BIMOS Logic Gate

This invention relates in general to semiconductor integrated circuit logic gates and, more particularly, to BIMOS logic gates having the low power dissipation, high input impedance, and high noise immunity of CMOS devices while maintaining the high drive capability associated with bipolar devices and having increased switching speed of the output signal.

Increased semiconductor technology has provided for the ability to fabricate a large number of semiconductor devices on one single chip. Bipolar circuits typically provide fast gate speeds, reduced delay per unit load, and have historically been the predominate technology applied in integrated circuits. CMOS (complimentory metal-oxide-semiconductor) structures provide high noise immunity, high input impedance, and low power requirements, and have rapidly gained acceptance in the industry. However, a large CMOS structure is required when driving large capacitive loads and in most cases, several stages of scaled CMOS inverters are necessary in order to minimize the total delay. For BIMOS arrays having a large number of devices, it is desirable that each MOS device be of small size. As the size of a MOS device is reduced, the transconductance of the device and consequently the ability to drive a heavy capacitive load is also reduced. Bipolar devices continue to be used for driving these capacitive loads due to their high current gain. In quiesent periods, the bipolar push-pull transistors do not dissipate power. During transient periods, the bipolar current gain allows faster charging and discharging of capacitive loads. This results in a significant decrease in metal and fanout delays. Furthermore, smaller CMOS devices may be used in the BIMOS circuit than those required in an all-CMOS device circuit. Attempts to combine bipolar and MOS technology to achieve all of these results have been numerous in recent years.

One previously known circuit combining bipolar and MOS devices comprises a pair of push-pull NPN transistors. The upper NPN transistor has a collector connected to a first voltage source and the source of a P-channel device, an emitter connected to an output terminal and the collector of the lower NPN transistor, and a base connected to an input terminal and the gates of the P-channel device and an N-channel device. The lower NPN transistor has an emitter connected to a second voltage source and the source of the N-channel device, and a base connected to the drains of the P-channel and the N-channel devices. However, this circuit has a low impedance at the input terminal since the input terminal is connected to the base of the upper NPN transistor, and as the output switches from low to high, the lower transistor is slow to turn off causing a slow transition to the high output.

Another previously known circuit is described in U.S. patent application Ser. No. 647,216 and "BI-MOS Technology for High-Performance VLSI Circuits", VLSI DESIGN, page 98, August 1984. A BIMOS circuit comprises upper and lower NPN push-pull transistors having an output terminal coupled therebetween. A P-channel device has a source and a drain connected to the collector and base, respectively, of the upper transistor. An N-channel device has a source and drain connected to the base and collector, respectively, of the lower transistor. The gates of the P-channel and N-channel devices are connected to an input terminal.

Yet another previously known circuit is described in "CMOS Digital Technology", ELECTRONIC DESIGN, page 106, Oct. 4, 1984. Upper and lower NPN push-pull transistors have an output terminal coupled therebetween. A P-channel device has a source and drain connected to the collector and base, respectively of the lower NPN transistor. An N-channel device has a drain and source connected to the base and emitter, respectively, of the lower NPN transistor. The gates of the P-channel and N-channel devices and the base of the upper NPN transistor are connected to an input terminal. Although this circuit provides high input impedance, low power dissipation, high noise immunity, and high drive capability, the switching speed is not optimum.

Thus, what is needed is an integrated circuit combining, CMOS and bipolar technology having a high input impedance, improved switching characteristics, low power requirements, high noise immunity, high drive capability and improved power dissipation.

Accordingly, it is an object of the present invention to provide an improved BIMOS logic gate.

Another object of the present invention to provide an improved BIMOS logic gate having improved output signal switching characteristics.

Still another object of the present invention to provide an improved BIMOS logic gate having a high input impedance.

A further object of the present invention to provide an improved BIMOS logic gate having low power requirements.

Yet another object of the present invention is to provide an improved BIMOS logic gate having high drive capability.

Another object of the present invention to provide an improved BIMOS logic gate having high noise immunity.

In carrying out the above and other objects of the invention in one form, there is provided a BIMOS circuit comprising an output terminal coupled between upper and lower NPN push-pull transistors for providing high current drive capability along with no d.c. power dissipation. A first MOS transistor circuit is coupled to the lower transistor for biasing the lower transistor. A second MOS transistor circuit is coupled between an input terminal and both the upper transistor and the first MOS transistor circuit for providing a high impedance at the input and for biasing both the upper transistor and the first MOS transistor circuit, wherein the first MOS transistor circuit is biased with a larger voltage than the upper transistor for improving the switching speed of the output signal.

The above and other objects, features, and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic of the preferred embodiment of the present invention.

FIG. 2 is a schematic of a second embodiment of the present invention.

Referring to FIG. 1, a circuit in accordance with the present invention is shown which is suitable to be fabricated in monolithic integrated circuit form. NPN transistors 11 and 12 comprise a pair of push-pull output transistors. Transistor 11 has its collector and emitter connected, respectively, to supply voltage terminal 13 and output terminal 14. Transistor 12 has its collector and emitter connected, respectively, to output terminal 14 and supply voltage terminal 15. Supply voltage terminal 15 is typically ground.

