Abstract:
An output circuit for a semiconductor integrated circuit is improved by reverse biasing the gates of non-selected output field effect transistors (MOSTs). A control MOST, when actuated by a chip-select signal, connects the gate of its associated output MOST with a negative voltage so that the non-selected output MOSTs are completely cut off. The invention avoids the problem which arises with the use of very short channel output MOSTs such that the channel cannot be completely cut off if a zero bias is applied to the gate.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an output circuit for a semiconductor integrated circuit for use with a field-effect transistor (hereafter referred to as a MOST). 
     FIG. 1 shows a system in which the output terminals of two semiconductor integrated circuits are connected to conventional output circuits. 
     It is generally well known to connect terminals of a plurality of semiconductor integrated circuits, as shown, in common with one another and to use such common terminals as output terminals for such system. 
     In FIG. 1, numeral 10a designates the output circuit component for a first semiconductor integrated circuit A. At 1a and 2a are its output terminal and a terminal to which a power source (high level) voltage is applied. At 3a is a pull up MOST connected between the power source terminal and the output terminal 1a. At 4a is a pull down MOST connected between the output terminal 1a and ground. Numeral 5a is a terminal to which a signal OUT for controlling the MOST 3a is applied. At 6a is a terminal to which a signal OUT for controlling the MOST 4a is applied, in complementary relation with the signal OUT. A MOST 7a is connected between the terminal 5a and ground, and a MOST 8a is connected between the terminal 6a and ground. A terminal 9a receives a signal CS1 for controlling the MOSTs 7a, 8a. 
     Terminals lb-9b in an output circuit 10b for a second semiconductor integrated circuit B correspond to those at 1a-9a in the output circuit 10a for the first semiconductor integrated circuit. At reference numeral 11 is a common output terminal of the output circuits 10a, 10b for the two semiconductor integrated circuits. 
     Now, the operation of the circuit shown in FIG. 1 will be explained. For convenience of illustration, all of the MOSTs used in the circuit may be taken as n-channel MOSTs. 
     In the system shown in FIG. 1, in which the output terminals of a plurality of the semiconductor integrated circuits are connected in common with each other, the output state of either one of the selected semiconductor integrated circuits may be present at the output terminal whereas the output state of the other semiconductor integrated circuit is in a non-selected condition and is controlled so as to exert no influence upon the output terminal 11. To this end, MOSTs 7a, 8a and 7b, 8b are provided to render each of the semiconductor integrated circuits selected or non-selected. Chip select signals CS1 and CS2 are transmitted to the respective gates of the MOSTs. Now, for transmission of the output state of only the semiconductor integrated circuit 10a (FIG. 1) from the output terminal 1a thereof to the output terminal 11, the chip select signals CS1 and CS2 are respectively set at &#34;0&#34; and &#34;1&#34;. Under such circumstances complementary signals are applied to the terminals 5a, 6a to transmit these signals to the gates of the MOSTs 3a, 4a, presenting a &#34;1&#34; or &#34;0&#34; to the output terminal 1a according to the level of these signals. For instance, if the signal OUT to the terminal 5a and the signal OUT to the terminal 6a are &#34;1&#34; and &#34;0&#34;, respectively, a &#34;1&#34; is output to the output terminal 1a since the MOST 3a is turned on and the MOST 4a is non-conductive. On the other hand, the MOSTs 7b, 8b conduct as the chip select signal CS2 is set at &#34;1&#34; on the side of the semiconductor integrated circuit B; however, the MOSTs 3b and 4b  do not conduct as the gates thereof are grounded, rendering the terminal lb floating. Accordingly, a signal transmitted to the terminals 5b, 6b is not present at the terminal 1b. If it is desired to transmit the output from the side of the semiconductor integrated circuit B, the chip select signals CS1 and CS2 are set at &#34;1&#34; and &#34;0&#34;. According to the system designated as above, control is performed using the chip select signals CS so as to turn off the MOSTs of one output circuit when the other output circuit is actuated. If the other output MOSTs are enabled, interference is caused between the two outputs, causing deterioration of the &#34;1&#34;, &#34;0&#34; levels at the terminal 11. 
     In a system of the class as aforementioned, the output MOSTs on the non-selected side are required to create a discontinuity. On the other hand, the output MOSTs 3, 4 are required to have short channel lengths to increase the switching speed thereof. However, a reduction in the channel lengths of the MOSTs 3, 4 lowers the threshold value of the MOSTs more than required due to the &#34;short channel effect&#34;. 
     In this instance, the MOSTs do not create a discontinuity to the full extent, i.e., the channel is not turned off completely, even if the terminals 5, 6 are grounded so as to maintain the gate voltages of the output MOSTs 3, 4 at the ground level. This phenomenon has been previously discussed by Nishizawa et al. in their article. &#34;A Limitation of Channel Length in Dynamic Memories&#34;, Journal of Solid-State Circuits, Vol. SC-15, No. 4, August 1980. The resistance of the MOSTs often amounts to a value of several MΩ--several hundred MΩ. 
     If 100 output circuits in a non-selected condition are connected to common output terminals, the common resistance amounts to a value of several tens of kΩ--several kΩ to thus involve an adverse effect upon the output level derived from the output circuit in a selected condition. 
