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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..
Sensing principle
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
Thick oxides
VI = +9V
VI = +9V
10 to 100,000 cGy
2 to 5000 cGy
B. Unbiased modes (sensor can be separate from machine during "expose"):
Medium-thickness oxides
VI = 0V
20 to 2 million cGy
Thick oxides
VI = 0V
4 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 disrupt wiring 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 www.radfet.com
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.doc
23 April 2008
[was tosp0601operationalspec01 30 Nov 2006]
ahs