Semiconductor device having a reverse bias voltage generator

Abstract: An output circuit of a semiconductor circuit has a reverse bias voltage generator which comprises a ring oscillating circuit, a reverse bias voltage generating circuit and a switching circuit. The output circuit is of the type having first and second output N-(P-)channel MOS transistors connected in series, an output terminal being provided at the intermediate position of the transistors. The ring oscillating circuit is activated for oscillation only when an output enabling signal is of a zero potential indicative of disabling outputting of an input data signal. The reverse bias voltage generating circuit generates a reverse bias voltage lower (higher) than the ground potential based on the oscillation output of the oscillating circuit. The switching circuit supplies a reverse bias voltage to the gate of the first output N-(P-)channel MOS transistor only when a bias instruction signal, i.e., a complementary signal of the output enabling signal, is of a positive (negative) potential, whereby the occurrence of ionization current in the first output N-(P-)channel MOS transistor can be effectively prevented even when a negative (positive) voltage is applied to the output terminal.

Description: BACKGROUND OF THE INVENTION The present invention relates to a semiconductor device and, more particularly, to an improvement in a semiconductor integrated circuit with an output circuit, formed on a semiconductor chip provided with a built-in substrate bias circuit. Fig. 1 illustrates an example of a conventional semiconductor integrated circuit having an output circuit. The conventional semiconductor integrated circuit having a built-in substrate bias circuit is, as seen in Fig. 1, so arranged that a first gate circuit comprises a NAND circuit NA₄₁ and an inverter IN₄₁ and a second gate circuit comprises a NAND circuit NA₄₂ and an inverter IN₄₂. Further, the output circuit comprises an output N-channel MOS transistor Q₄₁ (hereinafter referred to as an N-ch Tr) which is a first output N-channel MOS transistor and an N-ch Tr Q₄₂ which is a second output N-channel MOS transistor. In the NAND circuit NA₄₁, one input thereof is supplied with an input data signal DQ and the other input thereof is supplied with an output enabling signal DOE and, in the NAND circuit NA₄₂, one input thereof is supplied with a complementary signal DQ of the input data signal DQ and the other input thereof is supplied with the output enabling signal DOE. As for the next inverter IN₄₁, it inverts the output of the NAND circuit NA₄₁ and is connected to the gate of the N-ch Tr Q₄₁ and, as for the inverter IN₄₂, it inverts the output of the NAND circuit NA₄₂ and is connected to the gate of the N-ch Tr Q₄₂. Further, as for the N-ch Tr Q₄₁, its drain is connected to a power source V DD of a positive potential and its source is connected to an output terminal of the output signal D out and, as for the N-ch Tr Q₄₂, its drain is connected to the same output terminal of the output signal D out and its source is grounded. Thus, with respect to the conventional semiconductor integrated circuit as shown in Fig. 1, where the output enabling signal DOE is of a positive potential, the output signal D out of a positive potential is sent out from the output terminal when the input data signal DQ is of a positive potential and the complementary signal DQ of the input data signal is of a zero potential whereas the output signal D out of a zero potential is sent out from the output terminal when the input data signal DQ is of a zero potential and the complementary signal DQ of the input data signal is of a positive potential. Further, when the output enabling signal DOE is of a zero potential, the gate potential of each of the N-ch Trs Q₄₁, Q₄₂ becomes the zero potential which is close to the ground potential so that the N-ch Trs Q₄₁, Q₄₂ are all in an OFF state and the output terminal does not output any output signal D out and is in a high impedance state. With the conventional semiconductor circuit as explained above, it may occur that, when the output terminal of the output signal D out is connected to any other external circuits, a voltage close to -1 V which is the minimum value as a low level of an input voltage is applied to the output terminal under the high impedance state as mentioned above. In the conventional semiconductor integrated circuit as shown in Fig. 1, as already explained hereinabove, when the output enabling signal DOE is of a zero potential, the gate potential of each of the N-ch Trs Q₄₁, Q₄₂ is zero, close to the ground potential. When the potential of the output terminal for the output signal D out assumes a value close to -1 V, the threshold voltage V TN (in the order of 0.5 - 0.9 V) of the N-ch Trs Q₄₁, Q₄₂ is exceeded to allow the N-ch Trs Q₄₁, Q₄₂ to be turned to their ON state. Under the above state, the hole injection in the substrate is caused by the N-ch Trs Q₄₁, Q₄₂ and, when the capacity of the built-in substrate bias circuit is exceeded, the substrate potential rises so as to cause a malfunction in internal circuits. This phenomenon is known as a malfunction caused by an ionization current. The magnitude of the ionization current is proportional to the current capacity of a