P-channel MOS transistor 16 has its source and drain connected, respectively, to output terminal 14 and the base of transistor 12. N-channel MOS transistor 17 has its drain and source connected, respectively, to the base of transistor 12 and supply voltage terminal 15. P-channel MOS transistor 18 has its source connected to supply voltage terminal 13, and its drain both connected to the gates of transistors 16 and 17 and coupled to the drain of N-channel MOS transistor 19 by resistor 20. N-channel MOS transistor 19 has its drain and source connected, respectively, to the base of transistor 11 and supply voltage terminal 15. The gates of transistors 18 and 19 are connected to input terminal 21.

When the input signal on terminal 21 switches from a digital high to a digital low, transistors 18 and 11 are rendered conductive and the voltage on supply voltage terminal 13 is provided through transistor 11 to output terminal 14. Transistors 16 and 19 are rendered non-conductive. Transistor 17 is rendered conductive, sinking current from the base of transistor 12 that had been applied when the input signal was high. Transistor 17 may alternatively be a resistor.

When a digital high signal is applied to input terminal 21, transistors 18, 17 and 11 will be rendered non-conductive, thereby precluding the voltage on supply voltage terminal 13 from appearing on output terminal 14. Transistors 19, 16 and 12 will be enabled, thereby ensuring output terminal 14 will be pulled toward the potential of terminal 15. Input terminal 21 will have a high impedance since it is connected only to the gates of MOS transistors 18 and 19. Output terminal 14 will have the high current characteristics of the bipolar push-pull transistors, thereby reducing the delays associated with the long metal lines on an integrated circuit and the fan out to a number of devices which all contribute to the parasitic capacitance load on terminal 14. Resistor 20 creates a voltage difference between the gate of transistor 16 and the base of transistor 11, causing transistor 16 and transistor 12 to switch before transistor 11, insuring that output terminal 14 switches to a low state more rapidly and that transistors 11 and 12 are not conducting sumultaneously.

Referring to FIG. 2, a second embodiment of the invention comprises a pair of push-pull output transistors 22 and 23. Transistor 22 has its collector, base and emitter connected, respectively, to supply voltage terminal 24, node 30 and output terminal 25. Transistor 23 has its collector and emitter connected, respectively, to output terminal 25 and supply voltage terminal 26. Supply voltage terminal 26 is typically ground. P-channel MOS transistor 27 has its source and drain connected, respectively, to supply voltage terminal 24 and node 28. N-channel MOS transistor 29 has its drain and source connected, respectively, to node 30 and supply voltage terminal 26. Nodes 28 and 30 are coupled by resistor 31.

N-channel MOS device 32 has its drain and source connected, respectively, to output terminal 25 and the base of transistor 23. The gates of transistors 27, 29 and 32 are connected to input terminal 33. N-channel MOS transistor 34 has its drain and source connected, respectively, to the base of transistor 23 and supply voltage terminal 26. The gate of transistor 34 is connected to node 28.

When the input signal on terminal 33 switches from a digital high to a digital low, transistors 27, 34, and 22 are rendered conductive and the voltage on supply voltage terminal 13 is provided through transistor 22 to output terminal 25. Transistors 29, 23 and 32 are rendered non-conductive. Transistor 34 is rendered conductive, sinking current from the base of transistor 23 that had been applied when the input signal was high. Transistor 34 may alternatively be a resistor.

When a digital high signal is applied to input terminal 33, transistor 29 will be enabled, thereby pulling the base of transistor 22 down and ensuring a more rapid transition of transistor 22 to an off state, and transistor 27 is disabled so that no current is supplied to the base of transistor 22. Transistor 32 is enabled, thereby applying any current available on output terminal 25 to the base of transistor 23, thus pulling output terminal down to the voltage of supply voltage terminal 26. Input terminal 33 will have a high impedance since it is connected only to the gates of MOS transistors 27, 29 and 32. Output terminal 25 will have the high current characteristics of the bipolar push-pull transistors, thereby reducing the delays associated with the long metal lines on an integrated circuit and the fan out to a number of devices which all contribute to the parasitic capacitance load on terminal 25. Resistor 31 creates a voltage difference between the gate of transistor 34 and the base of transistor 22, causing transistor 34 and transistor 23 to switch before transistor 22, insuring that output terminal 25 switches to a low state more rapidly and that transistors 22 and 23 are not conducting sumultaneously.

Although the circuits described herein illustrate NPN transistors, P-channel and N-channel MOS transistor, various combinations of PNP transistors and MOS transistors may be used in a manner known to those skilled in the art in order to accomplish the teachings of this invention. Furthermore, although only two logic gates have been described, the invention may also be used to enhance other types of gates, i.e., NAND gates and NOR gates.

The invention described herein can be used for internal circuits to improve Very Large Scale Integration (VLSI) performance as well as output drivers. In a gate array or standard cell based VLSI, the device size is uniform for ease of physical design. As a result, the delay degradation per unit load for CMOS is generally different for different circuit functions due to the difference in output impedance of various circuit configurations. For the invention described herein, the unit load degradation is practically the same for all circuit functions because the bipolar push-pull transistors isolate the CMOS circuits from the loading. This leads to ease of applications in semi-custom environments. The bipolar transistors also make ECL input/output (i.e., high performance RAM's) much easier to achieve. The invention can also be used in word line drivers, bit line drivers and sense amplifiers to improve performance.

Another application is a gate array with mixed TTL and ECL input/output on the same chip. This feature is desirable in high performance disk drives, test systems and high speed graphics applications.

By now it should be appreciated that there has been provided a BIMOS circuit having improved output voltage switching speed, low power requirements, high input impedance, high noise immunity, and high current output capability.