**2. Mechanisms responsible for threshold voltage shift during irradiation**

The dosimetry of ionizing radiation using radiation‐sensitive MOSFETs is based on the thresh‐ old voltage shift, conversion into absorbed radiation dose *D* [25, 26]. This shift originates in the radiation‐induced electron‐hole pairs formed during irradiation. Namely, gamma and X‐rays interact with the electrons in SiO2 molecules releasing secondary electrons and holes, that is, photons break ≡  Si o − ihSi o  ≡ and ≡  Si o − Si o  ≡ covalent bonds in the oxide [27] (the index <sup>o</sup> is used to denote silicon atom in the oxide). The released secondary electrons, which are highly energetic, may be recombined by holes at the place of production or may escape recombination. The secondary electrons that escape recombination pass through the oxide bulk, break covalent bonds and create ≡  Si o − O •+ − Si o  ≡ complexes, where • denotes the unpaired electron. This complex is energetically very shallow and trapped holes can easily escape it. It is obvious that secondary electrons play a more important role in the bond break‐ ing than highly energetic photons, due to the difference in their effective masses, that is, in their effective cross section.

The ≡  Si o − O − Si o  ≡ mainly distributed near the Si/SiO2 interface, can also the broken by passing secondary electrons, usually created by non‐bridging oxygen (NBO) centers ≡  Si o − O and positively charged E ' centers, ≡  Si o + [28]. The main precursor of the traps in the oxide bulk and in interface regions is the NBO center, as an energetically deeper centre, and represents a more likely negative than positively charged amphoteric defect. Also, a secondary electron can also break ≡  Si o − Si o  ≡ bonds and create E <sup>γ</sup> ' centers, ≡  Si o • [27] by knocking out an electron.

Positive charge is formed in oxide by holes trapping, while electrons trapping lead to creation of negative charge. The concentration of positive charge in oxide is much higher since the hole trapping centers are more numerous compared to electron trapping centers. Moreover, trapped electrons and holes near Si/SiO2 interface have the strongest impact on channel carri‐ ers, hence on MOSFET characteristics.

Amphoteric defects Si 3  ≡  Si s • (index <sup>s</sup> is used to indicate a silicon atom in substrate) are marked as true interface traps and represent defects at the Si/SiO2 interface. At Si/SiO2 interface, a silicon atom ≡  Si s • back bonds with three silicon atoms from the substrate ≡  Si s and is mostly marked as ≡  Si s • or Si s • . Their creation can also originate from incident photons when they pass through the gate or substrate [27]; however, the amount can be neglected. Hydrogen released in the oxide (hydrogen‐released species model H‐model) [29, 30] is the main creator of interface traps. This model proposed that H ions released in the oxide by trapped holes at ≡  Si o − H and ≡  Si o − OH defects in the oxide drift toward the Si/SiO<sup>2</sup> interface under the positive electric field. When H<sup>+</sup> ions arrive at the interface, it picks up an electron from the substrate, becoming a highly reactive atom H<sup>0</sup> [31]. This atom reacts at the interface producing Si s • [32]. Dimerization of hydrogen atoms also exists near the Si/SiO<sup>2</sup> interface, what further leads to creation of H<sup>2</sup> molecule [31]. The increase in Si <sup>s</sup> • continues during annealing of irradi‐ ated MOSFETs for a long period of time [33].

Positive trapped charge in the oxide is called fixed traps (FT), and positive trapped charge near Si/SiO2 interface is called switching traps (ST) [27], where FT represents traps in the oxide that without the ability to exchange the charge with the channel within the MOSFET trans‐ fer/subthreshold characteristic measurement time frame. On the other hand, ST represents traps created near and at Si/SiO2 interface, and they do capture (communicate with) the carrier from the channel within the transfer/subthreshold characteristic measurement time frame. Furthermore, one can differentiate between slow switching traps (SST) created in the oxide near Si/SiO2 interface and fast switching traps (FST) created at Si/SiO2 interface also known as true interface traps (Si s • ).

Threshold voltage shift Δ *V*<sup>T</sup> during irradiation is a consequence of the increase in concentra‐ tion of FT, *Q*FT, and the increase in the concentration ST, *Q*ST. The threshold voltage *<sup>V</sup>*<sup>T</sup> can be expressed as follows [33]

$$V\_{\rm r} = V\_{\rm r0} - \frac{Q\_{\rm r\uparrow} + Q\_{\rm sr}}{Q\_{\rm sr}} = \left. V\_{\rm r0} + \Delta \right. V\_{\rm r\uparrow} \tag{1}$$

where *V*T0 is the value of *<sup>V</sup>*<sup>T</sup> before irradiation and *C*ox is the gate capacitance. In p‐channel MOSFETs, both FT and ST are positive and they contribute to the threshold voltage shift in the same direction, i. e. both *V*<sup>T</sup> and *V*T0 are negative. Also, the so‐called rebound effect [34] is absent in p‐channel MOSFETs: This phenomenon is due to the competitive effect of positive charge in the oxide and negative interface traps generated in n‐channel MOSFETs leading to a positive or negative Δ *V*<sup>T</sup> value dependence on the relative values of *Q*FT and *Q*ST. This is the rea‐ son why p‐channel MOSFETs instead of n‐channel MOSFETs are usually used in dosimetry of ionizing radiation.