transistor and this becomes maximum when the transistor is in a pinch-off state. Thus, the malfunction by the ionization current becomes worst when a high voltage is present between the drain and the source of the transistor and the gate voltage thereof is in the vicinity of the threshold voltage V TN . Further, it is known that the ionization current increases exponentially in accordance with the increase of the voltage between the drain and the source. In the conventional semiconductor integrated circuit as shown in Fig. 1, there were problems in that, although the ionization current by the N-ch Tr Q₄₂ was of a negligible magnitude, that by the N-ch Tr Q₄₁ was high thereby causing a malfunction in internal circuits due to the rise of the potential of the substrate. SUMMARY OF THE INVENTION Therefore, the main object of the present invention is to provide a semiconductor device in which the occurrence of an ionization current can be prevented even when an abnormal voltage, eg. a negative voltage (or a positive voltage depending on whether the transistors are N-channel or P-channel) is applied to the output under a high impedance state of the output terminal. A semiconductor device according to the present invention comprises an output circuit comprising means for receiving a first input data signal, means for receiving a second input data signal complementary to said first input signal, means for receiving an output enable signal taking one of first and second logic levels, a first output field effect transistor having a gate and a drain-source current path connected between a power voltage terminal and an output terminal, said power voltage terminal receiving a power voltage of a first polarity, a second output field effect transistor having a gate and a drain-source current path connected between said output terminal and a reference voltage terminal receiving a reference voltage, and a first gate circuit receiving said second input data signal and said output enable signal and having a first control node coupled to the gate of said second output transistor, said first control node assuming one of said logic levels corresponding to said second input data signal when said output enable signal is at said first logic level and an inactive level making said second output transistor non-conductive when said output enable signal is at said second logic level, said output circuit further comprising a second gate circuit receiving said first input data signal and said output enable signal and having a first control node connected to the gate of said first output transistor, said first control node assuming one of logic levels corresponding to said first input data signal when said output enable signal is at said first logic level and a high impedance state when said output enable signal is at said second logic level, and a reverse bias voltage generating circuit having a control node receiving said output enable signal and a bias output node connected to the gate of said first output transistor, said bias output node being set at a reverse bias voltage when said output enable signal is at said second logic level and at a high impedance state when said output enable signal is at said first logic level, reverse said reverse bias voltage being of an opposite polarity to the polarity of said power voltage, so that said reference voltage lies between said reverse bias voltage and said power voltage. In one embodiment the first and second transistors are N-channel transistors and the reverse bias voltage is a negative potential. In another embodiment the first and second transistors are P-channel transistors and the reverse bias voltage is a positive potential. The reverse bias voltage generator may comprise a ring oscillating circuit having a series circuit of a NAND circuit, a first inverter and a second inverter, one input of said NAND circuit being supplied with a complementary signal of said output enabling signal and the other input of the same being connected to an output of said second inverter. The reverse bias voltage generating circuit may comprise a serial diode connection of two MOS transistors, the source of one of said transistors being grounded and the gate and drain of the same being tied together and coupled to the output of said ring oscillating circuit through a capacitor and also connected to the source of the other transistor, the gate and drain of said other transistor being tied together for outputting said reverse bias voltage. The ring oscillating circuit may be such that, on receiving the complementary signal of the output enabling signal, it is activated and oscillates only when the complementary signal of the output enabling signal is of a positive potential (negative potential). The reverse bias voltage generating circuit may generate a reverse bias voltage lower (higher) than the ground potential based on the oscillation output of the activated ring oscillating circuit. The switching circuit may be such that, on receiving the complementary signal of the output enabling signal, it supplies the reverse bias voltage from the reverse bias voltage generating circuit to the gate of the first output N-(P-) channel transistor only when the complementary signal of the output enabling signal is of a positive potential (negative potential). Thus, the switching circuit may comprise a MOS transistor, the source thereof being connected to the output of said reverse bias voltage generating circuit, the gate thereof receiving said complementary signal of said output enabling signal and the drain being connected to the gate of said first output transistor. