3.1 The effect of total dose irradiation on threshold voltage

Fig.2 shows the measured I-V characteristics of the front-gate and back-gate for PDSOI NMOSFETs before and after irradiation (V_D=0.1 V). The X-axis indicates the gate applied voltage and the Y-axis indicates the logarithm of the drain current.

Fig.2Full-screen View

Measured transfer characteristics of body-grounded device for (a) front-gate and (b) back-gate

Both Fig.2(a) and (b) show that the off-state leakage current increases significantly when the irradiation dose is 300 krad(Si). The radiation bias is ON-state (i.e. VG=5 V), which forms a high electric field in the gate oxide. And then the radiation inducing electrons and holes quickly separate and migrate to the positive and negative electrodes in the oxide, respectively. Electron mobility is greater than hole mobility, so the recombination mechanism of free electrons and holes is weakened. As the density of positive charges increases, more and more effective oxide charges migrate to the SiO₂/Si interface and attract electrons near the interface. When the concentration of electrons is large enough, the parasitic conducting channel is formed, which causes the leakage current to increase.

Regarding the I-V characteristics of the front-gate and back-gate, shown in Fig.2, the leakage current reaches up to 10^-10~10^-9 A when the test gate voltage is 0 V. However, in the front-gate I-V characteristics, a greater number of positive charges migrate to the interface as the gate voltage increases, which leads to a large leakage current as a function of the total dose. When the total dose is 300 krad(Si), there is an exponential growth in the leakage current. However, the test voltage for back-gate I-V characteristics is applied on the substrate; substrate voltage has a slight effect on the electrons in the channel thanks to the isolation of buried oxide in SOI devices.

Using linear extrapolation, this work presents the degradation of threshold voltage as a function of total dose. Fig.3 (a) and (b) indicate the different phenomena that occur between the body-grounded devices and floating-body devices. For the body-grounded devices, shown in Fig.3(a), the negative shift quantity of the threshold voltage ($\Delta V_{th}$) increases after irradiation. When the total dose is 200 krad(Si), the shift is 30.4% for the front-gate. When the total dose is 300 krad(Si), the shift increases to 40.9%. For the back-gate, the degradation of threshold voltage is much more significant.

Fig.3Full-screen View

Normalization $\Delta V_{th}$ of front-gate and back-gate as a function of irradiation dose for (a) body-grounded devices, (b) floating-body devices, and (c) body-grounded devices in high dose conditions

When dealing with the floating-body devices, as shown in Fig.3(b), the $\Delta V_{th}$ of per unit dose shifts toward the negative direction at the outset. For the front-gate, the negative shift increases from 5.2% to 38.3% as the dose increases from 50 krad(Si) to 200 krad(Si). However, the threshold voltage then shifts toward positive values as the dose increases further. The negative shift decreases from 38.3% to 33.4% as the dose increases from 200 krad(Si) to 300 krad(Si).

The above phenomenon of negative drift turning to positive drift is referred to as a “rebound”. This rebound phenomenon has been mentioned in some studies, appearing for higher doses (i.e., up to 1 Mrad(Si)^[10,11]). However, in this paper, the phenomenon arises when the dose increases from 200 krad(Si) to 300 krad(Si). Fig.3(c) shows that the threshold voltage of body-grounded devices shifts in a negative direction as the dose increases, even as it reaches 1 Mrad(Si). For the front-gate, the negative shift increases from 38.2% to 62.2% as the dose increases from 200 krad(Si) to 500 krad(Si). The negative shift can increase even to 91.3% when the dose is 1 Mrad(Si).

The threshold voltage of PDSOI MOS devices is similar to that of bulk silicon devices. The threshold voltage of enhancement-mode nMOSFET is given by

where, $V_{FB0}$ is the flat band voltage, $N_A$ is the doping concentration in silicon film, ${\varphi }_F$ is femi potential, ${\varphi }_F=\frac{kT}{q}\ln \left(\frac{N_A}{n_i}\right)$, $x_{d\max }\left(=\sqrt{2{\epsilon }_{Si}\left(2{\varphi }_F+V_B\right)/q{N}_A}\right)$ is the maximal width of the depletion layer, $V_B$ is the voltage of the body region, which is equivalent to the substrate bias of bulk silicon devices, $C_{ox}$ is the oxide capacitance of front-gate.

