Abstract:
A programmable buffer circuit includes a first stage circuit which receives an input signal IN indicative of a first or second value, and a second stage circuit, responsive to an output of the first stage circuit, for outputting one of a value designated depending on the value of the input signal and a value designated regardless of the value of the input signal. The second stage circuit includes a plurality of MOS transistors whose conduction properties change depending on whether ion implantation is applied thereto. By applying the ion implantation selectively to the MOS transistors, the second stage circuit is operated as a transfer logical function circuit or an inverter logical function circuit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a buffer circuit and, in particular, to a buffer circuit which allows selection of operating states, that is, selection or non-selection, using an input signal, relative to a plurality of semiconductor devices, such as a semiconductor storage, according to a user&#39;s purpose. 
     Such a buffer circuit is used, for example, for making it possible to desirably select operating states of a plurality of semiconductor devices, such as a mask ROM, using an input signal according to a user&#39;s request. For example, a user may request that a semiconductor device be set to an operating state, that is, a selected state, when a low-level input signal is supplied. On the other hand, another user may request that a semiconductor device be set to an operating state when a high-level input signal is supplied. Further, another user may request that a semiconductor device be set to an operating state regardless of whether an input signal is at a low or high level. 
     For satisfying such requests, the buffer circuit is arranged so as to be capable of selecting one of three modes which correspond to the foregoing user&#39;s three kinds of requests, that is, the buffer circuit is arranged to be programmable. The buffer circuit of this type is disclosed, for example, in U. S. Pat. No. 4,612,459. 
     As will be described later in detail, this buffer circuit comprises a first stage circuit including an NOR circuit and a plurality of MOS transistors and a second stage circuit including an exclusive-OR circuit and a plurality of MOS transistors. Each of the foregoing MOS transistors changes from the enhancement type to the depletion type by ion implantation to change its conduction property. Specifically, the foregoing MOS transistors act as switching elements and are used for selecting one of the following three modes: 
     (1) In the first mode, a low-level output signal is produced as a selection signal when an input signal has a low level. This mode is called a low active mode. 
     (2) In the second mode, a low-level output signal is produced as a selection signal when an input signal has a high level. This mode is called a high active mode. 
     (3) In the third mode, a low-level output signal is produced as a selection signal irrespective of whether an input signal has a low or a high level. This mode is called a don&#39;t care active mode. 
     A user can designate one of the foregoing first to third modes. According to this designation, the ion implantation is selectively carried out relative to the foregoing plurality of MOS transistors during fabrication of the buffer circuit. For this reason, the foregoing buffer circuit is called a programmable buffer circuit. 
     However, the foregoing buffer circuit, particularly the second stage circuit thereof, requires the MOS transistors in large number since the exclusive-OR circuit is used. This results in a large chip area. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a programmable buffer circuit with a reduced number of transistors. 
     According to one aspect of the present invention, there is provided a programmable buffer circuit comprising a first stage circuit including an NOR logical function circuit which receives an input signal indicative of a first or second value; and a second stage circuit, responsive to an output of the NOR logical function circuit, for outputting one of a value designated depending on the value of the input signal and a value designated regardless of the value of the input signal. The second stage circuit includes a plurality of MOS transistors whose conduction properties change depending on whether ion implantation is applied thereto. By applying the ion implantation selectively to the MOS transistors, the second stage circuit is operated as a transfer logical function circuit or an inverter logical function circuit. 
     According to another aspect of the present invention, there is provided a programmable buffer circuit comprising a first stage circuit including an inverter logical function circuit which receives an input signal indicative of a first or second value; and a second stage circuit, responsive to an output of the first stage circuit, for outputting a value designated depending on the value of the input signal. The second stage circuit includes a plurality of MOS transistors whose conduction properties change depending on whether ion implantation is applied thereto. By applying the ion implantation selectively to the MOS transistors, the second stage circuit is operated as a transfer logical function circuit or an inverter logical function circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a conventional buffer circuit; 
     FIG. 2 is a diagram showing a buffer circuit according to a first preferred embodiment of the present invention; 
     FIG. 3 is a diagram showing a modification of a second stage circuit of the buffer circuit shown in FIG. 2; 
     FIG. 4 is a diagram showing a modification of the second stage circuit of the buffer circuit shown in FIG. 3; 
     FIG. 5 is a diagram showing another modification of the second stage circuit of the buffer circuit shown in FIG. 2; 
     FIG. 6 is a diagram showing a modification of the second stage circuit of the buffer circuit shown in FIG. 5; 
     FIG. 7 is a diagram showing voltages applied to a plurality of MOS transistors in the buffer circuits shown in FIGS. 2 to 6; and 
     FIG. 8 is a diagram showing a buffer circuit according to a second preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order to facilitate understanding of the present invention, the foregoing conventional buffer circuit will be first explained hereinbelow before describing preferred embodiments of the present invention. 