### **3. Response of pMOS dosimeters to gamma and X‐ray radiation**

#### **3.1. Important pMOS dosimetric parameters**

The most important parameters that characterize the pMOS dosimetric radiation response are sensitivity, dose linearity and room temperature long‐term stability [35, 36]. Sensitivity represents threshold voltage shift Δ *V*T and radiation dose *D* ratio (Δ *V*T/*D*) and could be con‐ trolled by the gate bias during irradiation. It is well known [37, 38] that an increase in sensitivity could be achieved with increase in gate bias during irradiation. In the case of positive gate bias, the sensitivity is higher, than in the case of negative gate bias and the lowest sensitivity being for zero gate bias [20]. Moreover, sensitivity increase can be achieved by increasing the gate oxide thickness [36–39] and by processing conditions which determine the FT density, their capture cross section and their location as well as the ST density [40].

silicon atom ≡  Si s

• or Si s •

substrate, becoming a highly reactive atom H<sup>0</sup>

ated MOSFETs for a long period of time [33].

• ).

*V*<sup>T</sup> = *V*T0 −

**3.1. Important pMOS dosimetric parameters**

positive electric field. When H<sup>+</sup>

traps created near and at Si/SiO2

leads to creation of H<sup>2</sup>

near Si/SiO2

near Si/SiO2

true interface traps (Si s

Threshold voltage shift Δ *V*<sup>T</sup>

where *V*T0 is the value of *<sup>V</sup>*<sup>T</sup>

positive or negative Δ *V*<sup>T</sup>

of ionizing radiation.

the same direction, i. e. both *V*<sup>T</sup>

expressed as follows [33]

marked as ≡  Si s

234 Radiotherapy

Si s • • back bonds with three silicon atoms from the substrate ≡  Si s

pass through the gate or substrate [27]; however, the amount can be neglected. Hydrogen released in the oxide (hydrogen‐released species model H‐model) [29, 30] is the main creator of interface traps. This model proposed that H ions released in the oxide by trapped holes

Positive trapped charge in the oxide is called fixed traps (FT), and positive trapped charge

that without the ability to exchange the charge with the channel within the MOSFET trans‐ fer/subthreshold characteristic measurement time frame. On the other hand, ST represents

from the channel within the transfer/subthreshold characteristic measurement time frame. Furthermore, one can differentiate between slow switching traps (SST) created in the oxide

interface and fast switching traps (FST) created at Si/SiO2

tion of FT, *Q*FT, and the increase in the concentration ST, *Q*ST. The threshold voltage *<sup>V</sup>*<sup>T</sup>

*<sup>Q</sup>*FT \_\_\_\_\_\_\_ + *<sup>Q</sup>*ST *C*ox

MOSFETs, both FT and ST are positive and they contribute to the threshold voltage shift in

absent in p‐channel MOSFETs: This phenomenon is due to the competitive effect of positive charge in the oxide and negative interface traps generated in n‐channel MOSFETs leading to a

son why p‐channel MOSFETs instead of n‐channel MOSFETs are usually used in dosimetry

The most important parameters that characterize the pMOS dosimetric radiation response are sensitivity, dose linearity and room temperature long‐term stability [35, 36]. Sensitivity

**3. Response of pMOS dosimeters to gamma and X‐ray radiation**

at ≡  Si o − H and ≡  Si o − OH defects in the oxide drift toward the Si/SiO<sup>2</sup>

[32]. Dimerization of hydrogen atoms also exists near the Si/SiO<sup>2</sup>

molecule [31]. The increase in Si <sup>s</sup>

. Their creation can also originate from incident photons when they

•

interface is called switching traps (ST) [27], where FT represents traps in the oxide

ions arrive at the interface, it picks up an electron from the

interface, and they do capture (communicate with) the carrier

during irradiation is a consequence of the increase in concentra‐

before irradiation and *C*ox is the gate capacitance. In p‐channel

value dependence on the relative values of *Q*FT and *Q*ST. This is the rea‐

and *V*T0 are negative. Also, the so‐called rebound effect [34] is

[31]. This atom reacts at the interface producing

= *V*T0 + Δ *V*T, (1)

continues during annealing of irradi‐

and is mostly

interface under the

interface, what further

interface also known as

can be

In practical applications, it is most convenient for pMOS dosimeters to have a linear response of threshold voltage shift Δ *V*<sup>T</sup> regarding observed radiation dose *D*. In this case, the sensitivity is the same for considered dose interval. It was shown that the response is linear for low doses and progressively saturates at a maximum values which respect to gate bias [40]. The linear dependence is given by [36]

$$
\Lambda \, V\_{\tau} = A \cdot D^\* \,\tag{2}
$$

where *A* is the constant and *n* is the degree of linearity. For *n* = 1 , the constant *A* represents sensitivity *S*:

$$\mathcal{S} = \Delta V\_{\sf T} / D. \tag{3}$$

Positive gate bias during irradiation reduces the recombination of produced electron‐hole pairs in SiO2 and as a consequence the pMOS dosimeters response becomes more linear and sensitive [33, 41].