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a circuit diagram of an example of a conventional semiconductor integrated circuit having an output circuit; Fig. 2 shows a circuit diagram of a first embodiment of the circuit used in a semiconductor integrated circuit formed on a P-type semiconductor substrate, according to the present invention; Fig. 3 shows a circuit diagram of the first example of the reverse bias voltage generator; Fig. 4 shows a circuit diagram of the second example of the reverse bias voltage generator; and Fig. 5 shows a circuit diagram of a second embodiment of the circuit used in the semiconductor integrated circuit, formed on an N-type semiconductor substrate. DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention are hereinafter explained with reference to the appended drawings. Fig. 2 shows a circuit diagram of a first embodiment of the circuit used in the semiconductor integrated circuit employing a reverse bias voltage generator according to the present invention. The circuit of Fig. 2 is formed on a P-type semiconductor substrate and incorporates therein a substrate bias circuit. First, as shown in Fig. 2, the first gate circuit comprises a NAND circuit NA₁, an output P-channel MOS transistor Q₃ (hereinafter referred to as, for example, P-ch Tr Q₃) and N-ch Trs Q₄, Q₅ and, the second gate circuit comprises a NAND circuit NA₂ and an inverter IN₂. Also, the output circuit comprises an N-ch Tr Q₁ as a first output N-channel MOS transistor and an N-ch Tr Q₂ as a second output N-channel MOS transistor. Further, a reverse bias voltage generator RBVG, which inputs a bias instruction signal X inverted by the inverter IN₃ in response to the output enabling signal DOE, comprises a ring oscillating circuit, a reverse bias voltage generating circuit and a switching circuit, hereinafter explained in detail. Fig. 3 shows a circuit diagram of a first example of the reverse bias voltage generator RBVG. The reverse bias voltage generator RBVG forms, as shown in Fig. 3, a ring oscillating circuit with a NAND circuit NA₃ and inverters IN₄, IN₅, a reverse bias voltage generating circuit with N-ch Trs Q₆, Q₇ receiving the output of the inverter IN₅ through a capacitor C₁ and, a switching circuit with an N-ch Tr Q₈. Hereinafter explained with reference to Figs. 2 and 3 is the function of the circuit of the first example. One input of the NAND circuit NA₁ is supplied with an input data signal DQ being of a positive potential in the presence of data and being of a zero potential in the absence of data and the other input thereof is supplied with an output enabling signal DOE being indicative of enabling outputting of the input-data signal DQ only when it is of a positive potential and, one input of the NAND circuit NA₂ is supplied with a complementary signal DQ of the input data signal and the other input thereof is supplied with the output enabling signal DOE whereby these NAND circuits send out respective NAND signals. Next, the P-ch Tr Q₃ which receives at its gate a NAND signal from the NAND circuit NA₁ has its drain connected to a constant-voltage power source V DD of a positive potential and its source connected to the drain of the N-ch Tr Q₄, the gate of the N-ch Tr Q₁ and the output of the reverse bias voltage generator RBVG so as to establish a gate signal Y. The N-ch Tr Q₄ which receives at its gate the output enabling signal DOE has its source connected to the drain of the N-ch Tr Q₅. The N-ch Tr Q₅ which receives at its gate a NAND signal from the NAND circuit NA₁ is grounded at its drain. Consequently, the P-ch Tr Q₃ and the N-ch Trs Q₄, Q₅ send to the gate of the N-ch Tr Q₁ a gate signal Y of a positive potential when the input data signal DQ is of a positive potential and also the output enabling signal DOE is of a positive potential and they send a gate signal Y of a zero potential when the input data signal DQ is of a zero potential and the output enabling signal DOE is of a positive potential and, when the output enabling signal DOE is of a zero potential, the P-ch Tr Q₃ and the N-ch Tr Q₄ are all in an OFF state thereby providing a high impedance state. Further, the inverter IN₂ which inverts the output of the NAND circuit NA₂ is connected to the gate of the N-ch Tr Q₂. Accordingly, the inverter IN₂ sends to the gate of the N-ch Tr Q₂ a gate signal of a positive potential only when the complementary signal DQ of the input data signal is of a positive potential and also the output enabling signal DOE is of a positive potential and, it sends a gate signal of a zero potential at other times. On the other hand, the ring oscillating circuit in the reverse bias voltage generator RBVG is so arranged that the NAND circuit NA₃, the inverters IN₄ and IN₅ are connected in series with one input of the NAND circuit NA₃ being supplied with the bias instruction signal X and the other input of the same being connected to the output of the inverter IN₅. Thus, the ring oscillating