Two-transistor SRAM semiconductor structure and methods of fabrication
A two-transistor memory cell based upon a thyristor 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.
1. The process for forming a pair of cross-coupled PNP and NPN bipolar transistors in a selected region of a first conductivity type semiconductor substrate of an integrated circuit, the integrated circuit also including field effect transistors outside the selected region, the process comprising:
introducing opposite conductivity type dopant into the selected region of the substrate to form a buried layer;
introducing first conductivity type dopant into the selected region of the substrate above the buried layer;
introducing additional opposite conductivity type dopant into a first portion of the selected region to form a shallow well vertically separated from the buried layer by the first conductivity type dopant;
in a same process as used elsewhere on the integrated circuit to form gates of field effect transistors outside the selected region, forming a first field effect transistor gate over only a portion of the shallow well, and a second field effect transistor gate entirely over the shallow well;
using the first field effect transistor gate and the second field effect transistor gate to mask underlying portions of the selected region, performing steps of:
introducing further first conductivity type dopant into the selected region on opposite sides of the first gate to thereby form a first doped region on one side of the first gate, and a second doped region on an opposing side of the first gate, the opposing side of the first gate being between the first gate and the second gate;
introducing more opposite conductivity type dopant into the selected region on only one side of the second gate, the only one side being on an opposite side of the second gate from the opposing side, to thereby form a third doped region; and
providing electrical connections to the first doped region, the second doped region, and the third doped region.
2. The process as in claim 1 further comprising a step of providing an electrical connection to the buried layer by introducing opposite conductivity type dopant into the substrate outside the selected region.
3. The process as in claim 2 further including the steps of:
providing a bit line connection to the buried layer; and
providing a word line connection to the third doped region.
4. The process as in claim 1 further including a step of forming regions of electrical insulating material to surround the selected region of the substrate.
5. The process as in claim 4 wherein the step of forming the regions of electrical insulating material is performed before the step of introducing the opposite conductivity type dopant into the substrate to form the buried layer.
6. The process as in claim 1 further comprising:
during the step of in the same process as used elsewhere on the integrated circuit to form the gates of the field effect transistors that are outside the selected region of the semiconductor substrate, a third field effect transistor gate is formed on the semiconductor substrate outside the selected region;
during the step of introducing more opposite conductivity type dopant, the more opposite conductivity type dopant is also introduced on opposite sides of the third field effect transistor gate to thereby provide a field effect access transistor for the pair of cross-coupled PNP and NPN bipolar transistors in the selected region.
7. The process as in claim 6 further including a step of providing electrical connections to the third field effect transistor gate, but not to either of the first field effect transistor gate or the second field effect transistor gate.
8. The process as in claim 4 wherein the buried layer extends under the electrically insulating material to outside the selected region.
9. The process as in claim 1 wherein the pair of cross-coupled PNP and NPN bipolar transistors provide a thyristor.
10. The process as in claim 1 further comprising forming a bipolar transistor outside the selected region to thereby provide a bipolar access transistor for the pair of cross-coupled PNP and NPN bipolar transistors.
11. The process as in claim 10 wherein the bipolar transistor outside the selected region is formed during the process steps for forming the pair of cross-coupled PNP and NPN bipolar transistors in the selected region.