Abstract:
An ESD protection circuit for protecting a circuit, comprising a lateral semiconductor-controlled rectifier, a MOS transistor, and a current-sinking device. The lateral semiconductor-controlled rectifier is coupled to the circuit and provided with a first common region and a second common region. The MOS transistor integrated with the lateral semiconductor-controlled rectifier includes the first common region The current-sinking device integrated with the lateral semiconductor controlled rectifier includes the second common region. The current-sinking device shunts the majority of a discharge current when the MOS transistor enters breakdown, thereby increasing the trigger current of the lateral semiconductor-controlled rectifier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrostatic discharge protection for semiconductor integrated circuitry. More particularly, the present invention relates to an electrostatic discharge protection circuit with a high trigger current. 
     2. Description of the Related Art 
     In sub-micron CMOS technologies, electrostatic discharge (ESD) protection has become a main concern in relation to the reliability of semiconductor integrated circuitry. Usually, MOS transistors near IC pads are designed with enlarged dimensions to provide on-chip ESD robustness. However, CMOS integrated circuits have become more vulnerable to ESD damage due to advanced processes such as the use of light-doped drains (LDDs) and clad silicide diffusions. Moreover, the fact that MOS transistors with enlarged dimensions occupy more layout area is contrary to the trend of scale miniaturization. 
     U.S. Pat. Nos. 5,012,317and 5,336,908disclose a lateral semiconductor-controlled rectifier (LSCR) as an on-chip ESD protection circuit. The required voltage for triggering conventional LSCR&#39;s relies heavily upon the junction breakdown between a substrate and the well region formed therein, being therefore in the range of about 30 ˜50V. Thus, the conventional LSCR&#39;s may not offer effective ESD protection for sub-micron CMOS devices because the gate oxides of the MOS transistors are permanently damaged before triggering. 
     To reduce the trigger voltage of the LSCR&#39;s without increasing the leakage current, a modified LSCR incorporating a field oxide device has been proposed by A. Amerasekera and C. Duvvury, as disclosed in “ESD in Silicon Integrated Circuits,” John Wieley &amp; Sons Press, 1998, p.90; furthermore, an LSCR triggered by a zener diode has been disclosed in U.S. Pat. No. 5,343,053. The trigger voltage can be further reduced to 10˜15V by replacing the aforementioned field oxide device with a thin oxide MOS transistor as disclosed in U.S. Pat. No. 5,465,189. The cross-sectional view of the low voltage triggering SCR (LVTSCR) is depicted in FIG.  1 . 
     In FIG. 1, the LVTSCR is fabricated onto a P-type semiconductor substrate  10  in which an N-well  11  is provided. A P-type doped region  12  and an N-type doped region  13  are spaced apart and formed in the N-well  11  while tied together to an IC pad  1  coupled to an internal circuit  2 . The internal circuit  2  denotes the core circuit of an integrated circuit to be protected by the LVTSCR. Another N-type doped region  14  and P-type doped region  15  are spaced apart and formed in the P-type semiconductor substrate  10  while tied together to a power node V SS  that is powered by a ground potential under normal operation. 
     In addition, an N-type doped region  16  is provided with one portion formed in the N-well  11  and another portion formed in the P-type semiconductor substrate  10  to span the P/N junction therebetween. A gate structure  17  is disposed on the P-type semiconductor substrate  10  between the N-type doped regions  14  and  16 . From bottom to top, the gate structure  17  comprises an oxide layer  18  formed on the P-type semiconductor substrate  10  and an electrode layer  19  connected to the power node V SS . 
     Accordingly, the P-type doped region  12 , N-well  11  and P-type semiconductor substrate  10  constitute the emitter, base, and collector of a parasitic PNP bipolar junction transistor  20 , respectively. Moreover, the P-type semiconductor substrate  10 , N-well  11 , and N-type doped region  14  constitute the collector, base, and emitter of a parasitic NPN bipolar junction transistor  21 , respectively. The equivalent circuit of FIG. 1 is illustrated in FIG. 2, wherein resistors  22   5  and  23  designate the associated parasitic resistance spread over the N-well  11  and the P-type semiconductor substrate  10 . In addition, reference numeral  24  designates the MOS transistor constituted by the N-type doped regions  14  and  16 , and the gate structure  17 . 
