IIRC, semiconductors are protected by an additional, thin silicon oxyde layer.

BPW34 photodiodes seem to be used for this application, as well.

http://www.sciencedirect.com/science/article/pii/0039602883903886

What about solar cells?

Good reference - thanks! It tells me what I need to know.

Solar cells have a glass surface which would block Alphas. (I think - I've got some raw cells to try, but I haven't gotten around to it. If they work, they would have a huge aperture for detection.)

Big photodiodes in metal cans will work, but you have to remove the glass cover.

I'm planning on posting circuits a little later. Since I'm not an EE, it's taking me a little while to get the op amp circuits working right. Lots of subtlety there, more than you get in a simple op-amp course as part of physics

:-)

especially in photovoltaic mode solar cells have a seriously large capacitance. You might be able to get an average current but certainly no event detection and then thermoelectric effects could just be on the same order of magnitude.

I'd rather take a dismantled BPX 61 as a starting point.

Si is rather benign but as with other material systems, passivation is almost always required and commercial devices usually come with silicon dioxide, silicon oxynitride or other dielectric passivation. 
The closer the passivation lattice constant is matched to the semiconductor substrate, the better the stability against further oxidation.

A bit of a problem could be exposed regions of different doping in humid conditions. Selective reactivity is observed particularly in wet chemical etching processes but I can imagine exposed transistor structures aren't that great under humid environmental conditions.

There are VUV and soft x-ray detectors on the market, I'm confident they'd come with the necessary handling recommendations if susceptible to moisture damage.

As for III-V semiconductors, there's a nice book from A. Baca, ISBN 978-0863413537. Perhaps someone knows an equivalent for group VI semiconductors.

interesting!

I'd say like humidity is actually more problem than oxidation.

By the way, VK2ZAY made experiments with 2N3055 for its large PN junction -

It looks interesting, but I'll stick to my mica covered geiger tube.

Chips usually have a "Passivation Layer".. The metal can / epoxy fill are mainly for excluding moisture and mechanical damages, as I recall..

After a quick Google search, I found this: http://www2.ece.gatech.edu/research/labs/vc/theory/oxide.html

Silicon does oxidize, but under high heat. That's what they use to insulate integrated circuits. 

I've done this (by accident) to a diode. It still works, but I never thought of using it as an alpha sensor. Would this work for diodes too?

I also found information on high temperature oxidation, but nothing on room temperature effects.

Yes, it should work for diodes. A PIN diode is nothing more than a regular diode with a special layer in the middle (the "intrinsic" layer, which is the "I" in PIN), which is only put there to decrease the capacitance. The "I" layer makes the P and N bits further apart, and so has less capacitance.

The problem you will find is aperture - the P/N junction on a regular diode is tiny, and an alpha has to pass through the junction to be measured.

I cut the top off of a metal 2N2222 transistor as an experiment. These are *really* tiny, but if you stick them right up next to an Americium source from a smoke detector it works fine. Your diode should work the same, the effect isn't based on the amplification of the transistor.

(Note that Alphas are blocked by almost everything, including 10cm of air. Put your detector right up next to your alpha source.)

I'll be posting circuits and notes in a separate project on my .io page soon. I use 2N5555 transistors, which have large B/E junctions for larger apertures.

Okay, thanks! Looks like I have an alpha sensor now. And it was a power diode, so the P/N junction is a little bigger than normal.