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Fast Memory Cells With Beta Roll-Off Transistors

IP.com Disclosure Number: IPCOM000036708D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Wong, RC: AUTHOR

Abstract

To improve stability against soft errors, bipolar memory cells are designed with saturated transistors. This technique degrades write performance. To restore some of the write performance, some double- diffused bases have been used to lower the beta of the flip-flop transistors. However, there are disadvantages with this beta lowering technique: 1. The degree of beta lowering is limited by the acceptable junction breakdown voltages. Breakdown voltages are becoming lower with the more advanced bipolar technologies and are close to the unacceptable limits. 2 Beta lowering degrades read performance and the stability against soft errors of the memory cells in the inactive states, where the cell currents are very low.

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Fast Memory Cells With Beta Roll-Off Transistors

To improve stability against soft errors, bipolar memory cells are designed with saturated transistors. This technique degrades write performance. To restore some of the write performance, some double- diffused bases have been used to lower the beta of the flip-flop transistors. However, there are disadvantages with this beta lowering technique:
1. The degree of beta lowering is limited by the

acceptable junction breakdown voltages. Breakdown

voltages are becoming lower with the more advanced

bipolar technologies and are close to the unacceptable

limits. 2 Beta lowering degrades read performance and the stability against soft errors of the memory cells in

the inactive states, where the cell currents are very

low.

It is proposed that these disadvantages can be avoided by replacing the beta lowering technique with a beta roll-off technique. The flip-flop transistors are designed with a beta that rolls off abruptly at the current level of the write access. Thus, the inactive cells with high beta remain stable against soft errors, but selected cells can be written quickly because of the beta roll-off. To guarantee that there are no read disturbs, the read current may need to be smaller than the write current. These are illustrated in the drawing.

In the advanced bipolar technologies all dimensions are scaled down and epi dopants are reduced. Thus, the conventional beta lowering technique with double base implants becom...