Cross-coupled thyristor SRAM semiconductor structures and methods of fabrication
A memory cell based upon thyristors for an SRAM integrated circuit is described together with a process for fabricating it. The memory cell can be implemented in different combinations of MOS and bipolar select transistors, or without select transistors, with thyristors in a semiconductor substrate with shallow trench isolation. Standard CMOS process technology can be used to manufacture the SRAM. Special circuitry provides lowered power consumption during standby.
1. A first semiconductor structure for a cross-coupled pair of first type and opposite type bipolar transistors comprising:
a semiconductor substrate of first conductivity type having an upper surface;
an insulating region extending into the substrate to surround a first portion of the upper surface of the substrate;
a buried layer of the opposite conductivity type to the first conductivity type disposed in the substrate beneath the first portion of the upper surface, the buried layer and the insulating region together providing a tub of the first conductivity type electrically isolated from remaining portions of the substrate;
a connecting region of the opposite conductivity type extending from the upper surface of the substrate to the buried layer to provide an electrical connection to the buried layer;
a shallow well region of the opposite conductivity type extending from the upper surface of the substrate over less than all of the first portion of the upper surface of the substrate, the shallow well region not extending to the buried layer;
a first connection of the first conductivity type adjacent the upper surface and extending into the tub to provide an electrical connection to the tub;
a field effect transistor gate electrode disposed over the shallow well region;
a second connection of the opposite conductivity type extending into the shallow well region to provide an electrical connection to the shallow well region disposed adjacent a first side of the field effect transistor gate electrode;
an emitter region of the first conductivity type disposed adjacent a second side of the field effect transistor gate electrode; and wherein:
the buried layer provides an emitter of the first type bipolar transistor;
the tub provides a base of the first type bipolar transistor;
the shallow well region provides a collector of the first type bipolar transistor and a base for the opposite type bipolar transistor; and
the emitter region provides an emitter for the opposite type bipolar transistor.
2. A semiconductor structure as in claim 1 providing a second cross-coupled pair of first type and opposite type bipolar transistors further including a second semiconductor structure comprising:
a second insulating region extending into the substrate to surround a second portion of the upper surface of the substrate;
a second buried layer of the opposite conductivity type disposed in the substrate beneath the second portion of the upper surface, the second buried layer and the second insulating region together providing a second tub of the first conductivity type electrically isolated from remaining portions of the substrate;
a second shallow well region of the opposite conductivity type extending from the upper surface of the substrate over less than all of the second portion of the upper surface of the substrate, the second shallow well region not extending to the second buried layer;
a third connection of the first conductivity type adjacent the upper surface and extending into the second tub;
a second field effect transistor gate electrode disposed over the second shallow well region;
a fourth connection of the opposite conductivity type extending into the second shallow well region, and disposed adjacent a first side of the second field effect transistor gate electrode;
a second emitter region of the first conductivity type disposed adjacent a second side of the second field effect transistor gate electrode; and wherein:
the second buried layer provides a second emitter of the first type bipolar transistor of the second pair of first type and opposite type bipolar transistors;
the second tub provides a base of the first type bipolar transistor of the second pair of first type and opposite type bipolar transistors;
the shallow well region provides a collector of the first type bipolar transistor of the second pair of first type and opposite type bipolar transistors and a base for the opposite type bipolar transistor of the second pair of first type and opposite type bipolar transistors;
the emitter region provides an emitter for the opposite type bipolar transistor of the second pair of first type and opposite type bipolar transistors
a first electrical connection between the first connection and the fourth connection; and
a second electrical connection between the second connection and the third connection.
3. A semiconductor structure as in claim 2 wherein:
the arrangement of the second semiconductor structure is rotated 180 degrees from the arrangement of the first semiconductor structure; and
the first and second electrical connections are straight lines.
4. A semiconductor structure as in claim 3 wherein a first voltage supply is coupled to each of the buried layer and the second buried layer, and a second voltage supply is coupled to the second connection and the fourth connection.
5. A semiconductor structure as in claim 4 wherein the first semiconductor structure and the second semiconductor structure together provide an SRAM cell.
6. A semiconductor structure as in claim 1 further comprising a field effect transistor outside the tub.
7. A semiconductor structure as in claim 6 wherein the field effect transistor includes opposite conductivity type source and drain regions.
8. A semiconductor structure as in claim 7 wherein one of the opposite conductivity type source and drain regions is electrically connected to the shallow well region.
9. A semiconductor structure as in claim 8 wherein:
a bit line is electrically connected to the other of the opposite conductivity type source and drain regions; and
a word line is electrically connected to a gate of the field effect transistor.
10. A semiconductor structure as in claim 1 further comprising an additional bipolar transistor outside the tub.
11. A semiconductor structure as in claim 10 wherein the additional bipolar transistor includes an emitter connected to the shallow well region.
12. A semiconductor structure as in claim 11 wherein:
a bit line is electrically connected to a collector of the additional bipolar transistor; and
a word line is electrically connected to a base of the additional bipolar transistor.
13. A semiconductor structure as in claim 1 wherein the first conductivity type is P and the opposite conductivity type is N.
14. A semiconductor structure as in claim 10 wherein the additional bipolar transistor is a vertical NPN bipolar transistor.