit diagram shown in FIG. 4, power supply VDD is connected to node  101  of RC delay circuit  300  and to node  21  of current discharge circuit  200 . When a positive ESD transient voltage is applied to output pad  34 , the diode D 13  is forward biased and begins to charge VDD nodes  21  and  100  up from near zero volts. During the period of delay time  91  shown in FIG. 3 the bipolar transistor B 3  and PMOS transistor P 1  are all in a conducting state and discharge the ESD transient current from VDD to ground. The sizes of the bipolar transistor B 3  and the PMOS transistor P 1  are designed to be large enough to be capable of discharging the ESD transient current without allowing the high ESD transient voltage or current to damage any devices elsewhere in the IC. In the normal mode of IC operation, the voltage of nodes  25  and  22  are near the potential of node  21  and the two discharge transistors P 1  and B 3  all not conducting current form VDD to ground. 
     Another example of circuit application which uses the present invention to discharge ESD transient current in order to prevent ESD damage of the IC is shown in FIG.  5 . The node  21  of discharge circuit  200  is directly connected to the output pad. A pull-up PMOS transistor P 81  and pull-down NMOS transistor N 82  are the driver transistors of the output buffer. The gate  87  of P 81  and gate  88  of N 82  are driven by the output buffer control circuitry which is not shown here. Diode D 83  is a diode connected from output pad to node  90  and D 84  is a diode connected from output pad to ground. Nodes  23 ,  24 ,  26 ,  86 , and  89  are connected to ground (zero volts). Nodes  90  and  101  of RC delay circuit  300  are connected to VDD (positive power supply). When the ESD transient current applies a positive voltage pulse to output pad shown in FIG. 5, the bipolar transistor B 3  and PMOS transistor P 1  are both turned on and conduct to discharge the ESD transient current from output pad to ground (primarily nodes  24  and  23 ) in the time period  91  in FIG.  3 . The sizes of the bipolar transistor B 3  and PMOS transistor P 1  are designed to be large enough to discharge the ESD transient current without allowing the high ESD transient voltage or current to damage any devices elsewhere in the IC. After the RC delay time  91 , the voltage of node  25  approaches VDD which renders transistor P 1  non-conducting. Then, the voltage between nodes  22  and  21  is nearly equal to zero which shuts off bipolar transistor B 3 . 
     FIG. 6 shows an alternative way to design the discharge circuit  201  according to this invention. Instead of using a P-channel MOS transistor P 1  in block  200  of FIG. 1, P 1  can be replaced by an N-channel MOS transistor N 51  as shown in FIG.  6 . The connectivity of the other components (R 52  and B 53 ) in block  201  is the same as R 2  and B 3  in block  200  in FIG.  1 . Since an N-channel MOS transistor requires an opposite gate voltage to conduct than a P-channel MOS transistor, the RC delay circuit 1   301  to generate the gate voltage  45  also needs to be modified. The detailed circuit diagram of RC delay circuit  102  is shown in FIG.  7 . 
     Node  46  in FIG. 7 is connected to ground which is the lowest potential in the chip, and node  102  is connected to the positive supply voltage VDD. When the power supply is not connected to the IC, the voltage level of  102  is near zero volts and the internal nodes  45  and  47  in FIG. 7 are also near zero volts. As VDD  102  rises driven by the ESD transient voltage, transistor P 56  and P 54  begin to conduct current from VDD (node  102 ) and charge up nodes  47  and  45 . With a combination of the high transistor on resistance of P 56  and a high gate capacitance value of N 57 , node  47  rises slowly in comparison to the rise time of VDD. In the time period  92  shown in FIG. 3, node  47  stays in a relatively low voltage which keeps transistor P 54  conducting, and the voltage level of node  45  follows VDD. With node  45  near greater than the threshold voltage of transistor N 51 , transistor N 51  of discharge circuit  201  conducts current from node  42  to ground  44  as shown in FIG.  6 . After a delay period  92  shown in FIG. 3, node  47  has been charged up to high voltage level which causes transistors P 54  and N 51  to be non-conducting which stops the current flow through R 2  and B 53  as shown in FIG.  6 . The delay time  92  shown in FIG. 3 is determined by the Ron of P 56  and the Cgate of the gate of N 57 . The values of Ron and Cgate can be adjusted by varying the channel widths and lengths of P 56  and N 57 . In the normal IC operation mode, the voltage level of node  102  is near the VDD value as is node  47 . Node  45  is near the zero volt level (ground). In the normal IC operation there is no current path from node  41  to ground in discharge circuit block  201  in FIG.  7 . 
     The voltage level of node  45  generated by RC delay circuit 1   301  is high enough to keep transistor N 51  conducting in the presence of ESD current pulse shown in FIG.  6 . The circuit operation and concept of the discharge circuit  201  in FIG. 6 function the same as discharge circuit  200  in FIG.  1 . Both circuits discharge the ESD transient current without letting the high ESD transient voltage or current damage devices elsewhere in the IC. 
     FIG. 8 shows a circuit application example which uses the present invention of circuit diagram shown in FIG. 6 to clamp the power supply to ground to prevent ESD damage to the IC. The power supply VDD is connected to the source of P 71 , node  41  of discharge block  201 , and VDD node of the RC delay circuit  301 . A pull-up PMOS transistor P 71  and pull-down NMOS transistor N 72  are the driver transistors of output buffer. The gate  76  of P 71  and gate  77  of N 72  are driven by the output buffer control circuitry which is not shown here. Diode D 73  is a diode connected from output pad  75  to VDD and D 74  is a diode connected from output pad  75  to ground. The nodes  43 ,  44 ,  78 ,  79 , and  46  are connected to ground (zero volts). 
     When a positive ESD transient voltage is applied to output pad  75 , VDD power supply bus  41  is charged up by the ESD transient voltage through diode D 73 , During a period of delay time  92  shown in FIG. 3, the bipolar transistor B 53  and NMOS transistor N 51  are all conducting and discharge the ESD transient current from VDD  41  to ground (primarily nodes  44  and  43  ). The sizes of the bipolar transistor B 53  and NMOS transistor N 51  are designed to be large enough to discharge the ESD transient current without allowing the high ESD transient voltage or current to damage devices elsewhere in the IC. In the normal mode of operation, the potential of node  42  is near VDD and node  45  is near zero. Transistor N 51  and B 53  are non-conducting and no current is discharged through block  201  in FIG.  8 .