CMOS integrated circuit for lessening latch-up susceptibility

[0002] 1. Field of the Invention

[0003] The present invention generally relates to semiconductor integrated circuit technology. More particularly, the present invention relates to a CMOS integrated circuit for lessening latch-up susceptibility.

[0004] 2. Description of the Related Art

[0005] Although CMOS-based integrated circuitry is characterized by low-power consumption and high-density integration, devices such as transistors or resistors parasitic onto semiconductor substrates raise a reliability issue to be considered. Referring to FIG. 1, a conventional CMOS circuit fabricated onto a semiconductor substrate is schematically illustrated in a cross-sectional view. In the drawing, reference numeral 10 denotes a P-type semiconductor substrate in which an N-well 11 is provided. A PMOS transistor 12 is formed on the N-well 11, whereas an NMOS transistor 13 is formed on the P-type semiconductor substrate 10. An N-type doped region 14 and a P-type doped region 15 are formed in the N-well 11 and the P-type semiconductor substrate 10 to form the contact regions, respectively.

[0006] In FIG. 1, a pair of spaced apart P-type doped regions 12S and 12D serve as the source and the drain of the PMOS transistor 12, while its gate 12G is disposed to cover a portion of the N-well 11 between the source 12S and the drain 12D. A pair of spaced apart N-type doped regions 13S and 13D serve as the source and the drain of the NMOS transistor 13, while its gate 13G is disposed to cover a portion of the P-type semiconductor substrate 10 between the source 13S and the drain 13D. The PMOS transistor 12 is configured with the gate 12G electrically connected to the gate 13G of the NMOS transistor 13 to form an input terminal V_IN, while the drains 12D and 13D are tied together to form an output terminal V_OUT. Both the source 12S of the PMOS transistor 12 and the N-type doped region 14 are powered by a voltage source V_DD, and the source 13S of the NMOS transistor 13 and the P-type doped region 15 is powered by another voltage source V_SS.

[0007] As shown in FIG. 1, the source 12S of the PMOS transistor 12, N-well 11, and P-type semiconductor substrate 10 constitute the emitter, base, and collector of a parasitic PNP bipolar junction transistor Q₁, respectively. Moreover, the source 13S of the NMOS transistor 13, P-type semiconductor substrate 10, and N-well 11 constitute the emitter, base, and collector of a parasitic NPN bipolar junction transistor Q₂, respectively. In the drawing, R_Wdesignates one parasitic resistor spread over the N-well 11, and R_SUBdesignates another parasitic resistor spread over the P-type semiconductor substrate 10.

[0008] However, when a voltage level higher than V_DDor lower than V_SSoccurs at the output terminal V_OUTon account of interference or noise, the emitter-base junctions of the parasitic transistors Q₁and Q₂will enter forward bias to conduct a current flowing therethrough. Even worse, the path between V_DDand V_SSis short-circuited so as to cause permanent damage to the integrated circuit. This is the so-called latch-up effect.

[0009] U.S. Pat. No. 4,947,227 discloses a latch-up resistant CMOS structure achieved by patterning a semiconductor substrate into a trench, on the inside surface of which an oxide insulating layer is thermally grown. Thereafter, amorphous silicon or polysilicon is deposited on the surface of the semiconductor and substantially fills the trench to form a well region. However, grain boundaries contained in the amorphous silicon or polysilicon will deteriorate the carrier mobility of transistors fabricated within such a well region.

[0010] In addition, U.S. Pat. No. 5,338,986 discloses a latch-up resistant CMOS output circuit achieved by increasing the source-gate spacing to dispose a resistance device at the source of the PMOS transistor or NMOS transistor and thus reduce the collector current of the parasitic transistor. However, U.S. Pat. No. 5,338,986 merely takes minority carriers into account, but secondary triggering induced by majority carriers results in a low holding-voltage and possibly a short-circuit of V_DD-V_SScausing permanent damage to the integrated circuit.

[0011] It is therefore an object of the present invention to provide a CMOS integrated circuit for lessening latch-up susceptibility.

[0012] The above object of the present invention can be accomplished with a CMOS integrated circuit. According to the present invention, the CMOS integrated circuit is formed on a P-type semiconductor layer and an N-type semiconductor layer in contact with the P-type semiconductor layer to establish a junction therebetween. A PMOS transistor is formed on the N-type semiconductor layer and configured with its source terminal connected to a first voltage source. An N-type contact region is formed in the N-type semiconductor layer and connected to the first voltage source. An NMOS transistor is formed on the P-type semiconductor layer and configured with its source terminal connected to a second voltage source. A P-type contact region is formed in the P-type semiconductor layer and connected to the second voltage source. Moreover, a P-type carrier-releasing region is provided with one portion formed in the N-type semiconductor layer and another portion formed in the P-type semiconductor layer to span the junction.