Room‐temperature long‐tem stability of irradiated pMOS dosimeters can be observed by cal‐ culating fading *F*. The percent of fading can be calculated as follows [27]:

$$F = \frac{V\_\mathrm{r}(0) - V\_\mathrm{r}(0)}{V\_\mathrm{r}(0) - V\_\mathrm{v}} = \frac{V\_\mathrm{r}(0) - V\_\mathrm{r}(0)}{\Delta V\_\mathrm{r}(0)}\,\mathrm{}\tag{4}$$

where *V*<sup>T</sup> (0) is the threshold voltage immediately after irradiation, *V*T0 is the pre‐irradiation threshold voltage, *V*<sup>T</sup> (*t*) is the threshold voltage after annealing time, *t* and Δ *V*<sup>T</sup> (0 )is the thresh‐ old voltage shift immediately after irradiation.

#### **3.2. Influence of gate bias on threshold voltage shift during irradiation**

**Figures 1** and **2** show the threshold voltage shift Δ *V*T of pMOS dosimeters with gate oxide thickness of 1 μm for X‐ray (energy of 140 keV) as a function of radiation dose *D* in the range from 0 to 1 0 cGy and from 0 to 1 Gy, while gate bias during irradiation was 0 and 5 V [35], respectively. Experimental data fitting with Eq. (2) for *n* = 1shows an almost linear response between Δ *V*T and *D*. Namely, for gate bias during irradiation of *V*irr = 0 V, correlation coef‐ ficient is *r* <sup>2</sup> = 0.98 , whereas for *V*irr = 5 V , correlation coefficient is *r* <sup>2</sup> = 0.99 .

**Figure 3** shows the threshold voltage shift Δ *V*T of pMOS dosimeters with gate oxide thickness of 1 μm as a function of gamma‐ray radiation dose *D* (gamma radiation originate from 60Co) in range from 0 to 1 Gy for gate bias during irradiation *V*irr = 0 V and *V*irr = 5 V [38]. The same dependence for gamma‐ray radiation dose in range from 0 to 5 Gy is given in **Figure 4** [38].

**Figure 1.** Threshold voltage shift *Δ V*T in pMOS dosimeters with 1‐μm‐thick gate oxide as a function of X‐ray radiation dose *D* in the 0–10 cGy range. Gate bias during irradiation *V*irr was 0 or 5 V.

Experimental data fitting presented in these figures using Eq. (2) for *n* = 1gives correlation coef‐ ficient *r* <sup>2</sup> = 0.99, so it is assumed that the linearity between Δ *V*T and *D* is satisfactory for practical application.

**Figure 5** shows the Δ *V*<sup>T</sup> = *f*(*D* ) dependence of pMOS dosimeters with gate oxide thickness of 1 μm for gamma‐ray radiation dose in the range from 0 to 50 Gy [36]. During the irradiation, the gate biases *V*irr were 0, 1.25, 2.50, 3.75 and 5 V. It can be seen that the threshold voltage shift for the same radiation dose increases with gate bias increase. The radiation dose up to 50 Gy did not significantly degrade the linearity of the pMOS dosimeters. Experimental data fitting

**Figure 2.** Threshold voltage shift *Δ V*T in pMOS dosimeters with 1‐μm‐thick gate oxide as a function of X‐ray radiation dose *D* in the 0–1 Gy range. Gate bias during irradiation *V*irr was 0 or 5 V.

**Figure 3.** Threshold voltage shift *Δ V*T in pMOS dosimeter with 1‐μm‐thick gate oxide as a function of gamma‐ray radiation dose *D* in the 0–1 Gy range. Gate bias during irradiation *V*irr was 0 or 5 V.

Experimental data fitting presented in these figures using Eq. (2) for *n* = 1gives correlation coef‐ ficient *r* <sup>2</sup> = 0.99, so it is assumed that the linearity between Δ *V*T and *D* is satisfactory for practical

**Figure 1.** Threshold voltage shift *Δ V*T in pMOS dosimeters with 1‐μm‐thick gate oxide as a function of X‐ray radiation

0 5 10

D (cGy)

**Figure 5** shows the Δ *V*<sup>T</sup> = *f*(*D* ) dependence of pMOS dosimeters with gate oxide thickness of 1 μm for gamma‐ray radiation dose in the range from 0 to 50 Gy [36]. During the irradiation, the gate biases *V*irr were 0, 1.25, 2.50, 3.75 and 5 V. It can be seen that the threshold voltage shift for the same radiation dose increases with gate bias increase. The radiation dose up to 50 Gy did not significantly degrade the linearity of the pMOS dosimeters. Experimental data fitting

0 20 40 60 80 100

D (cGy)

**Figure 2.** Threshold voltage shift *Δ V*T in pMOS dosimeters with 1‐μm‐thick gate oxide as a function of X‐ray radiation

application.

