4.1. HCI after TID
The total dose effect was first simulated in the transport state at an irradiation dose rate of 1 krad/s for 1000 s (1000 krad (SiO
2)), and then the hot carrier effect was simulated under the hot carrier stress condition with a voltage of 1.2 V applied to the drain and the gate of the device. The source substrate was grounded, and the simulation times were set to 10
3 s, 10
4 s, 10
5 s, and 10
6 s. The output device transfer characteristic curve is shown in
Figure 13.
After the simulation of the total dose and hot carrier coupling effect, the device output characteristic curve changes are similar to the transfer characteristic curve changes. From
Figure 13, it can be seen that the device output characteristic curve drifts upward after the simulation of irradiation with a total dose of 1 Mrad (SiO
2). Because the total dose effect has less influence on the saturation drain current, the output characteristic curve drifts less after simulating the total dose effect. After that, the simulated hot carrier effect output characteristic curve first drifts back, and when the applied hot carrier stress reaches 10,000 s, the output characteristic curve has basically drifted back to the position before irradiation. It then continues to move in the negative direction toward the
Y-axis until the stress time reaches 10
5 s, and the curve drift begins to converge to a saturation value. From
Figure 14, it can be seen that trap charge inside the device.
Due to the attraction of the trap charge in the buried oxygen layer to the channel electrons, a leakage channel is formed at the bottom of the channel, resulting in a decrease in the device threshold voltage and an increase in the off-state current. When the irradiation dose reaches 1 Mrad (SiO
2), the threshold voltage of the device is degraded from 0.233 V to 0.206 V, and the off-state current increases from 7.2 × 10
−9 A to 9.2 × 10
−8 A. The change rate of the threshold voltage is more than ten percent, and the leakage current increases to thirteen times the original. However, under the action of the hot carrier effect, a large number of hot electrons are rapidly formed in the channel, and most of the hot carriers tunnel into the gate oxygen. There will also be a small number of hot carriers in the buried oxygen layer under the action of the Coulomb force of the trap charge that enter the buried oxygen layer and neutralize the oxide layer trap charge. The gate oxygen in the trap charge and the interface state are negatively charged, which will offset some of the buried oxygen layer of the positive oxide layer charge, and the total dose effect on the device is reduced by the hot carrier effect. The influence of the total dose effect on the device is offset by the hot carrier effect; the threshold voltage begins to drift positively and the drain current gradually decreases.
Table 4 shows that the threshold voltage returns to the initial value before irradiation after applying hot carrier stress for more than 10,000 s, and the threshold voltage continues to increase after continuing to apply hot carrier stress. After applying hot carrier stress for 10
6 s, the threshold voltage starts to converge to a limiting value, which is less than five percent larger than that before simulating the coupling effect; the off-state current decreases to 1.0 × 10
−8 A, which is not yet restored to the initial value before irradiation, but is already in the same order of magnitude.
When the device is irradiated, a positively charged oxide trap charge is generated in the oxide layer. Under the action of the Coulomb force, the carrier (electron) in the channel is attracted and converges to the lower surface of the channel, forming a weak conductive channel, causing the device to still have a large leakage current when the gate voltage is zero. Then, only a gate voltage less than the threshold voltage can form a strong inverse layer in the channel. This also explains the effect of the oxide trap charge on threshold voltage and leakage current. When the irradiated device is under hot carrier stress, the radiation-generated charge in the buried oxide layer is neutralized by a small number of injected hot electrons in the channel, which restores the drift in parameters such as the threshold voltage due to the total dose effect [
12]. At the same time, some hot electrons in the channel cross the Si/SiO
2 barrier and form an interface state trap charge at the interface between the channel and the gate oxide layer. The negatively charged interface state will prevent the carrier (electron) in the channel from converging to form a conductive channel. Therefore, a voltage greater than the threshold voltage needs to be applied to the gate to offset the effect of the interface state and form a strong inverse layer [
13]. This also explains the effect of interface trap charges on threshold voltage.
4.2. TID after HCI
The hot carrier effect was first simulated under the stress condition of 1.2 V applied to the gate–drain and the source–substrate ground for 106 s. After that, the device was put in the transport state to simulate the total dose effect at an irradiation dose rate of 1 krad/s, and the simulation times were set to 100 s (100 krad (SiO2)), 500 s (500 krad (SiO2)), and 1000 s (1000 krad (SiO2)). After the simulation, the drain voltage was fixed at 0.8 V, the source and the substrate were grounded, and the gate voltage was swept from 0 to 0.8 V in the output transfer characteristic curve.
From
Figure 15, it can be seen that in the device simulation of the applied hot carrier stress, 10
6 s after the simulation of the total dose effect, the transfer characteristic curve first drifted in the positive direction of the
Y-axis, and then drifted back. At the irradiation time of about 500 s, the transfer characteristic curve drifted back to the position of the simulation of the hot carrier effect before the position. Continuing irradiation, the transfer characteristic curve also moved in the negative direction of the
Y-axis until the curve drifted towards a saturation value. When the irradiation time reached 500 s, the device off-state current returned to the same order of magnitude as before the simulated HCI, but the curvature of the transfer characteristic curve was smaller than before the simulated HCI, i.e., the subthreshold amplitude was larger.
