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Proud Mary

Sat Oct 16 2010, 09:58PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Could anyone kindly give me a part number for a small signal enhancement mode P-Channel MOSFET with an aluminium gate?

Thank you!

plazmatron

Sun Oct 17 2010, 12:04AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Any specific reason for having an Aluminium gate?
The original RADFET design doesnt seem to give a good reason for having an Aluminum gate, except, that at the time such gates were in common use.

From the stuff I have read, modern RADFET`s use p-channel mosfet structures with polysilicon gates. I would be inclined to try out a bunch of p-channel MOFETS and see what happens.

Proud Mary

Sun Oct 17 2010, 09:43AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Thank you for that, Les.

I'm still learning about oxide trapping, and the differences between thin and thick oxide as they relate to neutron and gamma dosimetry, so had assumed an aluminium gate was an actual necessity, as recent researchers are still talking about it.

Steve Conner

Sun Oct 17 2010, 04:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

As far as I know, only expensive RF power MOSFETs have aluminium gates nowadays. Aluminium has a lower resistivity than polysilicon, and that gives much better high frequency performance. If there were any cheap ones, Richie would have jumped on them for his Class-E SSTC work.

I don't see why it wouldn't work with a polysilicon gate, though.

Or maybe you meant Radfest, the radioactive equivalent of a Teslathon?

Proud Mary

Sun Oct 17 2010, 08:11PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Thanks for that Steve.

I'm going to use the circuit below, with a chopper stabilized op amp.

The MOSFET, inside an EM and thermal shield, will stick out of one end of a termperature controlled poured lead slab inside a diecast aluminium box.

From my beginner's understanding of the RADFET literature, this is yer actual Gy/hr gizmo, so I may be able to do a tertiary calibration job on it with my Philips X-ray Photometer - assuming I can find a MOSFET that works in this application.

1287345201 543 FT0 Radfet Circuit Upload

plazmatron

Sun Oct 17 2010, 08:56PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

"As an alternative to a RADFET, a commercial off-the-shelf transistor is used for the dosimeter 24 . Commercial off-the-shelf transistors are manufactured in high volume and sold for use in different applications. Parts are generally available and typically have a constant design over the many years that they remain in production. For example, p-channel transistors manufactured by Fairchild under product names BSS84 and FDC5614P are used. Other transistors by other manufacturers with the same, similar or different characteristics may be used. Since the dosimeter 24 is not used to regulate patient dosage, a device subject to regulatory approval may not be necessary. Other devices having desired linear, non-linear, experimentally determined or known characteristics as a function of dosage may be used. "

From US patent number: 7372942

Both types are available from RS components, at 20 odd pence a go.

Proud Mary

Sun Oct 17 2010, 09:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Why, thank you very much, Les.

Just what I was hoping to hear!

Reminds me of how I found out from a patent that the photodiode BPX65 could be used - and was being used at Harwell - in dosimetry right down to a few keV if you took the lens out.

plazmatron

Sun Oct 17 2010, 09:29PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Proud Mary wrote ...

Why, thank you very much, Les.

Just what I was hoping to hear!

Reminds me of how I found out from a patent that the photodiode BPX65 could be used - and was being used at Harwell - in dosimetry right down to a few keV if you took the lens out.

You're welcome!

I figured someone tighter than us might have tried to design something using off the shelf components, to save a buck!
Imagine the markup you could make on a 26p component, by calling it a radiation dosimeter in a glossy pamphlet!

Proud Mary

Mon Oct 18 2010, 11:04AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

plazmatron wrote ...

I figured someone tighter than us might have tried to design something using off the shelf components, to save a buck!
Imagine the markup you could make on a 26p component, by calling it a radiation dosimeter in a glossy pamphlet!

Farnell's best price for BSS84 is Â£0.166 each for UK stock. These prices would seem to bring all sorts of detector arrays within reach:

Stacked RADFETs for increased radiation sensitivity

O'Connell, B.; Kelleher, A.; Lane, W.; Adams, L.;

Radiation and its Effects on Components and Systems, 1995. RADECS 95., Third European Conference
18-22 Sep 1995 pp: 481 - 486

Abstract

Hitherto, pMOS Radiation Sensitive Field Effect Transistors (RADFETs) have not been able to detect doses in the milli-rad range, which are required for low dose clinical/personnel applications. This paper reports on further investigation of a design approach, where RADFETs are connected in a stacked sequence so that increased radiation sensitivity is obtained. The radiation sensitivity obtained for 40 stacked RADFETs is approximately 220 times the single RADFET sensitivity. This enables radiation sensitivities in the milli-rad range to be measured. Theoretical equations governing the threshold voltage of a MOS device as a function of bulk-source voltage are used to theoretically evaluate the output voltage of the stacked structure. Measurement and theory are found to agree closely in this analysis. % drift and % fading of the single RADFET, as a function of total radiation induced shift in VT, is similar to that of the stacked structure

SicelTech

Tue Oct 19 2010, 01:25AM

Registered Member #3333 Joined: Tue Oct 19 2010, 12:41AM
Location:
Posts: 1

Tyndall Labs in Cork or REM in UK sells RadFETs that have Al gates and 400nm up to 1000nm gate oxide oxides. The oxide thickness determines to a large extent the sensitivity of the device. You've found papers by Len Adams and you can also look for papers by Andrew Holmes-Siedle. The sensitivity is most directly tied to the thickness of the oxide and the bias (or E-field) across the gate. Sensitivities of 0.5mV/cGy (no bias) to 1.5mV/cGy (9V bias) are possible on these devices. The cGy unit is newer unit and is used because it's the same as the rad which was the old unit. Off the shelf devices which do not have a fairly thick oxide will not work as well and will not be as sensitive. The Adams stacked RadFET design was done using 400 and 1000nm oxides and got 84mV/cGy (rad).
Hope you operate at ZTC zero temperature coefficient (in your Adams paper). That will provide the most reliable data. The poly-Si gates work as well because the oxide is doing the trapping (oxide is a sea of dangling bonds which are oxide traps). The poly-si gates have a different threshold transition due to the different work function.

Cheers

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