Characteristics of a pMOSFET suitable for use in radiotherapy

P-channel MOSFET as ionizing radiation detector

In this study, the authors examine the responses of radiation-sensitive p-channel MOSFETs to irradiation and subsequent annealing at room temperature and higher temperatures to investigate their use as a dosimeter for measuring ionizing radiation. The response of these transistors to radiation was monitored based on the shift in the threshold voltage as a function of the absorbed dose of radiation. The results showed that the shift in the threshold voltage depended on the densities of traps formed during exposure to ionizing radiation in Si and at the Si-SiO₂ interface on which the charges were captured. We then analyzed the influence of these traps on the characteristics of the MOSFETs, and examined the influence of the gate bias, thickness of the gate oxide, energy of the ionizing radiation, and low doses on shifts in the threshold voltage. Moreover, we annealed the irradiated MOSFETs to determine their ability to preserve a certain dose of radiation over a long period as well as their potential for reuse. We explored the possibility of using commercial p-channel MOSFETs built into various electronic systems as sensors and dosimeters for measuring ionizing radiation. The results showed that they have approximately the same characteristics as radiation-sensitive MOSFETs with 100-nm-thick oxide layers.

Small dose effect in RADFET with thick gate oxide

In order to track "small dose effect" leading to stabilization of RADFETs with 1 μm thick oxide fabricated at Tyndall National Institute, Cork, Ireland irradiation was performed with gamma-rays in the radiation dose interval from 1 to 5 cGy, followed with room temperature annealing for the time of 28 days. Gate bias during irradiation was 0, 2.5 and 5 V. Threshold voltage shift ΔV_T determined from transfer characteristics in saturation were followed during irradiation and annealing. It was shown that there is significant increase in ΔV_T for 1 cGy radiation dose and those are 36.3, 43.3 mV and 45.7 mV for gate bias 0, 2.5 V and 5 V, respectively. For higher radiation doses ΔV_T also increases, however, such increase is much lower. RADFETs fading at room temperature lead to permanent decrease in ΔV_T and after 28 days the threshold voltage for devices is returned to virgin device value. Small dose effect is confirmed and radiation dose of at least 5 cGy is necessary for RADFET stabilization before their use in dosimetric application. After RADFETs stabilization they were irradiated in dose interval from 10 to 50 cGy with gate bias of 0, 2.5 and 5 V. It was shown that there is a linear dependence between ΔV_T and absorbed radiation dose D, for gate bias during irradiation 0, 2.5 and 5 V. Defects responsible for threshold voltage shift, formed during irradiation as well as their neutralization/compensation during annealing, are also discussed.