236 Radiotherapy

0.0

0

dose *D* in the 0–1 Gy range. Gate bias during irradiation *V*irr was 0 or 5 V.

1

2

VT (V)

3

4

<sup>5</sup> Virr = 0 V

Virr = 5 V

dose *D* in the 0–10 cGy range. Gate bias during irradiation *V*irr was 0 or 5 V.

0.2

0.4

VT (V)

0.6

0.8

Virr = 0 V Virr = 5 V

> using Eq. (2) for *n* = 1 gives correlation coefficient, *r* <sup>2</sup> = 0.98. Having that *r* <sup>2</sup> are very close to one, it can be assumed that there is a linear dependence between Δ *V*T and *D* and that the sensitivity of these devices for a given value of *V*irr is the same in the range from 0 to 50 Gy.

**Figure 4.** Threshold voltage shift *Δ V*T in pMOS dosimeter with 1‐μm‐thick gate oxide as a function of gamma‐ray radiation dose *D* in the 1–5 Gy range. Gate bias during irradiation *V*irr was 0 or 5 V.

**Figure 5.** Threshold voltage shift *Δ V*T in pMOS dosimeter with 1‐μm‐thick gate oxide as a function of gamma‐ray radiation dose *D* in the 0–50 Gy range. Gate bias during irradiation *V*irr was ranging from 0 to 5 V.

**Figure 6** shows the sensitivity *S* as a function of gate bias *V*irr during gamma‐ray irradiation to 50 Gy of pMOS dosimeters with gate oxide thickness of 1 μm [36]. The symbols stand for experimental data, whereas the solid lines represent fits, which are exponential.

**Figure 6.** Sensitivity of pMOS dosimeter with 1‐μm‐thick gate oxide as a function of gate bias *V*irr for 50 Gy gamma‐ray irradiation.

The increase in Δ *V*<sup>T</sup> with the increase in *V*irr is due to the increase in FT and ST. It is well known that with the number of holes which have avoided the recombination with elec‐ trons, the number of created FT and ST increases. When *V*irr = 0 V the electric field in the oxide is only due to work function difference between the gate and the substrate (zero bias conditions or dosimeter passive mode), so the probability for electron‐hole recombina‐ tion is higher than in the case when *V*irr > 0 V. For higher value of *V*irr , the large number of holes will escape the initial recombination, which further increase the probability for their capture at *E*′ , *E*<sup>γ</sup> ′ and NBO centers and increase FT and SST which leads to increase in Δ *V*T. Such conclusion is in agreement with results shown in **Figures 1**–**6**. It should be emphasized that during irradiation, the FT concentration is several times larger than ST concentration. This proves that the increase in Δ *V*<sup>T</sup> value during irradiation is mainly due to increase in FT [42].

#### **3.3. Influence of gate oxide thickness on threshold voltage shift during irradiation**

**Figure 6** shows the sensitivity *S* as a function of gate bias *V*irr during gamma‐ray irradiation to 50 Gy of pMOS dosimeters with gate oxide thickness of 1 μm [36]. The symbols stand for

**Figure 5.** Threshold voltage shift *Δ V*T in pMOS dosimeter with 1‐μm‐thick gate oxide as a function of gamma‐ray

was ranging from 0 to 5 V.

0 20 40 60

D (Gy)

012345 6

Virr (V)

**Figure 6.** Sensitivity of pMOS dosimeter with 1‐μm‐thick gate oxide as a function of gate bias *V*irr for 50 Gy gamma‐ray

experimental data, whereas the solid lines represent fits, which are exponential.

3560e2840 <sup>033</sup> *.S . .*

*VG*

0.05

0.10

0.15

S (V/Gy)

irradiation.

0.20

0.25

0.30

0

7

VT (V)

14

238 Radiotherapy

Virr = 5 V Virr = 3.75 V Virr = 2.5 V Virr = 1.25 V Virr = 0 V

radiation dose *D* in the 0–50 Gy range. Gate bias during irradiation *V*irr

**Figure 7** shows the threshold voltage shift Δ *V*T as a function of radiation dose *D* for pMOS dosimeters with gate oxide thicknesses of 400 nm and 1 μm [43]. Irradiation of these devices was performed with gamma‐ray irradiation in the dose range from 0 to 5 Gy when gate bias during irradiation was *V*irr = 5 V. It was shown that sensitivity Δ *V*T/*D* increases with gate oxide thickness increase and that there is a linear dependence between Δ *V*T and *D* (correlation coefficient *r* <sup>2</sup> = 0.99 ).

**Figure 7.** pMOS dosimeters threshold voltage shift *Δ V*T as a function of gamma‐ray dose in 0–5 Gy range. Gate bias during irradiation was *V*irr <sup>=</sup><sup>5</sup> V . Gate oxide thickness was 400 nm and 1  μm.