     In the conventional LVTSCR, the trigger voltage is reduced, and so is the trigger current. If the LVTSCR suffers from external overshooting or undershooting noises under normal operation, it may be triggered to turn on improperly. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an electrostatic discharge protection circuit with high trigger current for preventing the LSCR from improper conduction when suffering from the external overshooting or undershooting noises under normal operation. 
     The present invention achieves the above-indicated object by providing an ESD protection circuit for protecting a circuit, comprising a lateral semiconductor-controlled rectifier, a MOS transistor, and a current-sinking device. The lateral semiconductor-controlled rectifier is coupled to the circuit and provided with a first common region and a second common region. The MOS transistor integrated with the lateral semiconductor-controlled rectifier includes the first common region. The current-sinking device integrated with the lateral semiconductor controlled rectifier includes the second common region. 
     Therefore, the current-sinking device shunts the majority of a discharge current when the MOS transistor enters breakdown, thereby increasing the trigger current of the lateral semiconductor-controlled rectifier. When external overshooting or undershooting noises occur under the normal operation, the potential at the pad can be clamped to the snapback voltage of the PNP transistor because of the high trigger current provided by the ESD protection circuit in accordance with the present invention, thus not entering PNPN conduction. Therefore, the internal circuit is immune to function disorder or device damage. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The following detailed description, given by way of examples and not intended to limit the invention to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 schematically depicts a conventional LVTSCR in a cross-sectional view; 
     FIG. 2 schematically depicts the equivalent circuit off FIG. 1; 
     FIG. 3 schematically depicts one preferred embodiment of the present invention in a cross-sectional view; 
     FIG. 4 schematically depicts the equivalent circuit of FIG. 3; 
     FIG. 5 schematically depicts another preferred embodiment of the present invention in a cross-sectional view; and 
     FIG. 6 depicts the I-V curve of the electrostatic discharge protection circuit as shown in FIG. 3, while the I-V curve of the conventional LVTSCR is illustrated for comparison. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 3, the cross-sectional view of one preferred embodiment of the present invention fabricated onto a semiconductor substrate is schematically illustrated. In the semiconductor substrate a P-type semiconductor layer  30  and an N-type semiconductor layer  31  are provided adjacent to each other to form a P/N junction  40  therebetween. For example, the P-type semiconductor layer  30  and the N-type semiconductor layer  31  can be dual wells formed in the semiconductor substrate, a P-type substrate and an N-well formed therein, or a P-well and an N-type semiconductor substrate. While the dual wells are exemplified in the drawings, the scope of the invention is not limited to the following embodiments. 
     As shown in FIG. 3, a P-type doped region  32  and an N-type doped region  33  are spaced apart and formed in the N-type semiconductor layer  31 , wherein the P-type doped region  32  is closer to the junction  40  than the N-type doped region  33 . 
     Another N-type doped region  34  and P-type doped region  35  are spaced apart and formed in the P-type semiconductor layer  30 , wherein the N-type doped region  34  is closer to the junction  40  than the P-type doped region  35 . The P-type doped region  32  and the N-type doped region  33  are connected together to the IC pad  1 , which is coupled to the internal circuit  2  to be protected by the ESD protection circuit of the present invention. The N-type doped region  34  and the P-type doped region  35  are connected together to the power node V SS  that is powered by ground potential under normal operation. 
     In addition, an N-type doped region  36  is provided with one portion formed in the N-type semiconductor layer  31  and another portion formed in the P-type semiconductor layer  30  so as to span the junction  40  therebetween. A gate structure  37  overlies a portion of the P-type semiconductor layer  30  between the N-type doped regions  34  and  36 , comprising, from bottom to top, an oxide layer  38  and an electrode layer  39 . The oxide layer  38  is formed on the P-type semiconductor layer  30  where the electrode layer  39  is connected to the power node V SS . 
     Moreover, a P-type doped region  41  is provided in the N-type semiconductor layer  31  and formed between the P-type doped region  32  and the N-type doped region  36 . The P-type doped region  41  is connected to the power node V SS . 
