Andrew Holmes-Siedle

RADFET: A review of the use of metal-oxide-silicon devices as integrating dosimeters

Abstract Calibrated, radiation-sensitive metal-oxide-silicon field-effect transistors (RADFETs) have been launched into space and used in the laboratory to measure the doses from a variety of radiation sources. These experiments have demonstrated that the RADFET provides a convenient method for the continuous monitoring of total dose. The electrical output consists of a d.c. voltage which can be converted electronically to a value for accumulated dose. The voltage can be read remotely and displayed continuously. This review briefly outlines the physical mechanisms by which radiation dose is registered by RADFETs, describes the characteristics and performance of a practical RADFET and discusses applications. In addition to the name “RADFET”, these devices have been called “MOS Dosimeters”, “Mosimeters” and “Space Charge (SC) Transducers”.

The Development of an MOS Dosimetry Unit for Use in Space

A space qualified integrating radiation dosimeter, using a special extra-sensitive metal-oxide-semiconductor (named the TOT 201 transistor) as the sensing device has been put into geostationary orbit aboard a European ccmTunication satellite (OTS). This paper describes the fabrication and calibration of a specially sensitive MOS transistor (sensitivity of 3.8 10-4 Volt rad-1) and the design and calibration of × the flight circuit. This is the first known use of a special MOS device to measure the internal environment of a space vehicle. Weight size and power demand are so low that the unit could be widely used as a housekeeping monitor in future orbital payloads.

A Simple Model for Predicting Radiation Effects in MOS Devices

A mathematical model for the electrical consequence of charge buildup in metal-oxide-semiconductor devices has been investigated and found to be useful. It can be used (a) for comparing experimental data from different sources and (b) for making predictions of the lifetime of MOS circuits exposed to radiation in space or laboratory environments. A wide range of experimental data has been collected together and is presented in the form of a normalised oxide radiation-sensitivity parameter A, representing the probability of hole capture. The results show that the simple model, based upon a thin sheet of hole traps, must be modified when the rate of interface-state creation is commensurate with the trap filling rate but that this does not destroy its usefulness for a wide range of applications.

Thick oxide pMOSFET dosimeters for high energy radiation

Abstract This work examines the response of pMOSFET dosimeters to ionizing radiation. The dosimeters were fabricated with gate oxides having a range of thicknesses varying from 0.69 μm for the thinnest oxide up to 2.3 μm for the thickest oxide. In separate experiments the dosimeters were irradiated by 60Co γ-rays and linac X-rays both with biassed and unbiassed gates. The effects on transistor characteristics were measured and analysed to show the relative contributions to the shift in the threshold voltage from trapped charge in the oxide and charges trapped at the silicon-silicon dioxide interface.

Calibration and Flight Testing of a Low-Field pMOS Dosimeter

This paper presents the results obtained from pMOS dosimeters in the Geostationary Orbit for a period of 7 years, on board European spacecraft. One dosimeter monitored radiation dose in a microwave experiment on the OTS spacecraft and five others formed a self contained `dose vs depth monitor on the GEOS 2 spacecraft. The doses deduced from the measurements show good agreements with standard environmental prediction models for the case of thickly shielded devices (thickness above 3 mm aluminium). However, for thinner shields, dose values are generally lower than predicted. Furthermore, a large annual variation in dose is observed and this appears to have an inverse correlation with solar activity. Parallel laboratory studies included many experiments on the operation of pMOS dosimeters in zero bias (VIO) modes. At low doses, good linearity for threshold voltage shift vs. dose was found in this mode. These results were unexpected and may broaden the use of the MOS structure as a general-purpose dosimeter.

The Mechanisms of Small Instabilities in Irradiated MOS Transistors

When a metal-oxide semiconductor (MOS) device is irradiated, charge builds up in the oxide and new interface states are also created. The charge and discharge of slow interface states cause the flat-band and threshold voltage of a device held under bias to drift with time. The effect is distinctfrom ionic instabilities. Even small radiation-induced drift effects may be of importance in precision devices such as MOS comparators and regulators. Some recent data arising from experiments on precision MOS devices, - mainly using gamma irradiation are presented and methods of minimising radiation-induced slow drift effects are discussed.

MOS dosimeters-improvement of responsivity

Radiation-sensitive field effect transistors (RADFETs), based on the metal-oxide-semiconductor transistor, are used as remotely and non-destructively read dosimeters in spacecraft and in medicine. The RADFET has evolved from the simple MOS transistor device by a research on oxide thickness dependence of charge buildup and extensive dosimetric performance measurements. To develop a product line, RADFET devices with increasingly thick oxides have been made. Some results of this work are described.<<ETX>>

Diamond Vacuum Electronic Device Behavior After High Neutron Fluence Exposure

This paper reports the first neutron exposure on diamond electronic test structures for their possible application in very high fluence neutron conditions. The behavior of diamond lateral emission diodes after high neutron fluence of 4.4times1013 neutrons/cm2 is evaluated. No noticeable changes in the device structure and electrical behavior, specifically resistivity, dilation and emission characteristics were observed.

Dosimetric Silica Films: The Influence of Fields on the Capture of Positive Charge

The influence of oxide field and dose magnitude on the buildup of positive charge in dosimetric silica films has been studied in experiments using x-and gamma rays in the 0.1 to 100 Krad region. Specially-prepared metal-oxide-semiconductor (MOS) transistors and capacitors on n-type silicon were used. By the use of a normalisation method, the results have been compared with many earlier studies on MOS capacitors and commercial MOS transstors. At low fields, response rises sharply with field. At higher fields, a plateau region may or may not be followed by a region of negative slope. The relative importance of interface and bulk charge is discussed.

Stacked p-FET dosimeter for the STRV-2 MWIR detector

A stacked p-FET dosimeter consisting of a RADMON mother chip with three p-FETs and multiplexer and an attached RADFET has been developed for the STRV-2/MWIR detector. Calibration of the dosimeter, using an Am-241 source, indicates that the RADFET is about 20 times more sensitive to radiation than the RADMON. This dosimeter is expected to measure radiation dose from rads to Megarads.