TID (Total Ionizing Dose) Effect – Accumulated Ionization Damage
Charges generated by ionizing radiation can accumulate within a semiconductor, gradually altering its properties. This phenomenon is described by the term TID (Total Ionizing Dose). In CMOS devices, charges generated by ionizing radiation may become trapped in insulating layers or create defects in the crystal structure.
As gate oxide layers become thinner, the impact of radiation on the gate stack is reduced (due to less oxide thickness to trap charge). However, thick field oxides surrounding the conductive channel can still experience damage and charge trapping. In SOI (Silicon-On-Insulator) devices, charge trapping in the corner regions is particularly sensitive.
TID is primarily caused by ionizing radiation such as electrons, protons, gamma rays, and heavy ions. Each particle type exhibits different energy levels, penetration depths, and LET characteristics. While neutron-induced TID is generally negligible in sea-level or standard accelerator tests, ultra-cold neutrons and thermal neutrons are known to gradually induce semiconductor degradation via TID.
TID affects several parameters in CMOS devices such as threshold voltage (Vₜ), leakage current, and subthreshold slope. Threshold voltage may shift by tens to hundreds of millivolts, and leakage current can increase by several orders of magnitude. These changes lead to degraded switching characteristics, increased power consumption, or even device failure.
TID is quantified by multiplying the LET of the radiation particle by the fluence of a monoenergetic beam, yielding the total energy deposited per unit mass through ionization. TID is commonly expressed in units of rad(Si) or krad(Si).
1 krad(Si) corresponds to 10 Gy, and space-grade semiconductors are typically designed to withstand over 100–300 krad(Si).
In contrast, commercial CMOS devices may exhibit performance degradation even at several tens of Gy.
Schematic overview of TID damage mechanism
Image source: F. B. McLean and T. R. Oldham, HDL-TR-2129, via Sandia National Laboratories SAND2013-4379C
Related Articles