An In-Depth Study on Device Variability in 22 nm FD-SOI NMOSFETs Under X-Ray Exposure

Abstract

The effect of total-ionizing-dose (TID) on device-to-device variability in 22 nm fully depleted silicon-on-insulator (FD-SOI) nMOS transistors was experimentally quantified. Across ten W/L geometries, two device types (RVT, LVT), and three irradiation-bias conditions (Off-Stress, Work-Mode, Power-Off), ensembles of N=80 devices per geometry, 4800 transistors in total were irradiated to 300 krad(SiO2) and subsequently characterized. Pelgrom plots of Vth and the current-factor β were analyzed. TID consistently shifted Vth negative and broadened its distribution while preserving the 1/(WL)1/2 law; the extracted AVth increased by about 9%–47% depending on bias/type, with a robust ordering Off-Stress >Work-Mode> Power-Off. In contrast, β exhibited near-zero mean drift and only modest variance growth (typically ≤20 %), indicating a much weaker mobility-driven response. Size-segregated fits confirmed an area-dominated mechanism: small-area devices exhibited the largest variance increase, whereas large-area devices approached the measurement floor. A compact dose-aware Pelgrom model was established, adding a TID-dependent term to AVth that links the Poisson-distributed buried oxide (BOX) trapped charge and the bias-dependent electrostatic coupling, and captures the observed bias ordering and size scaling.