System and method to compensate for TID effect in fully-depleted SOI SRAMs
Publication Date: 2014-Jun-27
The IP.com Prior Art Database
Disclosed is a cell flipping method that enables, on average, all transistors in the Static Random Access Memory (SRAM) array to experience a similar total ionizing dose (TID) effect. Then, a back bias can be used to compensate for the Vt shift.
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System and method to compensate for TID effect in fully -
-depleted SOI SRAMs
depleted SOI SRAMs
Under total ionizing dose (TID) radiation, charges are created in the buried insulator (BOX) and lead to a Vt shift in fully depleted Silicon on Insulator (FDSOI) transistors. While it is possible to compensate the Vt shift with a back bias , since the Vt shift depends on the transistor bias, it is not possible to apply a global back bias suitable for a large Static Random Access Memory (SRAM) array.
The novel contribution is a cell flipping method that enables , on average, all transistors in the SRAM array to experience a similar TID effect. Then, a back bias can be used to compensate for the Vt shift.
Figure 1 shows how the TID effect depends on the bias condition of the transistor . As an example, For a negative Field Effect Transistor (NFET) FDSOI transistor biased with drain at a high voltage and gate grounded ("off" state), the Vt is reduced under TID;
while for a transistor biased with gate high and drain grounded ("on" state), the Vt change is negligible.
Figure 1: TID is bias-dependent. Vt shift is seen only in a transistor with Vg=0 and
In an SRAM cell, different transistors experience different bias conditions depending on the value stored in the cell. For example, with a logic value "1" stored on the cell shown in Figure 2, the left pull-down transistor M1 is biased in the "on" state, while the right pull-down transistor M2 is biased in the "off" state. Because of the TID effect, the Vt of the left partial discharge (PD) is decreased over time, while the Vt of the right PD is almost constant. This asymmetric Vt shift results in a Vt mismatch over time and reduces the cell Static Noise Margin (SNM).
Figure 2: Vt of M1 decreases upon radiation, but Vt of M2 does not change significantly
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With a cell flipping technique, the content of all cells is periodically reversed, and it is possible to reduce the asymmetry in Vt shift by biasing on both sides of the cell under similar conditions on average. However, cell flipping without considering the radiation dose is not optimum. If the content of the cells is flipped more often than needed , power is wasted; if it is flipped less often than needed , the Vt...