Electron Irradiation on Metal Oxide Silicon Field Effect Transistors in Space Applications

The space environment is hostile to most semiconductor electronic devices and components used for space applications. The radiation generally encountered in space are α, β, γ, x-ray, energetic electrons, protons, neutrons and ions of various kinds. Especially the Van Allen Belts consist of high energy electrons which are in continuous motion. Satellites systems operating in Low Earth Orbits (LEO) are prone to get exposed to high energy electrons. This paper describes the effect of electron beam irradiation on MOSFETs planned for space applications. The devices selected for the study are 2N6768 n- channel MOSFETs (JANTXV) procured from ISAC, Bangalore. The decapped MOSFETS are exposed to a beam of electrons for various doses ranging from 50 Gy to 10 KGy in the electron energy range 7 – 10 MeV using the electron accelerator facility at RRCAT, Indore. Pre and post- irradiation measurements of the electrical characteristics are undertaken to investigate the electron induced device degradation and damage. The dominant mechanism of the interaction of the electrons with semiconductors is to produce atomic displacement which can have serious impact on electrical characteristics. MOS (Metal oxide semiconductor) devices are the most sensitive of all semiconductor devices to radiation showing considerable degradation even for a relatively low dose of exposure to high energy electrons. The energy dissipated by electrons in semiconductors is sufficient to displace silicon atoms and causes a shift in the threshold voltage and leakage current. The study indicates that the gate threshold voltage substantially decreases with the increase in electron dose. The leakage current increases with dose which could have an impact on the performance of the device. The transconductance of the MOS transistor is found to decrease with increase in electron dose. The observed changes in the electrical characteristics upon electron irradiation are analysed and interpreted as due to trapped charges in the SiO2 layer and at the interface. The present investigation reveals that there is a remarkable degradation in threshold voltage and the leakage current displays substantial increase when the devices are exposed to electrons. This increase in leakage current can have significant impact on device performance in a radiation environment.