[0013] Therefore, the CMOS integrated circuit according to the present invention utilizes the P-type carrier-releasing region 27 as a majority-carrier guard ring as well as a minority-carrier guard ring. Even upon the occurrence of secondary triggering, higher holding voltage can be sustained because the P-type carrier-releasing region 27 acts as the majority-carrier guard ring. Therefore, V_DD-V_SSis not short-circuited and the integrated circuit is protected from permanent damage, thus lessening the latch-up susceptibility.

[0014] 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:

[0015] FIG. 1 schematically illustrates a cross-sectional diagram of a conventional CMOS circuit fabricated onto a semiconductor substrate; and

[0016] FIG. 2 schematically illustrates a cross-sectional diagram of one preferred embodiment of a CMOS circuit in accordance with the present invention fabricated onto a semiconductor substrate.

[0017] Referring to FIG. 2, one preferred embodiment of a MOS circuit according to the present invention fabricated onto a semiconductor substrate is schematically illustrated in a cross-sectional view. In FIG. 2, a P-type semiconductor layer 20 is in contact with an N-type semiconductor layer 21 to establish a P/N junction 26 therebetween. In this embodiment, a P-type semiconductor substrate (e.g., silicon substrate) is exemplified as the P-type semiconductor layer 20; an N-well formed in the P-type semiconductor substrate being exemplified as the N-type semiconductor layer 21. Of course, the N-type semiconductor layer 21 can be an N-type semiconductor substrate on which a P-well is formed as the P-type semiconductor layer 20. In addition, the P-type semiconductor layer 20 and the N-type semiconductor layer 21 can be a P-well and an N-well formed in the same semiconductor substrate, respectively.

[0018] As shown in FIG. 2, a PMOS transistor 22 is formed on the N-well 21, whereas an NMOS transistor 23 is formed on the P-type semiconductor substrate 20. An N-type doped region 24 and a P-type doped region 25 are formed in the N-well 21 and the P-type semiconductor substrate 20 to form contact regions, respectively.

[0019] In FIG. 2, a pair of spaced apart P-type doped region 22S and 22D serve as the source and the drain of the PMOS transistor 22, while its gate 22G is disposed to cover a portion of the N-well 21 between the source 22S and the drain 22D. A pair of spaced apart N-type doped regions 23S and 23D serve as the source and the drain of the NMOS transistor 23, while its gate 23G is disposed to cover a portion of the P-type semiconductor substrate 20 between the source 23S and the drain 23D. The PMOS transistor 22 is configured with the gate 22G electrically connected to the gate 23G of the NMOS transistor 23 to form an input terminal V_IN, while the drains 22D and 23D are tied together to form an output terminal V_OUT. Both the source 22S of the PMOS transistor 22 and the N-type doped region 24 are powered by the voltage source V_DD, and the source 23S of the NMOS transistor 23 and the P-type doped region 25 are powered by the voltage source V_SS. Usually, V_DDis about 5V or 3.3V, and V_SSis about 0V.

[0020] According to the present invention, a P-type carrier-releasing region 27 is provided with one portion formed in the P-type semiconductor layer 20 and another portion formed in the N-type semiconductor layer 21. In other words, the P-type carrier-releasing region 27 spans the P/N junction 26 between the P-type semiconductor layer 20 and the N-type semiconductor layer 21. Moreover, the P-type carrier-releasing region 27 is connected to the voltage source V_SS.

[0021] The operation of the CMOS circuit will be described with reference to FIG. 2 as follows.

[0022] When an voltage impulse lower than V_SSoccurs at the output terminal V_OUTon account of interference or noise, the junction between the drain 23D of the NMOS transistor 23 and the P-type semiconductor layer 20 enters forward bias and thus generates majority carriers, e.g., holes that will be discharged to V_SSthrough the P-type carrier-releasing region 27.

[0023] According to the present invention, the drain 22D of the PMOS transistor 22, the N-type semiconductor layer 21, and the P-type carrier-releasing region 27 constitute the emitter, base, and collector of a lateral PNP bipolar junction transistor Q₃, respectively. Accordingly, when an voltage impulse higher than V_DDoccurs at the output terminal V_OUTon account of interference or noise, the junction between the drain 22D of the PMOS transistor 22 and the N-type semiconductor layer 21 enters forward bias and thus generates minority carriers-holes which will be discharged to V_SSthrough the P-type carrier-releasing region 27. In other words, the minority carriers can flow through the transistor Q₃instead of flowing through the parasitic PNP bipolar junction transistor constituted by the source 22S, the N-type semiconductor layer 21, and the P-type semiconductor layer 20.

[0024] Therefore, the CMOS integrated circuit according to the present invention utilizes the P-type carrier-releasing region 27 as a majority-carrier guard ring as well as a minority-carrier guard ring. Even upon the occurrence of secondary triggering, higher holding voltage can be sustained because the P-type carrier-releasing region 27 acts as the majority-carrier guard ring. Therefore, V_DD-V_SSis not short-circuited and the integrated circuit is protected from permanent damage so as to lessen the latch-up susceptibility.

[0025] 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.