     In FIG. 1, the buffer circuit comprises a first stage circuit including an NOR circuit 11 and MOS transistors 12 and 13, and a second stage circuit including an exclusive-OR circuit 14 and MOS transistors 15 and 16. Each of the MOS transistors 12, 13, 15 and 16 changes from the enhancement type to the depletion type by the ion implantation to change its conduction property. The foregoing MOS transistors 12, 13, 15 and 16 act as switching elements and are used for selecting one of the following three modes: 
     (1) In the first mode, the ion implantation is applied to the MOS transistors 13 and 15. In this case, a low-level output signal OUT is produced as a selection signal when an input signal IN has a low level. This mode is called a low active mode. 
     (2) In the second mode, the ion implantation is applied to the MOS transistors 13 and 16. In this case, the low-level output signal OUT is produced as the selection signal when the input signal IN has a high level. This mode is called a high active mode. 
     (3) In the third mode, the ion implantation is applied to the MOS transistors 12 and 16. In this case, the low-level output signal OUT is produced as the selection signal irrespective of whether the input signal IN has the low or high level. This mode is called a don&#39;t care active mode. 
     A user can designate one of the foregoing first to third modes. According to this designation, the ion implantation is selectively carried out relative to the MOS transistors 12, 13, 15 and 16 during fabrication of the buffer circuit. For this reason, the foregoing buffer circuit is called a programmable buffer circuit. 
     Now, the number of the transistors required for constituting the foregoing buffer circuit is considered. In the first stage circuit, in addition to the MOS transistors 12 and 13, four transistors are necessary for constituting the NOR circuit 11. As a result, the first stage circuit requires six transistors. In the second stage circuit, in addition to the MOS transistors 15 and 16, six transistors are necessary for constituting the exclusive-OR circuit 14. As a result, the second stage circuit requires eight transistors. 
     Referring to FIG. 2, a buffer circuit according to the first preferred embodiment of the present invention will be described. In this embodiment, a first stage circuit of the buffer circuit is the same as the first stage circuit shown in FIG. 1. Specifically, the first stage circuit comprises an NOR circuit 11 and MOS transistors 12 and 13 whose conduction properties change by the ion implantation. A second stage circuit of the buffer circuit comprises a first circuit including a p- and n-channel common MOS transistor 21 whose conduction property changes by the ion implantation, a second circuit including a p-channel MOS transistor 22 and a MOS transistor 23 whose conduction property changes by the ion implantation, and a third circuit including an n-channel MOS transistor 24 and an MOS transistor 25 whose conduction property changes by the ion implantation. 
     The first circuit is a p- and n-channel common circuit, and the MOS transistor 21 is connected between an output terminal of the NOR circuit 11 and an output terminal of the output signal OUT. The second circuit is a p-channel circuit, and a series circuit of the MOS transistors 22 and 23 is connected between a power supply terminal Vcc and the output terminal of the output signal OUT. More specifically, the gate of the MOS transistor 22 is connected to the output side of the NOR circuit 11, while the drain thereof is connected to the power supply terminal Vcc. The drain of the MOS transistor 23 is connected to the source of the MOS transistor 22, while the gate and the source of the MOS transistor 23 are connected to the output terminal side of the output signal OUT. The third circuit is an n-channel circuit, and a series circuit of the MOS transistors 24 and 25 is connected between the ground terminal and the output terminal of the output signal OUT. Specifically, the gate of the MOS transistor 24 is connected to the output side of the NOR circuit 11, while the source thereof is connected to the ground terminal. On the other hand, the gate and the source of the MOS transistor 25 are connected to the drain of the MOS transistor 24, while the drain of the MOS transistor 25 is connected to the output terminal side of the output signal OUT. 