The hot carrier effect on the electrical characteristics of the device is mainly reflected in the two parameters of threshold voltage and saturated drain current, as can be seen from
Table 5. After the application of 10
6 s of hot carrier stress, the device threshold voltage increased by fifteen percent and the saturated drain current decreased by six percent. After the simulation of an irradiation dose of 1 Mrad (SiO
2), the threshold voltage was still higher than the initial value of the device, by two percent, but the saturated drain voltage reversed from negative to positive by two percentage points. The off-state current and subthreshold swing were not sensitive to the hot carrier effect, as the off-state current was halved and the subthreshold swing was reduced by only two percentage points after 10
6 s of hot carrier stress, both of which are relatively small changes. Afterwards, the simulation of an irradiation dose of 1 Mrad (SiO
2) dramatically increased the values of both parameters; the off-state current increased by orders of magnitude to 8.2 times the initial value, and the subthreshold amplitude of the pendulum increased by thirty percent compared with the initial value.
The hot carrier effect on the electrical characteristics of the device is mainly reflected in the two parameters of threshold voltage and saturated drain current, as can be seen from
Table 5. After the application of 10
6 s of hot carrier stress, the device threshold voltage increased by fifteen percent and the saturated drain current decreased by six percent. After the simulation of an irradiation dose of 1 Mrad (SiO
2), the threshold voltage was still higher than the initial value of the device by two percent, but the saturated drain voltage reversed from negative to positive by two percentage points. The off-state current and subthreshold swing were not sensitive to the hot carrier effect, as the off-state current was halved and the subthreshold swing was reduced by only two percentage points after 10
6 s of hot carrier stress, both of which are relatively small changes. Afterwards, the simulation of an irradiation dose of 1 Mrad (SiO
2) dramatically increased the values of both parameters; the off-state current increased by orders of magnitude to 8.2 times the initial value, and the subthreshold amplitude of the pendulum increased by thirty percent compared with the initial value.
Whether the total dose effect is simulated first and then the hot carrier effect is simulated, or the hot carrier effect is simulated first and then the total dose effect is simulated, these two coupling simulation methods will make the effects of TID and HCI on the device cancel each other out. The
Table 6 shows the degradation of electrical characteristics of the device under different coupling methods for the simulation of a 1000 krad (SiO
2) irradiation dose and 10
6 s of hot carrier stress. It can be seen that the difference lies in the threshold voltage and saturated drain current changes, which are smaller after the first HCI TID, and the threshold voltage under this coupling method also experiences positive growth; however, the growth is less than three percent. Saturated drain current experiences positive growth and the value of change is less than 2.5 percent. With TID first, followed by HCI, the off-state current and subthreshold swing changes are even smaller; the growth in off-state current under this coupling method is only 1.4%, and sub-threshold swing growth is 20%.
As the duration of HCI and TID action increases, the positively charged oxide trap charge induced by the total dose effect promotes channel carrier formation, whereas there is a negatively charged interfacial state induced by the hot carriers that hinders channel carrier formation. Thus, it is the reciprocal compensation between the two effects that leads to the mutual cancellation of the two effects.
4.3. Influence of Different Bias States on Coupling Effect
The hot carrier effect was first simulated under the stress condition of 1.2 V applied to the gate–drain and the source–substrate ground for 106 s. After that, the device was put in TG state, OFF state, N state, and NULL state to simulate the total dose effect at an irradiation dose rate of 1 krad/s, and the simulation time was set to 1000 s (1000 krad (SiO2)). After the simulation, the drain voltage was fixed at 0.8 V, the source and the substrate were grounded, and the gate was swept from 0 to 0.8 v in the output transfer characteristic curve.
From
Figure 16, it can be seen that regardless of the bias state of the device, the total dose effect can offset the influence of the hot carrier effect for the device after simulating HCI and then TID. When the device was put in the OFF state and TG state, the curve drifted back to a greater degree, followed by the NULL state, and the drift degree of the transfer characteristic curve was the least when the device was in the ON state. Compared with the initial transfer characteristic curve of the device, it can be seen that the damage of TID exceeds that of HCI when the device is irradiated for 1000 s in the OFF state and TG state.
Regardless of the bias state of the device, the change in the device threshold voltage shows a tendency of increasing and then decreasing after simulating HCI and then TID, as can be seen from
Table 7. When the hot carrier stress is applied for 106 s, the threshold voltage increases from the initial value of 0.233 V to 0.268 V. According to the JEDEC standard, the device is judged to have failed when the degradation of the electrical parameters exceeds 10%. In this case, the device threshold voltage increased by fifteen percent, indicating that the device failed. Subsequent irradiation simulations were performed, and the threshold voltage increase fell back to within 10% in all bias states, except the ON state, and the device returned to the non-failed state.
The
Table 8 shows the variation in the device off-state current and the drain saturation current, and it can be seen that the device off-state current and the drain saturation current show a tendency of decreasing and then increasing. The drain saturation current changes for the simulated coupling effect are generally small for the four bias states, with degradation in the plus or minus 5% range. In contrast to the drain saturation current, the off-state current varies more significantly, increasing more than sevenfold in both bias states, i.e., the OFF state and the TG state. The off-state currents degrade due to the coupling effect being dominated by TID.
From
Figure 17a, trap charges are mainly distributed in the buried oxygen layer below the channel when the device simulates the coupling effect in the TG state, ON state, and NULL state. When the device is in the OFF state, the trap charge is mainly distributed in the buried oxygen layer below the channel, in addition to some trap charge being distributed below the source region. From the data in
Figure 17b, it can be observed that the oxide trap charge concentration in the buried oxygen layer is significantly higher when the device is in the TG and OFF states, and the transfer characteristic curves drift backward to a higher degree. The NULL state is followed by a lower concentration of oxide trap charge in the buried oxygen layer when the device is in the ON state, and the curve drifts backward to the lowest degree.