Electrical and Computer Engineering ETDs

Publication Date

5-31-1968

Abstract

Previous investigations of the radiation effects in Metal­Oxide-Semiconductor (MOS) devices have shown that ionizing radiation produces a trapped space charge in the oxide which causes a shift in the electrical characteristics of the device. Several models have been proposed to explain the observed change in the MOS device characteristics as a function of nuclear radiation exposure. Although these models have qualitatively explained the change in device characteristics, they have not adequately explained the physical processes producing the charge buildup nor have they considered the detailed conduction and charge trapping processes which exist in the various regions of an MOS structure. This anlaysis of the MOS structure was undertaken to summarize and indicate the deficiencies which exist in the previous models, to investigate the charge trapping characteristics of the SiO2 layer, and to calculate the transient characteristics of an MOS structure. A study of the properties of the SiO2 layer has been performed wherein the applicability of using an energy band model for amorphous SiO2 has been qualitatively investigated. The charge trapping characteristics of the oxide as a function of the radiation exposure have been summarized and the similarities between the ionizing radiation effects in fused silica, quartz, and SiO2 are noted. A broken bond model for radiation effects in the three glass forms, has an advantage over other charge trapping models inasmuch as the trapping in relatively impurity-free SiO2 can be explained. The application of such a model offers plausible explanations for the buildup of the radiation-induced charge in the oxide and subsequent annealing of the charge as a function of temperature and exposure to ultraviolet radiation. In addition, possible radiation hardening techniques for preventing the charge buildup in the oxide are suggested; these are based upon the concepts presented in the discussion of the model. Numerical calculations are presented for the characteristics of an unirradiated one-dimensional MOS structure wherein capacitance, surface field, surface potential, and surface charge density in the silicon are determined for non-equilibrium conditions. These calculations are performed for non­uniform impurity doping profiles in the silicon and for various surface and bulk charge trapping distributions. The conduction characteristics of the oxide layer during and after irradiation are illustrated by both analytical and numerical solutions for the charge and electric field distributions as a function of time. As a result of the change in the electric field distribution produced by the relatively immobile positive charge, it is shown that the mobile charge can remain in the oxide for much longer time intervals than the normal transit time for carriers across the oxide layer.

Sponsors

The U.S. Atomic Energy Commision and Sandia Corporation through a contract with the University of New Mexico

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

William Jackson Byatt

Second Committee Member

Christopher Pratt Leavitt

Third Committee Member

Ahmed Erteza

Fourth Committee Member

Harold Dean Southward

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