The Δ *V*<sup>T</sup> <sup>=</sup> *<sup>f</sup>*(*<sup>D</sup>* ) dependence for pMOS dosimeters with gate oxide layer thicknesses of 100 nm, 400 nm and 1 μm is shown in **Figure 8** [36]. The gamma‐ray irradiation of these devices was performed in the dose range from 0 to 50 Gy, while the gate bias *V*irr = 5 V. It can be seen that the increase in gate thickness leads to the increase in Δ *V*T for the same radiation dose. It is mainly due to the increase in FT concentration [42]. Experimental data fitting using Eq. (2) for *n* = 1 , gives the correlation coefficient values, for pMOS dosimeters with 100 nm, 400 nm and 1 μm gate oxide thickness 0.99, 0.99 and 0.98, respectively, what proves linear dependence between Δ *V*T and *D*.

**Figure 8.** pMOS dosimeters threshold voltage shift *Δ V*T as a function of gamma‐ray dose in 0–50 Gy range. Gate bias during irradiation was *V*irr <sup>=</sup><sup>5</sup> V . Gate oxide thickness was 100 nm, 400 nm and 1  μm.

#### **3.4. Influence of photon energy on pMOS dosimetry sensitivity**

**Figure 9** shows the threshold voltage shift Δ *V*T as a function of radiation dose *D* for 1 μm gate oxide thickness pMOS dosimeters irradiated with gamma‐rays which originates from 60Co and X‐ray with energy 140 keV in dose range from 0 to 1 Gy for gate bias during irradiation *V*irr = 5 V [35, 38]. Experimental results fitting using Eq. (2) for *n* = 1gives the value of cor‐ relation coefficient *r* <sup>2</sup> = 0.99assuming that there is linear dependence between Δ *V*T and *D*, that is, sensitivity is the same for considered dose interval. It can be also seen from the figure that the sensitivity is much higher for X than for gamma radiation.

The Δ *V*<sup>T</sup> <sup>=</sup> *<sup>f</sup>*(*<sup>D</sup>* ) dependence for gamma and X‐rays for pMOS dosimeters with gate oxide thick‐ ness of 1 μm in dose range from 0 to 5 Gy and *V*irr <sup>=</sup> 5 V is shown in **Figure 10** [38]. Experimental results fitting using Eq. (2) for *n* = 1 , gives correlation coefficient for gamma and X‐rays 0.99 and 0.96, respectively. On the basis of these values, it can be concluded that for X‐rays, there is no linear dependence between Δ *V*<sup>T</sup> and *D*.

The Δ *V*<sup>T</sup> <sup>=</sup> *<sup>f</sup>*(*<sup>D</sup>* ) dependence for pMOS dosimeters with gate oxide layer thicknesses of 100 nm, 400 nm and 1 μm is shown in **Figure 8** [36]. The gamma‐ray irradiation of these devices was performed in the dose range from 0 to 50 Gy, while the gate bias *V*irr = 5 V. It can be seen that the increase in gate thickness leads to the increase in Δ *V*T for the same radiation dose. It is mainly due to the increase in FT concentration [42]. Experimental data fitting using Eq. (2) for *n* = 1 , gives the correlation coefficient values, for pMOS dosimeters with 100 nm, 400 nm and 1 μm gate oxide thickness 0.99, 0.99 and 0.98, respectively, what proves linear dependence

**3.4. Influence of photon energy on pMOS dosimetry sensitivity**

during irradiation was *V*irr <sup>=</sup><sup>5</sup> V . Gate oxide thickness was 100 nm, 400 nm and 1  μm.

100 nm 400 nm 1 m

that the sensitivity is much higher for X than for gamma radiation.

is no linear dependence between Δ *V*<sup>T</sup>

VT (V)

**Figure 9** shows the threshold voltage shift Δ *V*T as a function of radiation dose *D* for 1 μm gate oxide thickness pMOS dosimeters irradiated with gamma‐rays which originates from 60Co and X‐ray with energy 140 keV in dose range from 0 to 1 Gy for gate bias during irradiation *V*irr = 5 V [35, 38]. Experimental results fitting using Eq. (2) for *n* = 1gives the value of cor‐ relation coefficient *r* <sup>2</sup> = 0.99assuming that there is linear dependence between Δ *V*T and *D*, that is, sensitivity is the same for considered dose interval. It can be also seen from the figure

0 10 20 30 40 50

D (Gy)

**Figure 8.** pMOS dosimeters threshold voltage shift *Δ V*T as a function of gamma‐ray dose in 0–50 Gy range. Gate bias

The Δ *V*<sup>T</sup> <sup>=</sup> *<sup>f</sup>*(*<sup>D</sup>* ) dependence for gamma and X‐rays for pMOS dosimeters with gate oxide thick‐ ness of 1 μm in dose range from 0 to 5 Gy and *V*irr <sup>=</sup> 5 V is shown in **Figure 10** [38]. Experimental results fitting using Eq. (2) for *n* = 1 , gives correlation coefficient for gamma and X‐rays 0.99 and 0.96, respectively. On the basis of these values, it can be concluded that for X‐rays, there

and *D*.

between Δ *V*T and *D*.

