Comparison of the MOSFET Response at Exposed of the X-Ray and Gamma Radiation

Introduction. Electromagnetic or ionizing radiation has great penetrating power. Currently, in literature there is general opinion on complete radiation response of MOSFETs to various types of ionizing radiation. That is why radiation resistance of MOSFETs (metal–oxide–semiconductor field-effect transistors) and integrated circuits is of great interest. Objective. The purpose of this study is to compare the radiation response of MOSFETs to gamma and X-ray irradiation and assess the effect of applying external gate-substrate potential on dose dependences of the threshold voltage change in MOSFETs. Materials and methods. This study considers MOS transistors with a polysilicon gate with an oxide (silicon dioxide) thickness of 120 nm. Silicon dioxide acts as a dielectric in MOS structures. Cesium-137 radionuclides are the source of gamma radiation; an X-ray tube with a tungsten-rhenium cathode is a source of X-ray radiation . The change in the threshold voltage of n- and p-channel transistors is analyzed using dual transistor approach. Results. Strong influence of gamma and X-ray radiation brings the same effects in structures under investigation. Voltage applied to the MOS structures during X-ray irradiation had a strong effect on their radiation response. The maximum radiation response of MOS transistors was observed at high positive gate – substrate potentials. Proportionality coefficients are introduced to ensure that the initial sections of the dose dependences coincide for various applied gate – substrate potentials. The coefficients allow comparing active and passive modes of operation of the X-ray emitter. Conclusions. The values of the proportionality coefficients of the dependence between the threshold voltage change of the MOS transistor and the ionizing radiation dose are determined. A numerical correlation is estab-lished between the effects of gamma and X-ray radiation at doses up to 1.9·10 4 rad (the proportionality coefficient is 38.5). The proportionality coefficients are determined, which allow us to compare the passive gamma-ray irradiation mode (with no potential applied) and the active X-ray irradiation mode (with gate ‒ substrate potential applied). The correction coefficients obtained depend on the polarity of the applied gate - substrate potential. For the negative potential the proportionality coefficient is 38.5. With positive polarity applied the coefficient does not depend on the applied potential and equals 120.

Introduction. Comprehensive development of defense, space and nuclear industries implies using semiconductor integrated circuits (SIC) based on MOSFETs (metal-oxide-semiconductor field-effect transistors) which must remain operational when exposed to various types of irradiation for a long period of time [1]. Various types of ionizing radiation with high penetrability (gamma and X-ray radiation) are of prime interest.
Gamma rays are produced in the decay of radioactive isotopes, and X-rays are produced when a cathode is bombarded with high-energy particles when high voltage is applied to an Xray tube. Due to MOSFETs exposure to ionizing radiation short-term charge effects occur in metal and in the semiconductor, and long-term charge effects occur in the oxide [2,3].
When the gate oxide is exposed to ionizing radiation, electron-hole pairs (EHPs) are produced which are later separated by oxide-chargeinduced field. Due to their higher mobility, electrons leave the oxide in picoseconds and holes in accordance with the hopping mechanism move to the oxide phase interface, where they are captured by bulk traps, introducing a positive charge into the oxide [1,4]. Due to radiation-induced holes and bulk traps interaction, hydrogen is released, which is involved in the depassivation of amphoteric surface states [5,6].
When the threshold voltage is applied, surface states are charged positively for p-channel transistors and negatively for n-channel ones.
The oxide net charge determines the total radiation response of the MOSFET, which is expressed in the shift of the threshold voltage due to radiation effects [3]. Currently in literature there exist different views on the radiation response of MOSFETs when exposed to different types of ionizing radiation. In [7], the radiation responses of MOSFETs when exposed to γ-radiation from 137 Cs and X-ray radiation at energy of 10 keV are compared. The authors have shown that under gamma exposure the proportion of EHPs that escaped the initial recombination, and the change in the parameters of the structures under consideration, are greater than under X-ray irradiation. The results in [7,8] are identical. However, the experimental results in [9] contradict the results obtained in [7,8]. The authors studied the response of the threshold voltage of radiation sensors, which are p-channel MOSFETs with increased gate-oxide thickness when exposed to γray quanta from 137 Cs and x-ray quanta at energies of 90 and 140 keV with gate potential of +5 V and with no potential during irradiation. It is shown that the maximum radiation response is the given literature data were obtained at low radiation doses. At present in the literature there is no general consensus on the radiation response of MOSFETs exposed to small doses of various ionizing irradiation types when gate-to-substrate potential is applied, therefore this problem requires further detailed consideration.  The correlations between the change in the threshold voltage and the lengh of X-ray radiation exposure for n-and p-channel MOSFETs are shown in Fig. 3.
The correlaions for X-ray and gamma irradiation presented in fig. 2 and 3  where it has practically no effect on the surface It should be noted that the obtained proportionality coefficient between passive modes of gamma and x-ray irradiation, which is equal to 38.5, coincides with the proportionality coefficient between passive gamma-irradiation and various active modes of X-ray irradiation with a negative gate -substrate potential.
In active modes with positive polarity the coefficient is about three times the indicated value and amounts to 120. For both negative and positive polarity in the active X-ray mode, the proportionality coefficient in the considered gatesubstrate voltage range is independent of the potential.
Taking into account the correction proportionality factors, the dose dependences of the threshold voltage change with various potentials applied to the gate under X-ray and gamma exposure are shown in Fig. 7.
Results. Similar nature of dose dependences for gamma and X-ray irradiation, correlating with the data of the authors of [7], defines the relationships using a single approximating curve.
This indicates the equivalence of parameters of formation, transfer and capture processes of mo- Fig. 7. Dependences of threshold voltage shift for n-and р-channel MOSFETs on the gate-substrate potential applied when exposed to irradiation, adjusted for the correction proportionality coefficient. Solid curve is for X-ray irradiation, dashed curve is for gamma irradiation. Markers are for measurement results