# Radiation-Hardened 16T SRAM Cell with Improved Read and Write Stability for Space Applications
## 2 Proposed RHHS16T SRAM Cell
### SEU Recovery Analysis  
This subsection describes the characteristics of the proposed cell. A node is considered sensitive when it experiences an SEU, which occurs near the drain junction of an off-state transistor [25]. Generally, PMOS and NMOS transistors are known to induce positive and negative transient pulses, respectively [12]. Therefore, only the cases of '1'\(\rightarrow\)'0' for PMOS and '0'\(\rightarrow\)'1' for NMOS need to be considered. Assuming the initial state '1' for the proposed RHHS16T cell, the internal node S0 has an initial value of '0' and is surrounded only by NMOS transistors, so the only possible transient state in the event of an SEU is '0'\(\rightarrow\)'0'. Therefore, in the assumed case, S0 can be considered a non-sensitive node, while Q, QB, and S1 are sensitive nodes. (1) SEU at node Q: When Q is flipped by an SEU, stored data at node Q transition from '1' to '0', causing N7 to turn off and P3 to turn on. However, nodes QB and S0 maintain their initial state of '0' through the pull-down paths 'QB-N5-GND' and 'S0-N4-GND', respectively. Additionally, since both transistors N7 and N8 are in the off state, node S1 transitions to a high-impedance state with no pull-up or pull-down path connected. Figure 1: Schematic of the proposed RHHS16T cell. Figure 2: The cell layout of the proposed RHHS16T cell. Generally, the state of a node in a high-impedance state remains unaffected [4]. Therefore, node S1 maintains its initial state of '1'. Since nodes QB, S0, and S1, which are not directly affected by SEUs, maintain their initial state, node Q, which is affected by the SEU, can recover to its initial state '1' through the pull-up path 'Q-P5-P4-VDD'. (2) SEU at node QB: When QB is flipped by an SEU, stored data at node QB transition from '0' to '1', causing P5 to turn off and N3 to turn on. Since both transistors P5 and N6 are in the off state, stored data at node Q transition to a high-impedance state, maintaining its initial state of '1'. As Q maintains its value, N7 remains in the on state, and S1 maintains its initial state of '1' through the pull-up path 'S1-N7-P6-VDD'. This keeps transistor P1 in the off state, blocking the pull-up path 'S0-N4-GND'. As Q, S0, and S1 maintain their initial states, transistors P2 and P3 remain in the on state. Therefore, node QB, which is affected by the SEU, can recover to its initial state of '0' through the pull-down path 'QB-N5-GND'. (3) SEU at node S1: When S1 is flipped by an SEU, stored data at node S1 transition from '1' to '0', causing transistors P1 and P2 to turn on and N4 and N5 to turn off. However, the storage nodes Q and QB maintain their initial states through the paths 'Q-P5-P4-VDD' and 'QB-N5-GND', respectively. As QB maintains '0', transistor N3 remains in the off state, causing S0 to transition to a high-impedance state, maintaining its initial state of '0'. Therefore, S1 can recover to its initial state of '1' through the pull-up path 'S1-N7-P6-VDD'. (4) SEMNU at node pair Q-QB: When the storage node pair Q-QB is affected by an SEMNU, stored data at node Q transition from '1' to '0', and stored data at node QB transition from '0' to '1'. Consequently, transistors P5 and N7 turn off, and transistors P3 and N3 turn on. However, the internal node S1 transitions to a high-impedance state, maintaining its initial state of '1'. As S1 maintains its initial state, transistors P1 and N4 remain in the off and on states, respectively, and S0 maintains its initial state of '0' through the pull-down path 'S0-N4-GND'; as the voltage of QB drops sufficiently, transistor P5 turns on, allowing Q to recover to '1' through the pull-up path 'Q-P5-P4-VDD'. Therefore, the proposed cell can fully recover to its initial state even if soft errors affect nodes Q, QB, S1, or the storage node pair Q-QB, as shown in the simulation waveform in Figure 3. If sufficient charge accumulates at the node pairs Q-S1 or QB-S1, it can flip the state of the cell.