The EPAD threshold voltage is defined, by the manufacturer, as the value of the voltage at current

${I}_{D}=1\phantom{\rule{3.33333pt}{0ex}}\mathsf{\mu}\mathrm{A}$ [

22]. Zero temperature coefficient (ZTC) point is defined for current value of

${I}_{D}=68\phantom{\rule{3.33333pt}{0ex}}\mathsf{\mu}\mathrm{A}$. As the value of ZTC current is a very important parameter for the EPAD operation as a radiation sensor, the manufacturer’s claim was experimentally confirmed, and the graph is shown in the

Figure 11. We can observe the overlap at a current value of

$68\phantom{\rule{3.33333pt}{0ex}}\mathsf{\mu}\mathrm{A}$ of the transfer characteristics of EPAD measured at different temperatures. The threshold voltage of the measured EPAD was

${V}_{th}=1\phantom{\rule{3.33333pt}{0ex}}\mathrm{V}$. Using reader-circuit configuration in ZTC point can provide temperature independence of the radiation sensor. Thus, ZTC point is a significant parameter.

There are two big groups of EPADs in this experiment. The first group had a static bias, and second group had a dynamic bias during irradiation.

#### 3.1. Static Bias

Group of EPADs with static bias during the first phase of the experiment, consist of 8 EPADs, and have two subgroups: zero bias and static bias (higher values than zero). Their parameters such as bias during irradiation and initial threshold voltage are represented in

Table 1.

A subgroup of static-biased EPADs all have an initial threshold voltage of 4 V, and they are biased with values: 2.5, 5, 7.5 and 10 V during irradiation. Zero-biased subgroup of EPADs have initial threshold voltage values: 1, 2, 3 and 4 volts, their control gates are grounded during the experiment.

Degradation of EPAD transfer characteristic increases with the absorbed dose, and with higher bias value. We can observe in

Figure 12 the degradation of the transfer characteristic (EPAD8) with the absorbed dose. Characteristic is shifting from the right to the left because the floating gate is discharging, and thus threshold voltage is decreasing. As can be seen, the degradation is in the lower part of the transfer characteristic, and it is increasing during irradiation. Therefore, some parts of the characteristic cannot be read anymore. Threshold voltage can be read until the 82 Gy, and ZTC until the 180 Gy for the 10 V bias.

Because of this problem with degradation, we decided to make a Safe Operating Area of EPAD based on all information from the group of EPADs with static bias (

Figure 13). Safe Zone 1 is the area where the threshold voltage can be monitored, and Safe Zone 2 is the area where ZTC voltage can be monitored. Above curve that is representing the degradation of ZTC, it can be traced only higher than ZTC values. EPADs with zero bias are not degraded, and curves that are describing the degradation of Vth and ZTC are only converging to zero. Degradation of transfer characteristic of EPADs can be explained as a parallel resistive path that was opened in the floating-gate MOS structure [

27]. In our earlier work, we showed that after annealing at 70

${}^{\circ}\mathrm{C}$ transfer characteristics had recovered.

Because of the above mentioned, it is crucial to wisely choose the reader-circuit current for monitoring the voltage shift. We confirmed that if transfer characteristic is not degraded, there is no difference between results obtained with different reader-circuit currents. We call it a good example and this figure can be seen in

Appendix A,

Figure A1, we also show the bad example, where the characteristic is degraded (

Figure A2). In these figures, it is also shown the extrapolation method which is determined in MATLAB program packet by the transfer characteristics in saturation, as the intersection between

${V}_{G}$-axis and the extrapolated linear region of the

${\left({I}_{D}\right)}^{1/2}-{V}_{G}$ curves, using the least square method [

33].

Threshold voltage shift for a subgroup of EPADs with zero bias is shown in

Figure 14. We can observe a threshold voltage shift depending on the absorbed dose for four different values of the initial threshold voltage. The most significant voltage shift with the absorbed dose has EPAD with the highest initial threshold voltage. Thus, the sensitivity increases with the initial threshold voltage, i.e., with the amount of charge on the floating gate. We note that the dependence of the threshold voltage shift with the dose is not linear, but it decreases, and thus the sensitivity decreases during irradiation. It can be better seen in

Figure 15, where is shown the sensitivity of these EPADs with absorbed dose. The sensitivity was calculated as the change in threshold voltage divided by the value of the absorbed dose. As the floating gate is discharging during irradiation, its electric field weakens and, consequently, a smaller generation of electron-hole pairs occurs, resulting in less discharging of the floating gate. For this reason, the sensitivity of EPADs with only an internal electric field (from the FG) decreases with the absorbed dose.