     SUMMARY OF THE INVENTION 
     This invention has been provided in light of the above problems. It is an object of the present invention to lower the electric potential of a source electrode of the MOSTs 7, 8 controlled by the chip select signal to a value lower than the earth potential, so as to fully cut off the output MOSTs 3, 4 when in a non-selected condition, to thereby cause no effect on the common output terminals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing essential components of an output circuit of a semiconductor integrated circuit made according to the prior art; 
     FIG. 2 is a circuit diagram showing essential components of an output circuit of a semiconductor integrated circuit according to one embodiment of the present invention; 
     FIG. 3 is a circuit diagram showing one example of a source for generating a predetermined voltage V N  for the embodiment shown in FIG. 2; and 
     FIG. 4 is a circuit diagram of the essential components of an output circuit of a semiconductor integrated circuit according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the invention will now be described in the following in conjunction with FIG. 2, wherein the same reference numerals used in FIG. 1 are used to designate the same or corresponding parts. In this connection, it will be noted that the arrangement is made so that the source electrodes of the MOSTs 7, 8 are connected to a voltage generating source generating a predetermined voltage V N  (in this instance, -V TH ) lower than the ground potential. 
     Since the basic performance of the circuit shown in FIG. 2 is similar to that shown in FIG. 1, only the differences in performance will be described. The electric potentials of the gate electrodes of the MOSTs 3, 4 (FIG. 2) on the non-selected side amount to -V TH  (the threshold voltage of the output MOSTs) lower than the ground level to thus make the output MOSTs completely discontinuous or cut off, thus, presenting no effect at the output terminal 11. However, on the selected side, the chip select signal CS is at &#34;0&#34; so that the MOSTs 7, 8 are opened to some extent for the same reason as the MOSTs 3, 4 on the non-selected side were previously to possibly deteriorate the levels at the terminals 5, 6. In general, the circuit is adapted to supply the CS signal sufficiently faster than the OUT, OUT signals. For this reason, the MOSTs 7, 8 are not required to be very quickly actuated as is the case with the MOSTs 3, 4, and thus it is possible to provide these MOSTs with longer channel lengths than those of the MOSTs 3, 4. Further, the threshold voltage value may be intentionally increased by ion injection and the like in the formation of the gate electrodes of these MOSTs to make it possible to create full discontinuity when the chip select signal CS is at &#34;0&#34;. 
     A voltage generating source for generating a predetermined potential V N  less than the ground potential as in FIG. 2 will now be explained with reference to FIG. 3. 
     This voltage generating source may be formed on the same substrate as that of the output circuit. As shown in FIG. 3, 32 is an output terminal for producing the voltage -V TH . At 33 is a clamp MOST for setting the output voltage at -V TH . This device is connected between the output terminal 32 and ground, with the gate being grounded. A first rectifier MOST 34 has one main electrode and the gate connected to the output terminal 32. A second rectifier MOST 35 has one main electrode connected to the other main electrode of the first rectifier MOST 34 and its own gate, the other main electrode being grounded. At 36 is a rectifier node, 37 being a combined capacitor connected between a terminal 38, to which a clock signal φ c  is applied, and the other main electrode of the rectifier MOST 34. 
     How this source is actuated will now be explained. In this connection, the circuit without the clamp MOST 33 is generally well known. The voltage amplitude V of the clock signal φ c  and a voltage V TH  equal to the threshold value of the first and second rectifier MOSTs 34, 35 produce a negative voltage of -(V-2V TH ) at the output terminal 32 side. As shown in FIG. 3, the clamp MOST 33 is connected to the output 32, and a current flows from the ground point through the clamp MOST 33 when the voltage at the output terminal 32 is more negative than -V TH , whereby the voltage at the output terminal 32 is clamped at -V TH . That is, a constant voltage -V TH  is supplied to the output terminal 32. 
     FIG. 4 is a circuit diagram showing another embodiment of the invention, wherein like reference numerals designate like or corresponding parts. This circuit may be produced by eliminating from each of the output circuits 10a, 10b a circuit (composed of the MOSTs 3, 7) on the pull up side, and by providing a common pull up resistor 14 between the output terminal 11 and a power source terminal 13. This produces an output circuit system of the so-called &#34;open drain output&#34; type. The circuit as shown is actuated in the same manner as in FIG. 2. As is readily suggested from the performance of the circuit in FIG. 2, the &#34;1&#34; level at the output terminal 11 is prevented from being lowered by making the output MOST 4 in the non-selected condition fully discontinuous or cut off. 
     Although the device as described hereinabove refers to the instance wherein an n-channel MOST is employed, the invention is also applicable to the case where a p-channel MOST is used. 
     As above set forth, according to the present invention, the output circuit is constructed so that the control MOST is connected between a low potential point and the gate of the output MOST for powering either of the binary logic voltages in response to the gate output and so that the control voltage supplied to the gate of the control MOST makes the output MOST discontinuous, with the electric potential of the aforementioned low potential point being made lower than the value of the binary logic voltages. As a result, it is possible to make the output MOST fully cut off or discontinuous if required, but no interference is involved between the outputs even if a plurality of such output MOSTs are used in parallel with each other.