Investigation of Oxide and Interface Trapped Charge on Threshold Voltage Shift in Gamma Irradiated NMOSFET by Subthreshold Method

Abstract

In this paper, the oxide trapped charge density (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OT</sub> ) and interface trapped charge density (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IT</sub> ) on the threshold voltage shift with the total dose under gamma irradiation of NMOSFET are investigated. The subthreshold technique (mV/dec) is used to separate the irradiation induced threshold voltage shifts into voltage shifts due to the interface trapped charge. The linear extrapolation method is used to extract the threshold voltage from I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> -V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> curve at low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> of typically 100 mV. The results showed that the threshold voltage was shifted from 0.69 V to 0.45 V, the threshold swing was increased from 95 to 107 mV/dec, the oxide charge and interface charge density were found to be increase almost by 1 order in magnitude for a total gamma dose of 10 kGy.