tosp0804opnspec01.doc23 April 2008 [was tosp0601operationalspec01 30 Nov 2006]ahs

REM "SPACEFET": OPERATIONAL SPECIFICATION FOR USE IN SPACECRAFT

Introduction : The RADIATION-SENSING FIELD-EFFECT TRANSISTOR (abbreviation: RADFET) is a microminiature, integrating radiation dosimeter, well suited to aerospace and physics applications. The appropriate missions involve environmental radiation doses in the rad to megarad range. They include space and military missions, high-energy physics experiments amd nuclear power. The type described here is designed specifically for these applications is called "SpaceFET". RADFETs have a microscopically small sensor volume, measured in cubic micrometres, which offers opportunities for radical new designs of miniature radiation sensing systems. The sensing principles are described below. Compared to other detector systems, a RADFET system is compact and easily coupled with computer power. This document is based on frequently-asked questions from the aerospace sector. It covers the latest RADFET sensor available from REM, the Type TOT600, now well tested in many environments. The new chip carrier is an important part of the design and is based on previous well-proven designs by REM.

History: The first “special dosimetric MOSFET”, REM’s TOT200, flew in space in 1978. A quarter of a century later, commercial interest centres on the clinical MOSFET dosimeter but there is also a specialist market in space applications. REM originated the concept and has been in the "specialist MOSFET dosimeter" business throughout this period. Because the sensor die is minute and readout is convenient, radical new designs of dosimeter are possible. REM is working with semiconductor and medical teams to produce and apply those new designs in space, high-energy physics, nuclear power, medicine and "first reponder " in nuclear emergencies. Over its 40-year career, the RADFET has been tested with a comprehensive range of particles and photons at energies from tens of eV to 23 GeV..

Sensingprinciple

The Chip: The dosimetric information, "oxide trapped charge", QOT, is generated by radiation and stored in the gate oxide film of the device, which weighs a few micrograms.Other trapping phenomena in the structure [so-called "side effects"] are minimized by the design of the silicon device and the software. The charge storage is stable, losses with time being a few percent or less in the normal period of interest (see below).

The Package: The chip carrier materials and geometry are chosen to minimize the stopping or scattering of incoming radiation while providing adequate protection during handling. The weight of the sensor is well below a gram.(see below).

The Readout: The field produced when space-charge is trapped in the gate oxide layer of the FET yields a change in the threshold voltage which can be converted to a value of dose in rad or Gray(Si). The "reader" is a simple "threshold tracker" circuit using microwatts of power. Lookup tables or software can be used to "linearize" the charge buildup curve.

OPERATING PARAMETERS

1. Sensor size and weight:.Dimensions of the die: 1.25x0.65x0.5mm. Dimensions of the sensor head (Carrier Type CCS ): 0.3 x 8.7 x 17 mm, giving a weight of about 100 mg excluding sockets and cables.

2 System power, weight and telemetry requirement:The sensor head is connected via a cable to a reader board of less than 30 x 30 mm. Whole dosimeter system can weigh less than 100 gm. Power drain: milliwatts. Telemetry and command interfaces employ simple protocols, a very low data rate [e.g. one to ten 16-bit voltage readings per sensor per day].

3.Dose ranges :

Total dose range - about 1E6 cGy.

A. Biased modes

Medium-thickness oxides VI = +9V10 to 100,000 cGy

Thick oxidesVI = +9V2 to 5000 cGy

B. Unbiased modes (sensor can be separate from machine during "expose"):

Medium-thickness oxides VI = 0V20 to 2 million cGy

Thick oxides VI = 0V4 to 200,000 cGy

4. Operational Temperature (conservative working range): -20 to +50 deg C. Outside this range, carriers may (a) freeze (b) slowly escape [thermal anneal]

5. Particle Measurement Energy Ranges :Well covered in the literature. Responds to any charged particle which will penetrate the encapsulation used and to photons in the range soft X-rays to gamma rays.Relatively insensitive to neutrons.

6.Viewing constraints in space; collimators may be designed to give any desired acceptance cone up to 270 degrees.

7. Arrays :Very compact arrays of FETs may be made in 1D, 2D or 3D (pitch ~ 0.3 mm).

8. Instrument Position and Configurationn: Sensors placed within electronic boxes of vehicles or may be mounted on the outside of a spacecraft with a variety of slab or spherical absorbers to give a "dose vs. depth curve" for a given orbit.

9. Instrument Reliability & Lifetime. Space environments do not cause any special failure mechanisms. Well proven in missions of well over 5 years in orbit. Dose information in the form of "oxide trapped charge" is stored for many tens of years with only a predictable, small amount of charge loss ("fade). In the case of the REM sensor, fade is 2-3 percent in the first few months after storage and a reduced rate thereafter.

10. Other Operational Constraints :High voltage stress on the gate causes electrostatic breakdown of the thin sensor film;the normal well-known "ESD" protective measures are adequate. As with any package, severe mechanical stress and shock may disruptwiring or crack the chip carrier.The reader analog signal (VT) must be read with millvolt resolution but is tolerant of electrical noise.

REM is available to supply technical advice on the above questions and on the redesign of RADFETS for special applications..

REFERENCES and NOTES on RADFETs

1. Articles on RADFET applications: see

2. tosp0802prbRSPspec02.doc . Technical Note REM-TOSP-08-02prbRSPspec02, REM'S RADFET dosimeter sensor probes - availability, with radiation response curves..." by Andrew Holmes-Siedle, (REM Oxford Ltd, January 30, 2008)

3.Bibliography of research on RADFETs: Appendix D of A. Holmes-Siedle and L. Adams, "Handbook of Radiation Effects" (Oxford University Press, 2nd Edition 2002). ISBN 019850733X.

this report:tosp0804opnspec01.doc; "REM "SPACEFET": OPERATIONAL SPECIFICATION FOR USE IN SPACECRAFT"(REM Oxford, UK April 23, 2008)

tosp0804opnspec01.doc23 April 2008 [was tosp0601operationalspec01 30 Nov 2006]ahs