240 Radiotherapy

**Figure 9.** Threshold voltage shift *Δ V*T in pMOS dosimeter with 1‐μm‐thick gate oxide as a function of gamma and X‐ray radiation dose *D* in the 0–1 Gy range. Gate bias during irradiation *V*irr was ranging from 0 to 5 V.

**Figure 10.** Threshold voltage shift *Δ V*T in pMOS dosimeters as a function of gamma and X‐ray radiation dose *D* in the 0–5 Gy range. Gate bias during irradiation was *V*irr <sup>=</sup> 5 V.

From **Figures 9** and **10**, it can be seen that increasing in Δ *V*T is much higher in the case when pMOS dosimeters are irradiated with X‐rays (140 keV photon energy) than in the case of gamma‐rays originating from 60Co (energies of photons of 1.17 and 1.33 MeV). This is a conse‐ quence of different photon energies which lead to ionization of SiO<sup>2</sup> molecules. Namely, X‐ray photons energy of 140 keV lead to molecule ionization by both photo effect and Compton's effect, while gamma‐ray photons with energies of 1.17 and 1.33 MeV lead to SiO<sup>2</sup> molecules ionization only by Compton's effect [38]. A direct change in Δ *V*T values is caused by a larger number of FT and ST, which are formed during X‐ray irradiation compared to gamma‐ray irradiation, the reason being the probability for molecule ionization by photoeffect is signifi‐ cantly higher than by Compton's effect.

## **4. Fading of irradiated pMOS dosimeters**

As a dosimeter radiation sensitive MOSFET must satisfy a crucial demand, which implies compromising between sensitivity to irradiation and stability with time after irradiation. Stability represent insignificant change in ΔVT of an irradiated MOSFET at room tempera‐ ture for a long‐time period (saved dosimetric information) [43]. Having that immediate dose readout is not always possible, also the exact moment of irradiation is often unknown as in the case of individual monitoring the radiation dose measurements must be performed peri‐ odically. Room temperature stability of irradiated pMOS dosimeters can be determined by calculating fading using Eq. (4).

Fading results for pMOS dosimeters with gate oxide thickness of 400 nm and 1 μm, at room temperature previously irradiated with X‐ray (energy 140 keV) up to 1 Gy for *V*irr <sup>=</sup> <sup>0</sup> V and *V*irr = 5 V are presented in **Figures 11** and **12**, respectively [35]. It can be seen that fading of pMOS dosimeters with gate oxide thickness of 400 nm (**Figure 11**), which were irradiated with gate bias *V*irr = 5 V, is about 40% in the first 7 days, whereas those of pMOS dosimeters irra‐ diated without gate bias during irradiation have 22% fading also in the first 7 days. For the time period between 7 and 28 days, fading of pMOS dosimeters irradiated with gate bias 5 V increased for about 3%, whereas fading of pMOS dosimeters irradiated without gate bias dur‐ ing irradiation had a nearly constant value. Fading of 1 μm thick gate oxide pMOS dosimeter (**Figure 12**), which were irradiated up to 1 Gy with gate bias *V*irr = 5 V , in the first 7 days was 14%, whereas for the time period between 7 and 28 days, it increases about 1%. pMOS dosime‐ ters with the same gate oxide thickness, which were irradiated without gate bias the first 7 days, have fading increase for about 1%, and this value is kept up to 28 days. From **Figures 11** and **12**, it can be concluded that fading is lower when the gate oxide of pMOS dosimeters is thicker which in accordance with early study [44] showed that fading decreases with the increase in gate oxide thickness.

The decrease in the positive trapped charge causes fading of pMOS dosimeters. This decrease orig‐ inates from electron tunneling from Si into SiO2 ; once captured at positive oxide trapped charge, which lead to their neutralization/compensation and change in threshold voltage shift [45].

From **Figures 9** and **10**, it can be seen that increasing in Δ *V*T is much higher in the case when pMOS dosimeters are irradiated with X‐rays (140 keV photon energy) than in the case of gamma‐rays originating from 60Co (energies of photons of 1.17 and 1.33 MeV). This is a conse‐

photons energy of 140 keV lead to molecule ionization by both photo effect and Compton's

ionization only by Compton's effect [38]. A direct change in Δ *V*T values is caused by a larger number of FT and ST, which are formed during X‐ray irradiation compared to gamma‐ray irradiation, the reason being the probability for molecule ionization by photoeffect is signifi‐