circuit is activated for oscillation as a consequence of the bias instruction signal X turning to a positive potential when the output enabling signal DOE is of a zero potential indicative of disabling outputting of the input data signal DQ. Further, in the N-ch Tr Q₆ of the reverse bias voltage generating circuit, the output of the inverter IN₅ is coupled to the gate as well as the drain through the capacitor C₁ and the source is grounded whereas, in the N-ch Tr Q₇, the source is connected to the gate as well as the drain of the N-ch Tr Q₆ and the gate and the drain are connected to the source of the N-ch Tr Q₈ of the switching circuit so that the oscillating output of the ring oscillating circuit activated causes the generation of a reverse bias voltage lower than the ground potential which resultant reverse bias voltage is forwarded to the source of the N-ch Tr Q₈. Therefore, with the N-ch Tr Q₈ of the switching circuit which receives the bias instruction signal X at its gate, the drain supplies to the gate of the N-ch Tr Q₁ with the reverse bias voltage from the reverse bias voltage generating circuit as a gate signal Y only when the bias instruction signal X is of a positive potential. On the assumption that the threshold voltage is V TN , when the potential of the gate signal Y becomes more negative than -V TN , the N-ch Tr Q₄ is turned ON. Thus, by the arrangement wherein the current capacity of the reverse bias voltage generator RBVG is made smaller than that of the N-ch Trs Q₄, Q₅, the potential of the gate signal Y will be in the order of -V TN . Thus, the N-ch Tr Q₁ functions such that, with its drain being connected to the constant-voltage power source V DD of a positive potential and its source being connected to the output terminal, it becomes conductive and outputs the output signal D out of a positive potential only while the gate signal Y is of a positive potential where the input data signal DQ is of a positive potential and the output enabling signal DOE is also of a positive potential. When the output enabling signal DOE is at a zero potential, the gate of the N-ch Tr Q₁ is supplied from the reverse bias voltage generator RBVG a reverse bias voltage, as the gate signal Y, in the order of -V TN which is lower than the ground potential so that, unless a negative voltage greater in the absolute value than a value in the order of -2V TN is applied externally to the output terminal, the N-ch Tr Q₁ will not be turned ON. The threshold voltage V TN is in the order of 0.5 - 0.9 V and the negative voltage -2V TN is greater in the absolute value or more negative than -1 V so that, even when a negative voltage smaller in the absolute value than -1 V is applied externally to the output terminal of the output signal D out , there is no fear of the occurrence of an ionization current due to the N-ch Tr Q₁. Further, the N-ch Tr Q₂ receives at its gate a gate signal sent from the inverter IN₂, has its source grounded and has its drain connected to the output terminal so that, only while the gate signal is of a positive potential where the complementary signal DQ of the input data signal is of a positive potential and the output enabling signal DOE is also of a positive potential, the N-ch Tr Q₂ becomes conductive and outputs the output signal D out of a zero potential close to the gro8nd potential. Next, Fig. 4 shows a circuit diagram of a second embodiment of the reverse bias voltage generator covered by the embodiments of the present invention. This second embodiment is different from the reverse bias voltage generator of the first example shown in Fig. 3 in the point that, instead of the diode connection of the N-ch Trs Q₆, Q₇ used in the first example, the diode connection of P-ch Trs Q₉, Q₁₀ is employed in this second embodiment. In the second embodiment shown in Fig. 4, since P-ch Trs are used instead of N-ch Trs, it is an advantage that an ionization current due to N-ch Trs can be suppressed. Although the above explanation is made only with respect to the embodiments and examples wherein circuits are formed all on a P-type semiconductor substrate, it is to be understood that the circuits according to the present invention can be formed similarly on an N-type semiconductor substrate with N-ch Trs and P-ch Trs substituted respectively for each other as shown in Fig. 5. As described above, in the circuit of the present embodiments and examples, it is so arranged that, in the absence of the output enabling signal, the gate potential of the output N-(P-)channel MOS transistor is caused to be in a negative potential (positive potential) so that no ionization of current can occur even when a negative voltage (positive voltage) is applied to the output terminal. As explained above, the advantage of the present invention resides in that, when there is no output enabling signal, it is possible for the gate potential of the output N-(P-)channel MOS transistor to be rendered a negative potential (positive potential) whereby the occurrence of ionization current can be effectively prevented even when a negative voltage (positive voltage) is applied to the output terminal. While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that the changes within the scope of the appended claims may be made without departing from the invention.