     The buffer circuit thus arranged is programmable with the following three kinds of modes: 
     (1) By applying the ion implantation to the MOS transistors 13, 23 and 25, a low-level output signal OUT is produced as a selection signal when an input signal IN has a low level. This means that the buffer circuit is programmed into the low active mode. 
     (2) By applying the ion implantation to the MOS transistors 13 and 21, the low-level output signal OUT is produced as the selection signal when the input signal IN has a high level. This means that the buffer circuit is programmed into the high active mode. 
     (3) By applying the ion implantation to the MOS transistors 12 and 21, the low-level output signal OUT is produced as the selection signal irrespective of whether the input signal IN has the low or high level. This means that the buffer circuit is programmed into the don&#39;t care active mode. 
     As appreciated from FIG. 2 and the foregoing description, in this embodiment, by applying the ion implantation selectively to the MOS transistors 12, 13, 21, 23 and 25, the second stage circuit can be operated as an inverter circuit or a transfer circuit. Particularly, since the second stage circuit uses no exclusive-OR circuit as shown in FIG. 1, it can be realized by five transistors. Even if the MOS transistor 21 is counted as two transistors, the second stage circuit can be realized by six transistors. 
     FIG. 3 shows a modification of the second stage circuit shown in FIG. 2. In this modification, the p- and n-channel common MOS transistor 21 shown in FIG. 2 is replaced with an n-channel MOS transistor 21&#39; whose conduction property changes by the ion implantation. Further, n-channel MOS transistors 23&#39; and 25&#39; are used in the second and third circuits as MOS transistors to be subjected to the ion implantation. 
     FIG. 4 shows a modification of the second stage circuit shown in FIG. 3. In this modification, positions of the MOS transistors 22 and 23&#39; shown in FIG. 3 are replaced with each other, and positions of the MOS transistors 24 and 25&#39; shown in FIG. 3 are replaced with each other. 
     FIG. 5 shows another modification of the second stage circuit shown in FIG. 2. In this modification, the MOS transistor 21 shown in FIG. 2 is replaced with a p-channel MOS transistor 21&#34; whose conduction property changes by the ion implantation. Further, p-channel MOS transistors 23&#34; and 25&#34; are used in the second and third circuits as MOS transistors to be subjected to the ion implantation. 
     FIG. 6 shows a modification of the second stage circuit shown in FIG. 5. In this modification, positions of the MOS transistors 22 and 23&#34; shown in FIG. 5 are replaced with each other, and positions of the MOS transistors 24 and 25&#34; shown in FIG. 5 are replaced with each other. 
     In each of the buffer circuits shown in FIGS. 2 to 6, voltages Vt applied to the respective MOS transistors upon the enhancement or depletion type become as shown in FIG. 7. 
     Now, referring to FIG. 8, a buffer circuit according to the second preferred embodiment of the present invention will be described. In this embodiment, the buffer circuit is programmable with the low active mode and the high active mode. This buffer circuit differs from the buffer circuit of the first preferred embodiment in that a first stage circuit in this embodiment is constituted only by an inverter 30. The buffer circuit thus arranged is programmable with the following two kinds of modes: 
     (1) By applying the ion implantation to the MOS transistors 23 and 25, a low-level output signal OUT is produced as a selection signal when an input signal IN has a low level. This means that the buffer circuit is programmed into the low active mode. 
     (2) By applying the ion implantation to the MOS transistor 21, the low-level output signal OUT is produced as the selection signal when the input signal IN has a high level. This means that the buffer circuit is programmed into the high active mode. 
     As appreciated, a second stage circuit of the buffer circuit shown in FIG. 8 may be arranged as shown in FIGS. 3 to 6, respectively. 
     The NOR circuit, the inverter circuit and the transfer circuit are not limited to those shown in the figures, but may be realized by various logical function circuits. 
     In the buffer circuit according to the present invention, the second stage circuit uses no exclusive-OR circuit, but comprises the plurality of MOS transistors which can be converted from the enhancement type to the depletion type by the ion implantation. By applying the ion implantation selectively to the plurality of MOS transistors, the second stage circuit is operated as an inverter logical function circuit or a transfer logical function circuit. This can reduce the number of the transistors required for constituting the second stage circuit at least by two as compared with the conventional buffer circuit. Thus, the design of the buffer circuit can be facilitated and the chip area can be reduced. Further, the speed-up of signal transmission can be expected corresponding to the reduction in number of the transistors.