     Accordingly, the P-type doped region  32 , N-type semiconductor layer  31  and P-type doped region  41  constitute the emitter, base, and collector of a first PNP bipolar junction transistor  42 A, respectively, while the P-type semiconductor layer  32 , N-type semiconductor layer  31  and P-type semiconductor layer  30  constitute the emitter, base, and collector of a second PNP bipolar junction transistor  42 B, respectively. In other words, the first PNP transistor  42 A and the second PNP transistor  42 B share the same emitter and base. Moreover, the P-type semiconductor layer  30 , N-type semiconductor layer  31 , and N-type doped region  34  constitute the collector, base, and emitter of an NPN bipolar junction transistor  43 , respectively. The equivalent circuit of FIG. 1 is illustrated in FIG.  4 . 
     In FIG. 4, resistors  44  and  45  designates the associated parasitic resistance spread over the N-type semiconductor layer  31  and the P-type semiconductor layer  30 . Moreover, reference numeral  46  designates the MOS transistor constituted by the N-type doped regions  34  and  36 , and the gate structure  37 . As shown in FIG. 4, the PNP transistors  42 A and  42 B have a common emitter and base, where the first PNP transistor  42 A has its collector (the P-type doped region  41 ) connected to the power node V SS , and the second PNP transistor  42 B is configured with its collector (the P-type semiconductor layer  30 ) coupled to the power node V SS through the resistor  45 . 
     The operation of the ESD protection circuit in accordance with the present invention will be described in conjunction with FIGS. 3 and 4. 
     When the ESD stress occurring to the pad  1  is high enough to cause the drain junction of the MOS transistor  46  (i.e., the junction between the N-type doped region  36  and the P-type semiconductor layer  30 ) to enter avalanche breakdown, the first and second PNP transistors  42 A and  42 B are turned on to clamp the potential at the pad  1  to the snapback voltage V sb of the PNP transistor. Therefore, the trigger voltage V t of the ESD protection circuit according to the present invention is the breakdown voltage of the MOS transistor  46  at the drain junction, which is in the range of 10˜15V. Because the first PNP transistor  42 A is provided with the collector connected to the power node V SS  but the second PNP transistor  42 B is configured with the collector connected to the power node V SS  through the resistor  45 , a majority of the current flows through the first PNP transistor  42 A, while a minority of the current flows through the second PNP transistor  42 B to the P-type semiconductor layer  30 . At this time, the NPN transistor  43  is non-conductive. 
     When the current flowing through the second PNP transistor  42 B is sufficient to forward bias the junction between the P-type semiconductor layer  30  and the N-type doped region  34 , the LSCR constituted by the second PNP transistor  42 B and the NPN transistor  43  is triggered to turn on. Accordingly, the potential at the pad  1  can be clamped to a further low holding voltage V h , and the conductive LSCR provides low resistance to bypass the ESD stress occurring at the pad  1 . 
     The I-V curve of the ESD protection circuit of FIG. 3 is depicted in FIG. 6, with the conventional LVTSCR I-V curve is illustrated for comparison. In FIG. 6, the solid line denotes the I-V curve of the ESD protection circuit in accordance with the present invention, and dotted line designates the conventional one. 
     As shown in FIG. 6, the ESD protection circuit of the present invention makes use of the first PNP transistor  42 A as a current-sinking device. When avalanche breakdown occurs to the drain junction of the MOS transistor  46 , the PNP transistor  42 A sinks a majority of the current to increase the trigger current I t1 , thereby turning on the LSCR constituted by the second PNP transistor  42 B and the NPN transistor  43 . In comparison, the conventional LVTSCR forward biases the junction between substrate  10  and the N-type doped region  14  as soon as the drain junction of the MOS transistor  24  enters breakdown, as shown in FIG.  1 . Therefore, the trigger current I t2  of the conventional LVTSCR is much smaller than I t1 . 
     Accordingly, when external overshooting or undershooting noises occurs under normal operation, the potential at the pad  1  is clamped to the snapback voltage of the PNP transistor because of the high trigger current provided by the ESD protection circuit in accordance with the present invention, thus not entering PNPN conduction. Therefore, the internal circuit  2  is immune to function disorder or device damage. 
     Referring to FIG. 5, the cross-sectional view of another preferred embodiment of the present invention fabricated onto a semiconductor substrate is schematically illustrated. As compared with the above embodiment, the N-type doped region  36  is connected to the pad  1  in this embodiment. 
     While the invention has been described with reference to various illustrative embodiments, the description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those person skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as may fall within the scope of the invention defined by the following claims and their equivalents.