The fading represents the deviation percentage of the threshold voltage during the second phase of the experiment, and it was calculated using the following equation [

31]:

where

${V}_{th}\left(0\right)$ is the threshold voltage immediately after irradiation,

${V}_{th}\left(t\right)$ is the threshold voltage after spontaneous recovery, and

${V}_{th0}$ is the pre-irradiation threshold voltage.

Figure 16 shows fading for the first 1100 h after irradiation. We notice that as the initial threshold voltage of the EPAD increases, the fading decreases. We assume that when the electric field from the floating gate is weaker, the holes are more trapped somewhere in the oxide and, after irradiation, create instability.

EPAD with the highest initial threshold voltage, which has the highest sensitivity, also has the lowest fading. This result is in excellent agreement with our previous research on EPAD [

27]. After exactly 1107.7 h (about a month and a half), EPAD with zero bias and a 4 V initial threshold voltage has a fading of 2.02%.

Looking at EPADs with static bias during irradiation in

Figure 17, we can see that they have a more significant voltage shift than EPADs with zero bias. All EPADs have here the same initial threshold voltage, 4 V. Thus, the influence of the external electric field from the control gate helps in the generation of electron-hole pairs during irradiation, as expected. However, during the first 350 Gy, we see that the EPAD with zero bias has a larger shift.

If we look at the sensitivity of these transistors in

Figure 18, we notice that the zero-biased EPAD has a much higher sensitivity at the beginning of the experiment. This phenomenon has already been noted before [

27], but we could not explain it at the time. In the discussion, we will try to explain this unusual occurrence in semiconductor dosimeters. We can also note on the same graph that static-biased EPADs have a more linear sensitivity dependence during irradiation compared to zero-biased EPADs, which may be useful in some applications. Still, it should be careful about possible degradation that can occur.

When looking at fading for this subgroup of EPADs (

Figure 19), we can see that again the smallest fading has EPAD with zero bias, and that fading increases with bias. Thus, higher bias does more damage to the EPAD structure, as already demonstrated by the Safe Operation Area.

#### 3.2. Dynamic Bias

Looking at the previous section in the

Figure 8, we can see that the dynamic bias depends on the two parameters: the supply voltage

${V}_{DD}$ and the resistor

${R}_{S}$ in the voltage divider. Assuming that EPAD in a voltage divider is always charged to the same threshold voltage value, in our case it is

${V}_{th}=4\phantom{\rule{3.33333pt}{0ex}}\mathrm{V}$. The first subgroup in the dynamic group has a voltage value of

${V}_{DD}=6\phantom{\rule{3.33333pt}{0ex}}\mathrm{V}$ and the second subgroup of

${V}_{DD}=12\phantom{\rule{3.33333pt}{0ex}}\mathrm{V}$. Depending on the choice of resistor value

${R}_{S}$, each EPAD is biased in a different voltage range. Information on EPADs is provided in

Table 2.

Looking at the first subgroup of dynamic biased EPADs compared to EPADs with 0 V and 5 V bias, we can see that the whole subgroup has a very similar dependence of the threshold voltage shift on the absorbed dose (

Figure 20). From here we can take EPAD no. 7 with a dynamic range of 1.61–4.93 V as the most linear and with the highest sensitivity. It is also interesting to note that the shift of threshold voltages of this subgroup are distributed between the endpoints of 0 V and 5 V biased EPADs, which corresponds to their dynamic range that they had during irradiation.

The second subgroup of dynamic biased EPADs is shown in

Figure 21. Like the first subgroup, they also have very similar dose-dependence changes in threshold voltage. Unfortunately, for one unknown reason, one of the transistors in this subgroup (EPAD no. 15) failed during the experiment, and we will not discuss it further. This subgroup has a higher sensitivity than the first subgroup of dynamic and group of static-biased EPADs. Linearity is also better than for the first subgroup. It is difficult to determine the best in this subgroup because they have very similar characteristics, we consider it to be EPAD no. 9 because it has the closest linear dependence.

When it comes to fading for a whole group of dynamic biased EPADs, some distributions can be observed due to different biasing (

Figure 22). With the higher values of the dynamic range we have higher fading, as the subgroups themselves have very similar characteristics, it is difficult to draw such a conclusion because there are overlaps. It is important to emphasize that for the whole group of dynamic biased EPADs the value of the fading after 1100 h does not exceed 9%.