Additionally, the threshold voltage for the irradiation condition is given by

where $N_{ot}$ is the density of the oxide charge and $N_{it}$ is the density of the interface state.

The factor that causes the threshold voltage shift is divided into two parts: the trapped oxide charges and interface states, given by

where $\Delta V_{it}\left(=q{N}_{it}/C_{ox}\right)$ is the contribution of the threshold voltage shift due to the interface states, and $\Delta V_{ot}\left(=q{N}_{ot}/C_{ox}\right)$ is the contribution of the threshold voltage shift due to the trapped oxide charges.

From the I-V characteristic of MOSFET, the drain current is given by

where $I_D$ and VDS are the drain current and voltage, respectively, W/L is the gate width/length, $n_i$ is the intrinsic carrier concentration, β=q/kT, and φ is the surface potential.

The constant $\alpha$ is given by

where $t_{ox}$ is the oxide thickness, $L_D$ is the Debye length given by $L_D={\left[{\epsilon }_{Si}/(\beta q{N}_A)\right]}^{1/2}$, ${\epsilon }_{Si}$ and ${\epsilon }_{ox}$ are the dielectric constants of Si and SiO2 respectively, and $\mu$ is the carrier mobility, which is a radiation sensitive parameter. The relationship between threshold and carrier mobility can be shown as

If the $V_{DS}$ is very small (i.e. $V_{DS}\ll 2\left(V_{GS}-V_{th}\right)$), (6) can be simplified into (7):

The carrier mobility $\mu$ can be obtained by equation (7) according to the experiment data of $I_D$ and $V_{GS}$. The midgap current is defined to be the current, which occurs when the surface potential equals φF (i.e., $\varphi ={\varphi }_F(=\left(kT/q\right)\ln \left(N_A/n_i\right))$). Using the carrier mobility $\mu$ and femi potential ${\varphi }_F$ for equation (4), we can obtain the midgap current. Because the midgap current belongs in the range of 10-14~10-13A, it is essential that the sub-threshold of the I-V curve is linearly extrapolated in the negative direction.

The midgap voltage shift in sub-threshold curves due to trapped oxide charges, which is given by

where the labels 2 and 1 refer respectively to subthreshold-current curves at different radiation levels.

The difference between the threshold voltage and midgap voltage is defined as stretchout voltage Vso, given by

The threshold voltage shift due to the interface state is as follows:

Using the sub-threshold separation technology, the factors $\Delta V_{ot}$ and $\Delta V_{it}$ for the floating-body and body-grounded devices are shown in Fig.4 (a) and (b) respectively, which is a function of the irradiation dose. For the body-grounded devices, the $\Delta V_{it}$ shifts toward a negative value in the first instance. When the dose increases from 150 krad(Si) to 200 krad(Si), this change occurs slowly. The “rebound” phenomenon arises when the dose reaches more than 200 krad(Si). Meanwhile, the $\Delta V_{ot}$ shifts toward a negative value as a function of the dose, the quantity of which is larger than it is for $\Delta V_{it}$. The above factors cause $\Delta V_{th}$ to shift toward a negative value as a function of the dose. For the floating-body device, when the dose increases to over 200 krad(Si), the “rebound” phenomenon arises for both $\Delta V_{it}$ and $\Delta V_{ot}$, causing the threshold voltage to shift toward a positive value.

Fig.4Full-screen View

$\Delta V_{th}$, $\Delta V_{it}$, and $\Delta V_{ot}$ as a function of irradiation dose of the front-gate for (a) body-grounded and (b) floating-body

Because of the special structure of SOI devices, the total dose radiation induces a large number of positive charges in the buried oxide layer. For floating-body devices, the potential of buried oxide increases as the charges increase. This attracts electrons to move near the interface, which weakens the channel’s formation. However, for body-grounded devices, the incremental potential of buried oxide has little effect on the body because of its constant voltage. Thus, the threshold voltage shifts in a negative direction.

3.2 The effect of the dose rate on threshold voltage

Fig.5 shows front-gate and back-gate I-V curves for PDSOI nMOSFETs. There are discrepancies under different dose rates (i.e. the high dose rate is 50 rad(Si)/s and the low dose rate is 1 rad(Si)/s). A lager shift of the sub-threshold current at the low dose rate is observed than that at the high dose rate. The total dose radiation induces positive charges that are proportional to the thickness of the oxide. For a measured characteristic of the back-gate, more serious degradation can be found at lower dose rates.

