Circuit Configuration for Protecting Terminals of Integrated Circuits

The invention relates to a circuit configuration for protecting terminals of integrated circuits, in particular CMOS circuits, in which bypass transistors are connected between the circuit terminals and supply potential terminals.

In the context of the invention, the term "terminal" includes not only inputs, outputs, and combined inputs/outputs, but also supply potential connections.

Circuit configurations of this generic type are known and are described in more detail below in the description of the drawings.

Experiments on which the invention is based have shown that using MOS transistors of unlike channel types results in dissimilar response behavior in the transistors and therefore in poor protection against electrostatic voltages, or ESD protection.

It is accordingly an object of the invention to provide a circuit configuration for protecting terminals of integrated circuits, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which has improved ESD protection behavior for integrated circuits, in particular CMOS circuits.

With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for protecting terminals of integrated circuits, in particular CMOS integrated circuits, comprising bypass transistors connected between the circuit terminals and supply potential terminals, the bypass transistors being thin-oxide transistors of the n-channel type and having zero gate-to-source voltage.

In accordance with a concomitant feature of the invention, the circuit configuration includes an output driver operatively associated with the integrated circuit, the output driver including MOS transistors of the same channel type, and an inverter connected upstream of one of the transistors of the output driver.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a circuit configuration for protecting terminals of integrated circuits, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

FIG. 1-3 are schematic and block circuit diagrams of three known structures of a protective circuit configuration; and

FIG. 4 is a circuit diagram of a particular embodiment of a protective circuit configuration according to the invention.

Referring now to the figures of the drawing in detail and first, particularly, to FIGS. 1-3 thereof, there is seen the fundamental structure of known circuit configurations of this generic type. Each of FIGS. 1-3 schematically shows an integrated circuit 10 (a CMOS circuit), with supply potential terminals that are conventionally identified as V.sub.ss and V.sub.dd. schematically illustrated output of the integrated circuit 10 is shown at reference numeral 11.

In order to protect against electrostatic voltages (that is electrostatic discharge) at the output 11, FIGS. 1-3 each have shunt paths, which are provided in FIG. 1 by diodes 12-14, in FIG. 2 by MOS transistors 20 and 21, and in FIG. 3 by a combination of thick oxide transistors 30, 31 and one diode 32. A structure of the kind shown in FIG. 1 is known, for instance, from the publication entitled "Electrostatic Discharge Control", by Tarak N. Bhar and Edward J. McMahon, (published by Hayden Book Company, Inc., New Jersey, 1983), pp. 82 and 83.

Previously, the MOS bypass transistors 20 and 21 for CMOS circuits of FIG. 2 were provided as transistors of unlike channel types as is known, for instance, from the publication entitled "Electrical Overstress Protection for Electronic Devices" by Robert Antinone (New Jersey, 1986), part II, section V, part A, and particularly Fig. V.A.7, on page 148 thereof.

As mentioned above, it has been empirically determined that the use of MOS transistors of unlike channel types results in dissimilar response behavior in the transistors and accordingly in poor protection against electrostatic voltages, or ESD protection.

An integrated circuit 10 is provided in FIG. 4 which is supplied by way of supply potential terminals VDD and VSS. The integrated circuit 10 includes an output 11 and an input 12. Both the output 11 and the input 12 are each assigned a protective circuit according to the invention. Transistors 21 and 20, respectively, are connected between the output 11 and the supply potential terminals VDD and VSS. Similarly, transistors 24 and 25 are connected between the input 12 and the supply potential terminals. A resistor R is connected in series between the input 12 and the integrated circuit 10. At the node between the resistor R and the integrated circuit 10 protective transistors 22 and 23 connect to the respective supply potential terminals. On the output side, the integrated circuit 10 is connected with the output 11 through an output driver with transistors 40 and 41.

The transistors 20 to 25 of the protective circuit are formed as thin-oxide transistors of the n-channel type. The gate and the source of the transistors 20, 22 and 24 are connected to the supply potential VSS. Similarly, the gate and the source of the respective transistors 21, 23 and 25 are directly interconnected. This means that the gate-to-source voltage of the protective transistors is always zero. Accordingly, the MOS-line of the protective transistors is never utilized. The protective function of the protective transistors 20 to 25 is based on its function as a bipolar transistor in the case of heavy-current effects. The gate-length of the transistors, which corresponds to the parasitic base-width of the parasitic bipolar transistor, plays a role in this. The breakdown or starting potential of the transistors depends on the gate-length; the breakdown potential is adjusted such that optimum protection is provided for the terminals of the integrated circuit.

A further feature which is also shown in the circuit configuration of FIG. 4, is that an output driver which is typically present in the integrated circuit 10 and has transistors 40, 41, is shown separately. Through the use of this output driver, the ESD protection can be further improved if the transistors 40, 41 are of the same type. Once again, the transistors 40, 41 are either n-channel MOS transistors or npn bipolar transistors. In order to achieve a p-channel transistor in a conventional output driver, a transistor, such as the transistor 40, may be preceded by an inverter 42.

This last-described further feature can be achieved either alone or together with the transistors 20, 21.