# Study on Single-Event Transient Hardness of Semi-Enclosed Gate NMOS
## 3 Analysis of Simulation Results
### Analysis of Heavy Particle Irradiation with Different LETs and Internal Mechanisms  
From Figure 6, it can be observed that when heavy particles are incident at an angle of 150\({}^{\circ}\), with the direction of incidence parallel to the channel, the transient current peaks reach their maximum at around 100 ps for LET values ranging from 20 to 70. The peak transient currents for LET values of 20 MeV\(\cdot\)cm\({}^{2}\)/mg, 30 MeV\(\cdot\)cm\({}^{2}\)/mg, 40 MeV\(\cdot\)cm\({}^{2}\)/mg, 50 MeV\(\cdot\)cm\({}^{2}\)/mg, 60 MeV\(\cdot\)cm\({}^{2}\)/mg, and 70 MeV\(\cdot\)cm\({}^{2}\)/mg are 2.1959 mA, 3.1112 mA, 4.2419 mA, 5.3875 mA, 6.3796 mA, and 7.3675 mA, respectively. This clearly indicates that the peak transient current increases with the LET value of the incident energy. Additionally,  
Figure 4: Drain transient characteristics between semi-enclosed gate NMOS and conventional NMOS under identical simulation conditions. Figure 5: The comparison of the charge quantity at the drain between semi-enclosed-gate NMOS and conventional NMOS over time. the time required for the semi-enclosed gate NMOS device to recover to a stable state after a single event transient also increases with higher LET values. To further analyze the underlying mechanisms of the semi-enclosed gate NMOS device, we examine the heavy ion-induced charge density gradient (Heavyion charge density) in Figure 7. The figure shows that the heavy particle enters from the center of the gate and diagonally penetrates towards the drain, with its energy center almost entirely covering the conductive channel. Due to the obstruction by the gate, the energy of the 20 MeV\(\cdot\)cm\({}^{2}\)/mg particle slightly diminishes as it enters the conductive channel region, with a small portion covering the drain and an even smaller portion dispersing towards the source. The ionization density is uniformly distributed along the incident trajectory in a cylindrically symmetric form, with its concentration decreasing gradually from 9.934 \(\times\) 10\({}^{20}\) cm\({}^{-3}\) to zero. The region of maximum energy gradient diffusion occurs within the range of from 6.622 \(\times\) 10\({}^{20}\) cm\({}^{-3}\) to 3.311 \(\times\) 10\({}^{20}\) cm\({}^{-3}\). When heavy ions pass through the sensitive region, their ionization density exhibits a positive correlation with the Linear Energy Transfer rate (LET), and electron-hole pairs are continuously generated along their motion trajectory. The total charge generated, \(\mathrm{Q_{total}}\), is proportional to LET:  
\[\mathrm{Q_{total}}=\mathrm{k}\cdot\mathrm{LET}\cdot\mathrm{D} \tag{1}\]  
k is a proportionality constant, D is the thickness of the sensitive region traversed by the ion, LET is in MeV\(\cdot\)cm\({}^{2}\)/mg, and we assume a constant LET along the path for simplicity. This assumes the ion travels a fixed distance D through the device, depositing energy uniformly in a cylindrical region around its trajectory. The drain collects these carriers with a characteristic collection time constant \(\tau_{c}\), leading to a current that rises as follows:  
\[\mathrm{I(t)=I_{0}\cdot(1-e^{-t/\tau_{c}})}\quad\mathrm{0<t<T} \tag{2}\]  
Figure 6: Output waveforms of drain transient current induced by heavy-ion bombardment with different LETs. After the ion exits at t = T, no further charge is generated, and the current decays as the collected carriers deplete:  
\[\text{I}(\text{t})=\text{I}_{0}\cdot[\text{e}^{-(\text{t }-\text{ T})/\text{rc}}-\text{e}^{-\text{t}/\text{rc}}]\quad\text{t }>\text{T} \tag{3}\]  
Based on this, it can be inferred that, when the particle first strikes, the device has not yet undergone ionization, and no current is generated, resulting in the drain current being at its minimum. As the heavy particle penetrates, the energy deposition gradually spreads, increasing the affected area and causing the internal ionization range within the semi-enclosed gate NMOS device to expand.