Switching circuit utilizing a high voltage transistor protection technique for integrated circuit devices incorporating dual supply voltage sources

[0002] The present invention is related to the subject matter disclosed in U.S. patent application Ser. No. 09/803,315 for: “Negative Voltage Driver Circuit Technique Having Reduced Current Flow to the Negative Supply Voltage Source” filed on even date herewith and assigned to the assignee of the present invention, the disclosure of which is herein specifically incorporated by this reference.

[0003] The present invention relates, in general, to the field of integrated circuit (“IC”) devices. More particularly, the present invention relates to a switching circuit utilizing a high voltage transistor protection technique for integrated circuit devices having dual voltage supplies.

[0004] In certain integrated circuit devices (e.g. some memory ICs) a high voltage supply level (“VCCP”) may be required for proper device operation. Should this voltage be equal to or less than twice the supply voltage (“VCC”, i.e. VCCP≦2*VCC ), then a switching circuit comprising a relatively thick gate oxide P-channel transistor in series with a pair of series connected N-channel transistors may be coupled between VCCP and a reference voltage level (VSS or circuit ground) with the gate of the P-channel device coupled to a switched source of VCCP, the gate of the intermediate N-channel device coupled to VCC and the gate of the remaining N-channel device coupled to a switched source of VCC.

[0005] However, in those applications wherein the high voltage supply level is more than twice the device supply voltage (i.e. VCCP>2*VCC) and relatively thick gate oxide thickness transistors are used, the associated thin gate oxide transistors must still be protected from possible high gate oxide voltage stress.

[0006] The switching circuit incorporating a high voltage transistor protection technique of the present invention extends the maximum pumped voltage (“VCCP”) for reliable MOS transistor operation to VCCP=VTN+(2*VCC). This is effectuated by adding an additional relatively thick gate oxide transistor in series with the thin gate oxide MOS N-channel transistors in a conventional high voltage switching circuit to increase the reliable maximum voltage for the high voltage power supply.

[0007] Particularly disclosed herein is an integrated circuit device including a switching circuit comprising first and second series coupled MOS transistors having a gate oxide of a first thickness, each of the first and second MOS transistors having a gate terminal thereof. Third and fourth MOS series coupled transistors are also provided having a gate oxide of a second thickness lesser than the first thickness, each of the third and fourth MOS transistors having a gate terminal thereof. The first, second, third and fourth MOS transistors are coupled in series between a high voltage source and a reference voltage source with an output node defined intermediate the first and second MOS transistors. A first signal input is coupled to the gate terminal of the first transistor for receiving a first input signal capable of transitioning between a level of the high voltage source and a level of the reference voltage source with the gate terminal of the second transistor being coupled to the high voltage source. A supply voltage source having a voltage level lesser than that of the high voltage source is coupled to the gate terminal of the third transistor. A second signal input is coupled to the gate terminal of the fourth transistor for receiving a second input signal capable of transitioning between a level of the supply voltage source and a level of the reference voltage source.

[0008] The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein:

[0009] FIG. 1 illustrates a conventional switching circuit configuration appropriate for those circumstances wherein VCCP≦2*VCC;

[0010] FIG. 2 illustrates a corresponding switching circuit configuration appropriate for those circumstances wherein VCCP≧2*VCC but in which the intermediate N-channel devices presents an undesirably high impedance; and

[0011] FIG. 3 illustrates a switching circuit in accordance with the present invention appropriate for those circumstances wherein VCCP≧VTN+(2*VCC);

[0012] FIG. 4A illustrates an alternative embodiment of a switching circuit in accordance with the present invention having N-channel transistors provided and functioning in a manner corresponding to those in the preceding FIG. 3 configured in a NOR logic configuration; and

[0013] FIG. 4B illustrates another alternative embodiment of a switching circuit in accordance with the present invention having N-channel transistors provided and functioning in a manner corresponding to those in the preceding FIG. 3 configured in a NAND logic configuration.

[0014] With reference now to FIG. 1, a conventional switching circuit 10 is shown appropriate for applications wherein VCCP≦2*VCC. VCC may range between approximately 1.4 volts to 1.6 volts with a nominal value of 1.5 volts while VCCP may range substantially between 2.8 volts to 3.2 volts with a nominal value of 3.0 volts. The switching circuit 10 comprises a P-channel transistor 12 having a relatively thick gate oxide coupling a pumped voltage source VCCP to an output node. The transistor 12 is coupled in series with a pair of series connected, relatively thin gate oxide N-channel transistors 14 and 16 coupling the output node to circuit ground (or “VSS”). The relatively thin gate oxide may be substantially 32-37 Angstroms (“Å”) in thickness with a nominal value of 35 Å while the relatively thick gate oxide may be in the range of 75-88 Å with a nominal value of 82 Å.

[0015] The gate of the P-channel transistor 12 is coupled to a switched VCCP source while the gate of the lower N-channel transistor 16 is coupled to a switched VCC source. The gate of the intermediate N-channel transistor 14 is coupled to the supply voltage source VCC.