Claims: An output circuit comprising means for receiving a first input data signal (DQ), means for receiving a second input data signal ( DQ ) complementary to said first input signal, means for receiving an output enable signal (DOE) taking one of first and second logic levels, a first output field effect transistor (Q1) having a gate and a drain-source current path connected between a power voltage terminal (VDD) and an output terminal (DOUT), said power voltage terminal receiving a power voltage of a first polarity, a second output field effect transistor (Q2) having a gate and a drain-source current path connected between said output terminal and a reference voltage terminal receiving a reference voltage, a first gate circuit (NA2,IN2) receiving said second input data signal and said output enable signal and having a first control node coupled to the gate of said second output transistor, said first control node assuming one of said logic levels corresponding to said second input data signal when said output enable signal is at said first logic level and an inactive level making said second output transistor non-conductive when said output enable signal is at said second logic level, and a second gate circuit (NA1, Q3-Q5) receiving said first input data signal and said output enable signal and having a first control node connected to the gate of said first output transistor, said first control node assuming one of logic levels corresponding to said first input data signal when said output enable signal is at said first logic level characterised in that said first control node assumes a high impedance state when said output enable signal is at said second logic level and, in that said output circuit further comprises a reverse bias voltage generating circuit (IN3,RBVG) having a control node receiving said output enable signal and a bias output node connected to the gate of said first output transistor, said bias output node being set at a reverse bias voltage when said output enable signal is at said second logic level and at a high impedance state when said output enable signal is at said first logic level, said reverse bias voltage being of an opposite polarity to the polarity of said power voltage, so that said reference voltage lies between said reverse bias voltage and said power voltage. An output circuit according to Claim 1, wherein said reverse bias voltage generating circuit comprises a ring oscillating circuit having a series circuit of a NAND circuit (NA₃), a first inverter (IN₄) and a second inverter (IN₅), one input of said NAND circuit being supplied with a complementary signal (X) of said output enabling signal (DOE) and the other input of the same being connected to an output of said second inverter. An output circuit according to Claim 2, wherein said reverse bias voltage generating circuit comprises a serial diode connection of two MOS transistors (Q₆,Q₇), the source of one of said transistors being grounded and the gate and drain of the same being tied together and coupled through a capacitor (C₁) to the output of said ring oscillating circuit (NA₃,IN₄,IN₅) and also connected to the source of the other transistor, the gate and drain of said other transistor being tied together for outputting said reverse bias voltage. An output circuit according to any preceding claim, wherein said reverse bias voltage generating circuit comprises a switching circuit comprising a MOS transistor (Q₈), the source thereof being connected to the reverse bias voltage, the gate thereof receiving the complementary signal (X) of said output enabling signal and the drain being connected to the gate of said first output transistor (Q₁). An output circuit according to Claim 1, wherein the first and second output transistors (Q₁,Q₂) are N-channel transistors and the reverse bias voltage is a negative potential. An output circuit according to Claims 3 and 5, wherein the serial diode connected transistors (Q₆,Q₇) are N-channel type. An output circuit according to Claim 1, wherein the first and second output transistors (Q₁,Q₂) are P-channel transistors and the reverse bias voltage is a positive potential. An output circuit according to Claims 3 and 7, wherein the serial diode connected transistors (Q₉,Q₁₀) are P-channel type.

Inventors: NAKADA KAZUHIRO, NAKADA, KAZUHIRO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1989
Main classification: H03K19
IPC codes: G05F3, H01L21, H01L27, H03K19