As a dosimeter radiation sensitive MOSFET must satisfy a crucial demand, which implies compromising between sensitivity to irradiation and stability with time after irradiation. Stability represent insignificant change in ΔVT of an irradiated MOSFET at room tempera‐ ture for a long‐time period (saved dosimetric information) [43]. Having that immediate dose readout is not always possible, also the exact moment of irradiation is often unknown as in the case of individual monitoring the radiation dose measurements must be performed peri‐ odically. Room temperature stability of irradiated pMOS dosimeters can be determined by

Fading results for pMOS dosimeters with gate oxide thickness of 400 nm and 1 μm, at room temperature previously irradiated with X‐ray (energy 140 keV) up to 1 Gy for *V*irr <sup>=</sup> <sup>0</sup> V and *V*irr = 5 V are presented in **Figures 11** and **12**, respectively [35]. It can be seen that fading of pMOS dosimeters with gate oxide thickness of 400 nm (**Figure 11**), which were irradiated with gate bias *V*irr = 5 V, is about 40% in the first 7 days, whereas those of pMOS dosimeters irra‐ diated without gate bias during irradiation have 22% fading also in the first 7 days. For the time period between 7 and 28 days, fading of pMOS dosimeters irradiated with gate bias 5 V increased for about 3%, whereas fading of pMOS dosimeters irradiated without gate bias dur‐ ing irradiation had a nearly constant value. Fading of 1 μm thick gate oxide pMOS dosimeter (**Figure 12**), which were irradiated up to 1 Gy with gate bias *V*irr = 5 V , in the first 7 days was 14%, whereas for the time period between 7 and 28 days, it increases about 1%. pMOS dosime‐ ters with the same gate oxide thickness, which were irradiated without gate bias the first 7 days, have fading increase for about 1%, and this value is kept up to 28 days. From **Figures 11** and **12**, it can be concluded that fading is lower when the gate oxide of pMOS dosimeters is thicker which in accordance with early study [44] showed that fading decreases with the increase in

The decrease in the positive trapped charge causes fading of pMOS dosimeters. This decrease orig‐

which lead to their neutralization/compensation and change in threshold voltage shift [45].

; once captured at positive oxide trapped charge,

effect, while gamma‐ray photons with energies of 1.17 and 1.33 MeV lead to SiO<sup>2</sup>

molecules. Namely, X‐ray

molecules

quence of different photon energies which lead to ionization of SiO<sup>2</sup>

cantly higher than by Compton's effect.

242 Radiotherapy

calculating fading using Eq. (4).

gate oxide thickness.

inates from electron tunneling from Si into SiO2

**4. Fading of irradiated pMOS dosimeters**

**Figure 11.** Fading *F* at room temperature for 30 days of pMOS dosimeter with 400 nm gate oxide thickness previously irradiated with X‐ray (140 keV) radiation dose of 1 Gy. Gate bias during irradiation was *V*irr <sup>=</sup> <sup>0</sup> V and *V*irr <sup>=</sup><sup>5</sup> V.

**Figure 12.** Fading *F* at room temperature for 30 days of pMOS dosimeter with 1 μm gate oxide thickness previously irradiated with X‐ray (140 keV) radiation dose of 1 Gy. Gate bias during irradiation was *V*irr <sup>=</sup> <sup>0</sup> V and *V*irr <sup>=</sup> 5 V.

#### **5. pMOS dosimeter reuse**

For a while, it was widely thought that pMOS dosimeters could not be used for subsequent determination of radiation dose. They were, namely, just used to determine the maximum radiation dose, after which they would be replaced. However, studies on the pMOS dosim‐ eter reuse are given in [46] for radiation dose 400 Gy. Recent work has shown that irradi‐ ated pMOS dosimeters manufactured in Tyndall National Institute, Cork, Ireland, could be annealed at room and elevated temperature and reused for ionizing radiation measurements. **Figures 13** and **14** show the threshold voltage shift Δ *V*T as a function of gamma radiation dose *D* for gate bias *V*irr = 5 V and *V*irr = 0 V , respectively, for both the first and second irradiation [47, 48]. After the first irradiation, the pMOS dosimeters were annealed at room temperature for 5232 h without gate bias. Latter, the annealing process was continued at 120 o Cwithout gate bias for 432 h. The pMOS dosimeters were then irradiated under the same conditions. It can be seen from **Figure 13** that the values of Δ *V*T during the first and second irradiation are very close. For pMOS dosimeters irradiated with the gate bias *V*irr = 0 V (**Figure 14**), the val‐ ues of Δ *V*T are higher for the second than for first irradiation. Such results are contradictory with earlier results [46] for pMOS dosimeters irradiated up to 400 Gy where it was shown that the values of Δ *V*T during the first irradiation (for *V*irr = 5 V and *V*irr = 0 V) were higher than the values obtained during the second irradiation.

**Figure 13.** Dependence of the threshold voltage shift *Δ VT* in pMOS dosimeters with 400 nm gate oxide thickness on the gamma‐ray radiation dose *D* in the 0–35 Gy range during the first and second irradiation with gate bias *V*irr <sup>=</sup><sup>5</sup> V.

**5. pMOS dosimeter reuse**

244 Radiotherapy

the values obtained during the second irradiation.