Fig.5Full-screen View

Measured transfer characteristics as a function of irradiation dose for (a) front-gate and (b) back-gate under different dose rate conditions

Fig.6 shows the threshold voltage shift for the front-gate and back-gate at different dose rates. At the beginning of irradiation, we observed a similar shift of threshold voltage at different dose rates. When the dose value increases further, the degeneration of threshold voltage worsens under low dose rate conditions. In addition, as the irradiation dose reaches 50 krad(Si), the threshold voltage shift under low dose rate conditions is larger than the shift under high dose rate conditions. For obtaining the same threshold voltage shift, more dose are needed for high dose rate conditions than that of low dose rate conditions. These results indicate enhanced low dose rate sensitivity (ELDRS) of H-gate partially depleted SOI MOSFET devices for 60Co γ-irradiation.

Fig.6Full-screen View

Normalization $\Delta V_{th}$ of (a) front-gate and (b) back-gate under different dose rate conditions

Fig.7 shows the shift of $\Delta V_{th}$, $\Delta V_{it}$ and $\Delta V_{ot}$ for the front-gate using sub-threshold separation technology. It is noted that the trapped oxide charges, as well as the interface states, are responsible for the ELDRS effects in SOI devices. Additionally, the interface states play a significant role in the threshold shift under low dose rate conditions. This threshold voltage shift as a result of the interface states is much larger than those occurring as a result trapped oxide charges (i.e., when the irradiation dose is 100 krad(Si), the $\Delta V_{ot}$ is -0.07 V and the $\Delta V_{it}$ is -0.19 V in the lower dose rate experiments).

Fig.7Full-screen View

$\Delta V_{th}$, $\Delta V_{it}$ and $\Delta V_{ot}$ of the front-gate as a function of irradiation dose under low dose rate conditions

The interface is a transition region that constitutes oxide-nonoxide and ordered-disordered structures. This includes the bending bond of oxide cavities $\equiv S{i}_3-Si\equiv$ and the bond of $\equiv S{i}_3-OH$ or $\equiv S{i}_3-H$, which is formed during the thermal growth process. Both of these are easily broken to form the dangling bond of tri-valent silicon (i.e. the interface state) under ionizing radiation. Edward H Poindexter[19] gave the name “dangling bond” to the Pb center. In most cases, the density of weak bonds is higher when the distance to the interface is shorter.

Irradiation induced excitons react with oxide to form the neutral hydrogen atoms $H^0$. Consequently, the trapped hole becomes $H^{+}$, which shifts toward the interface because of the additional electric field ($V_G\ge 0$). This reaction is given by[20]

The reaction produces the center of the interface state Pb. The above process is called the $H^{+}$ model.

Moreover, the relationship between the generation of electron-hole pairs and dose rate is given by

where $g_0$ is the generation rate of electron-hole pairs, $D$ is the dose rate, $E$ is the electric field, and $E_0$, $E_1$ and $m$ are constants.

Because hole mobility is greater than the mobility of $H^{+}$, trapped oxide holes form electrostatic barriers, which can prevent $H^{+}$ from reaching the Si/SiO2 interface and generating interface states. At the lower dose rate, the buildup process of oxide charges is weakened. Both the holes and $H^{+}$ can transport to nearby the Si/SiO2 interface, and participate in the formation of trapped oxide charges and interface states.

By using the experimental data from ID and VGS, the carrier mobility under varying irradiation doses can be obtained from equation (7), as shown in Fig.8, where obvious mobility degradation can be observed. As the trapped oxide charges and interface states increase, the degradation of mobility should be taken into account by ^[21]

Fig.8Full-screen View

Normalization $\mu$ as a function of irradiation dose under different dose rates

where ${\alpha }_{ox}$ and ${\alpha }_{it}$ are fitting parameters.

According to equations (11)-(13), when the density of the interface state increases along with dose, this enhances the scattering effect of the interface state on carriers in the channel, leading to a decrease in the carriers’ mobility, as shown in (14).

Fig.8 also shows that the degradation of carrier mobility worsens at a low dose rate, which is similar to the conclusion of [22]. Thus, this work’s mobility data further verifies that the interface state is the one of the primary factors to cause differences in characteristic degradation under total dose radiation at different dose rates, and also plays an important role in the effect of ELDRS on SOI devices.