[0016] As previously mentioned, in those instances wherein VCCP≦2*VCC, the switching circuit 10 may be successfully implemented. However, care must be taken such that the voltage across the gate oxide of the thin gate oxide transistors 14 and 16 does not exceed VCC while the gates of the thin oxide devices may also not transition with a drain-to-source voltage of greater than VCC. The switching circuit 10 effectuates this if VCCP≦2*VCC. Transistor 14 is “on” at all times due to the constant VCC level applied to its gate with a small voltage drain-to-source (“VDS”) and a voltage drain-to-gate (“VDG”) or voltage source-to-gate (“VSG”)≦VCC. Transistor 16 drain voltage (“VD”) equal to VCC−VTN (threshold voltage) and its maximum VDG (or VSG)<VCC.

[0017] For those cases wherein VCCP≧2*VCC, another solution must be found as if the switching circuit 10 were used, then VDG for transistor 14 would be greater than VCC when the output node is at VCCP.

[0018] With reference additionally now to FIG. 2, a switching circuit 20 is shown appropriate for applications wherein VCCP≧2*VCC. The switching circuit 20 comprises a similar P-channel transistor 12 having a relatively thick gate oxide coupling a pumped voltage source VCCP to an output node. The transistor 12 is coupled in series with a pair of series connected N-channel transistors 24 and 16 coupling the output node to circuit ground (or “VSS”). In this instance, while the transistor 16 still has a relatively thin gate oxide, the transistor 24 has replaced the transistor 14 of the preceding figure with one including a relatively thick gate oxide.

[0019] As previously, the gate of the P-channel transistor 12 is coupled to a switched VCCP source while the gate of the lower N-channel transistor 16 is coupled to a switched VCC source. The gate of the intermediate N-channel transistor 24 is coupled to the supply voltage source VCC.

[0020] The transistor 24 in the switching circuit 20 has a relatively thick gate oxide in this instance and replaces the relatively thin gate oxide transistor 14 of FIG. 1. For transistor 24, the maximum VDS, VDG or VSG can be at a level of VCCP and still be reliable. Nevertheless, the transistor 24 will have a very high impedance because the gate voltage applied to it is only VCC. The gate oxide thickness of thick oxide transistors is generally more than twice the thickness of corresponding thin oxide transistors. Also, the channel lengths of thick oxide transistors are also generally greater than twice the channel length of thin oxide devices. These two factors will cause the transistor 24 to have a much higher impedance than that of the corresponding transistor 14 of FIG. 1 (on the order of four times greater). This will cause the output node to pull down at an unacceptably slow rate and while transistor 24 can also be made very large (i.e. a greater channel width), this would be inherently self-limiting since increasing the size of transistor 24 will also add additional capacitance to the pull-down path.

[0021] With reference additionally now to FIG. 3, a switching circuit 30 in accordance with the present invention is shown appropriate for those circumstances wherein VCCP≧VTN+(2*VCC). The switching circuit 30 comprises a P-channel transistor 32 having a relatively thick gate oxide coupling a pumped voltage source VCCP to an output node. The transistor 32 is coupled in series with a string of series connected N-channel transistors 34, 36 and 38 coupling the output node to circuit ground (or “VSS”). In this instance, the transistors 36 and 38 have a relatively thin gate oxide, while the transistors 32 and 34 have a relatively thick gate oxide.

[0022] As in the previous circuits, the gate of the P-channel transistor 12 is coupled to a switched VCCP source while the gate of the lowest N-channel transistor 38 is coupled to a switched VCC source. The gate of the upper intermediate N-channel transistor 34 is coupled to the pumped supply voltage source VCCP while the gate of the lower intermediate N-channel transistor 36 is coupled to the supply voltage source VCC.

[0023] Unlike the switching circuits 10 and 20 shown in the preceding figures, the switching circuit 30 includes an added transistor 34 to protect transistor 36. This circuit will function reliably if VCCP−VTN ≦2*VCC, where VTN is the threshold voltage of a relatively thick oxide transistor, typically on the order of about 0.9 volts. The VDS of transistor 34 is small and the VDG of transistor 36 is less than or equal to VCC for the condition described above. While the pull-down rate on the output node is somewhat impacted due to the addition of the series connected transistor 34, but not nearly as much as that which would be experienced with the switching circuit 20 of FIG. 2. The switching circuit 30 extends the safe maximum VCCP voltage from 2*VCC to (2*VCC)+VTN.

[0024] With reference additionally now to FIG. 4A, an alternative embodiment of a switching circuit 40 in accordance with the present invention is shown having N-channel transistors 34, 36 and 38 provided and functioning in a manner corresponding to those depicted and described with respect to the preceding FIG. 3 (wherein P-channel transistor 32 functions in a normal manner) configured in a NOR logic configuration in which one or more N-channel transistors 38A, 38B through 38N may be utilized. Correspondingly, FIG. 4B illustrates yet another alternative embodiment of a switching circuit 50 in accordance with the present invention wherein any number of N-channel transistors 38A, 38B through 38N may be coupled in series to configure the switching circuit 50 into a NAND logic configuration. Any combination of serial and parallel coupled transistors 38A, 38B through 38N may be used to configure appropriate logic switching circuits for a specified application.

[0025] While there have been described above the principles of the present invention in conjunction with a specific switching circuit implementation, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.