1st 2nd

0

2

VT (V)

4

For a while, it was widely thought that pMOS dosimeters could not be used for subsequent determination of radiation dose. They were, namely, just used to determine the maximum radiation dose, after which they would be replaced. However, studies on the pMOS dosim‐ eter reuse are given in [46] for radiation dose 400 Gy. Recent work has shown that irradi‐ ated pMOS dosimeters manufactured in Tyndall National Institute, Cork, Ireland, could be annealed at room and elevated temperature and reused for ionizing radiation measurements. **Figures 13** and **14** show the threshold voltage shift Δ *V*T as a function of gamma radiation dose *D* for gate bias *V*irr = 5 V and *V*irr = 0 V , respectively, for both the first and second irradiation [47, 48]. After the first irradiation, the pMOS dosimeters were annealed at room temperature for 5232 h without gate bias. Latter, the annealing process was continued at 120 o Cwithout gate bias for 432 h. The pMOS dosimeters were then irradiated under the same conditions. It can be seen from **Figure 13** that the values of Δ *V*T during the first and second irradiation are very close. For pMOS dosimeters irradiated with the gate bias *V*irr = 0 V (**Figure 14**), the val‐ ues of Δ *V*T are higher for the second than for first irradiation. Such results are contradictory with earlier results [46] for pMOS dosimeters irradiated up to 400 Gy where it was shown that the values of Δ *V*T during the first irradiation (for *V*irr = 5 V and *V*irr = 0 V) were higher than


D (Gy)

**Figure 13.** Dependence of the threshold voltage shift *Δ VT* in pMOS dosimeters with 400 nm gate oxide thickness on the gamma‐ray radiation dose *D* in the 0–35 Gy range during the first and second irradiation with gate bias *V*irr <sup>=</sup><sup>5</sup> V.

**Figure 14.** Dependence of the threshold voltage shift *Δ VT* in pMOS dosimeters with 400 nm gate oxide thickness on the gamma‐ray radiation dose *D* in the 0–35 Gy range during the first and second irradiation with gate bias *V*irr <sup>=</sup> <sup>0</sup> V.

## **6. Low‐cost commercial p‐channel MOSFETs as pMOS dosimeters**

In recent years, many investigations were driven toward application of low‐cost commercial p‐channel MOSFETs as a dosimeter in radiotherapy [49]. Asensio et al. [50] show results of some most important dosimetric parameters (sensitivity, linearity, reproducibility and angu‐ lar dependence) for power p‐channel MOSFETs 3N163. These transistors were irradiated by gamma‐rays originating from 60Co up to 55 Gy. These devices were irradiated without gate bias (*V*irr = 0 V ). **Figure 15** shows the Δ *V*<sup>T</sup> = *f*(*D* ) dependence for 15 devices. The data showed excellent linearity with a mean sensitivity value of 29.2 mV/Gy and reasonable good reproducibility. Moreover, the angular and dose rate dependencies are similar to those of other, more specialized pMOS dosimeters. The authors of this paper concluded that power p‐channel MOSFET 3N163 would be an excellent candidate for low‐cost system capable of measuring gamma‐radiation dose.

The possibility of vertical diffusion MOS also called double‐diffusion MOS transistor or simple DMOS as a sensor of electron beam was also investigated [51] These devices were DMOS BS250F, ZVP3306 and ZVP4525, manufactured by Diodes Incorporated (Plano, USA). The irradiation was performed by an electron beam of 6 MeV energy without gate bias. The same authors investigated the behavior of p‐channel MOS transistors from integrated circuit CD4007 (Texas Instruments, Dallas, USA and NXP Semiconductor Eindhowen, Netherlands) under 6 MeV energy electron beam. In **Figure 16**, the Δ *V*T versus *D* is plotted four samples of the ZVP3306 DMOS transistors. The results for other type DMOS transistors are similar. As it can be seen, there is a linear depen‐ dence between Δ *V*<sup>T</sup> and *D* to radiation dose of 25 Gy. Values of sensitivity for BS250F, ZVP4525 and ZVP3306 are 3.1, 3.4 and 3.7 mV/Gy, respectively. It was also shown [51] that p‐channel MOS

**Figure 15.** Threshold voltage shift *Δ V*T in p‐channel MOSFETs 3N163 as a function of gamma‐ray radiation dose *D* in the 0–58 Gy range. Gate bias during irradiation was *V*irr <sup>=</sup> <sup>0</sup> V.

transistors from integrating circuits CD4007 during irradiation without gate bias (*V*irr = 0 V) pre‐ sented the sensitivity 4.6 mV/Gy with a very good linear behavior of the threshold voltage shift compared to the radiation dose. Moreover, with the possibility of applying thermal compensa‐ tion, this transistor may be a promising candidate in radiotherapy.

**Figure 16.** Threshold voltage shift *Δ V*T in DMOS ZVP3306 as a function of 6 MeV electron beam radiation dose *D* in the 0–25 Gy range.
