Radiation modelling of vacuum field emission devices

Abstract

Recent advances in micro and nanofabrication techniques have enabled modern vacuum field emission devices (VacFEDs) and have been demonstrated in the laboratory for use as diodes and transistors. Modern VacFEDs operate through cold emission of electrons across a vacuum gap. It has been proposed that these devices are "radiation insensitive" since they do not have a solid state junction as in other modern electronic devices. Radiation testing has been conducted to characterize the radiation response for these devices however, minimal supporting modeling has been performed. This thesis attempts to model and quantify the radiation effects of modern VacFEDs.

It focuses primarily on two effects associated with ionizing radiation exposure to a VacFED diode materials and structure: 1) The production of a net electron Direct Drive (DD) current in conductive layers due to imbalance in ionization rates in device layers and 2) Radiation Induced Conductivity (RIC) due to creation and drift of electron-hole pairs across an electric field of a dielectric insulating layer. These currents are treated as a noise sources that compete with the output signal of the device. Two radiation transport codes are used quantify interaction, electron charge and energy deposition of consequence to direct drive and RIC effects: 1) CEPXS/ONEDANT: a 1-dimensional electron-photon discrete ordinates code package and 2) MCNP6: a general-purpose, continuous-energy, generalized-geometry, time dependent, Monte Carlo radiation-transport code. RIC response was found to have the greatest current for all device models considered over all energies.

This thesis found a dose rate of 6 x 106 rad(Si)/s at the surface of a VacFED diode is required to cause a 0.1 [mu] A noise current in a device designed to operate at 1.0 [mu]A. This finding suggests that VacFED technology has the capability to operate continuously in a modern pressurized water nuclear reactor core gamma ray environment, which has an approximate dose rate of 3 x 105 rad(Si)/s.