Integrated Circuit CMOS Inverter Structure

The present invention relates to an inverter structure and circuit in integrated circuit form in which two complementary MOS transistors have their outputs coupled back to the input by means of a feedback circuit to positively fix the d-c working point of the inverter circuit.

It has previously been proposed to provide a feedback circuit in CMOS inverters having a high resistance ohmic resistor. The resistance value is in the order of magnitude of 10 meg-ohms or more, preferably higher. This structure has the disadvantage that the ohmic resistor must be constructed as a separate discrete element which is externally connected to the CMOS inverter.

It is an object of the present invention to provide a fully integrated inverter circuit in which all circuit components can be integrated on a single chip.

Briefly, the feedback circuit for complementary transistors is formed of two oppositely poled serially constructed semiconductor diodes. The two serially connected semiconductor diodes are integrated with the two complementary MOS transistors on a common semiconductor chip.

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic circuit diagram of the CMOS inverter;

FIG. 2 is a detailed electrical diagram of the CMOS inverter;

FIG. 3 is a top view of the integrated circuit structure of the inverter of FIGS. 1 and 2; and

FIG. 4 is a cross section along lines IV--IV of FIG. 3.

The inverter 1 (FIGS. 1, 2) has two complementary MOS field effect, transistors 2, 3 (FIG. 2). The transistor 2 is a p-channel transistor; the transistor 3 is an n-channel transistor. The control electrodes of the two transistors 2, 3 are connected together and form the input 4 of the inverter. The drain electrodes of the two transistors are also connected together and form the output connected to terminal 5 of the inverter. The source electrode 8 of the p-channel transistor 2 is connected to the positive terminal of a supply voltage U.sub.dd ; the source electrode 9 of the n-channel transistors is connected to the negative supply voltage -U.sub.ss. A feedback circuit is connected from the output terminal 5 of the inverter 1 to its input terminal 4.

In accordance with the present invention, the feedback circuit from the output terminal 5 to the input terminal 4 has two oppositely poled serially connected semiconductor diodes 6, 7. The entire circuit structure, including the diodes 6, 7, can be integrated on a single semiconductor chip substrate.

One possible form of a layout for the inverter is shown in FIGS. 3 and 4. The entire circuit arrangement, schematically illustrated in FIG. 1 and in greater detail illustrated in FIG. 2, is placed on a single semiconductor chip, for example of silicon. This chip may contain additional circuit elements, not shown. A monolithic integrated circuit structure is thus provided. The semiconductor substrate 11 preferably is n-conductive silicon.

A narrow strip 12 (FIG. 3), at which the substrate chip material 11 appears at the surface thereof, divides the chip into a left portion L and a right portion R. The right portion R of the semiconductor structure is embedded into a guard band 13 which is p.sup.+-conductive. The left portion L is embedded in an n.sup.+-conductive guard band, which includes a second p.sup.+-conductive guard band 22 as an island structure formed in a window thereof.

The right portion R of the semiconductor structure has the n-channel transistor 3 located within the p.sup.+-conductive guard band 13. A first p-well diffusion zone 16 is diffused in the p.sup.+-conductive guard band 13 and in the n-conductive semiconductor substrate 11 therebeneath. Diffusion is from the top surface of the semiconductor chip. The drain zone 17 of the n-channel transistor is diffused in the first p-well diffusion zone 16 spaced from the p.sup.+-conductive guard band 13. The N.sup.+-conductive source zone 18 of the n-channel transistor 3 is diffused into the p-well diffusion zone 16 partly without being spaced from the p.sup.+-conductive guard band 13.

The left portion L of the semiconductor structure includes the p-channel transistor 2 within the n.sup.+-conductive guard bands 14. The p.sup.+-conductive drain zone 19 and the p.sup.+-conductive source zone 20 of the p-channel transistor 2 are diffused from the top side of the semiconductor chip within the guard bands 14 into the substrate 11 which, in this region, is surrounded in ring-shaped manner by the guard band 14. The p.sup.+-conductive source zone 20 of the p-channel transistor 2 is located, in part, without spacing with respect to the n.sup.+-conductive guard band 14.

Semiconductor diodes 6, 7 are also located at the left portion L of the semiconductor structure. A special region or zone is provided for the semiconductor diodes 6, 7, forming an island within the n.sup.+-conductive guard bands 14. The outer edge of this island is formed by a ring-shaped narrow strip 21 along which the n-conductive semiconductor substrate material of the substrate chip 11 is exposed at the surface thereof. A second p.sup.+-conductive guard band 22 is diffused in the semiconductor substrate material within the ring-shaped narrow strip 21. A second p-well diffusion zone 23 is diffused within guard band 22 into the semiconductor substrate material which forms the common anode zone of the two semiconductor diodes 6, 7, poled oppositely with respect to each other, and serially connected. The second p-well diffusion zone 23 has first and second n.sup.+-conductive diffusion zones 24, 25 diffused therein. The first diffusion zone 24 forms the cathode for the semiconductor diode 6 and the second diffusion zone 25 forms the cathode for the semiconductor diode 7.

A silicon dioxide layer 26 (FIG. 4) covers the semiconductor chip at its surface. This layer 26 has been omitted from FIG. 3 for clarity of illustration thereof. The silicon dioxide layer 26 is etched away at those portions of the semiconductor chip which form contact windows for connection to the circuit elements contained on the semiconductor chip.

The n-channel transistor 3 is formed with a first contact window 27 at its N.sup.+-conductive drain zone 17 and with a second contact window 28 at its n.sup.+-conductive source zone 18.

The p-channel transistor 2 is formed with a first contact window 29 at its p.sup.+-conductive drain zone; it is fromed with a second contact window 30 at its p.sup.+-conductive source zone 20.

Semiconductor diode 6 has a contact window 31 at its cathode zone 24. Semiconductor diode 7 has a contact window 32 at its cathode zone 25.

A contact window 33 is formed in the vicinity of contact window 28 on the first p.sup.+-conductive guard band 13. The n.sup.+-conductive guard band 14 has a contact window 34 in the vicinity of contact window 30.

A metallizing layer is applied on the silicon dioxide layer 26. This metal layer contacts the semiconductor surface at those zones or regions in which the layer 26 is etched off. The metallizing coating forms a first conductive strip 35 which connects contact windows 27, 29, 32. The right end of the contact strip 35 forms the output terminal 5 of the inverter. The metallizing coating also forms a second conductive strip 36 which connects contact window 31 with the cathode zone 34 of the semiconductor diode 6. It does not contact the semiconductor surface at any other position, however. The second contact strip 36 is carried over the zones of the channels of the two MOS transistors 2 and 3 and forms the control electrodes for these two transistors. The second conductive path 36 has a branch 36a which forms the input terminal 4 of the inverter. The conductive layer also is formed into third and fourth conductive bridges 27, 38. The bridge 37 forms the source electrode 9 of the n-channel transistor 3 and connects the two contact windows 28, 33. The fourth conductive bridge 38 forms the source electrode 8 for the p-channel transistor 2 and connects the two contact windows 30, 34. The third conductive strip 37 is connected to the negative supply voltage -U.sub.ss and the fourth conductive strip 38 is connected to the positive supply voltage +U.sub.dd.

Various structural and layout changes may be made within the scope of the inventive concept.