On 2020-05-21 17:26, Phil Hobbs wrote:
On 2020-05-21 16:39, John Larkin wrote:
On Thu, 21 May 2020 14:12:43 -0400, Phil Hobbs
wrote:
On 2020-05-21 13:33, John Larkin wrote:
On Sun, 17 May 2020 14:15:31 -0000 (UTC), Cursitor Doom
wrote:
Gentlemen,
I have some old OC171 transistors from an old Eddystone short wave
radio
I'm restoring. The problem is I suspect they've developed whiskers, as
the resistance readings from e,c and b to the screen connection are
all
far too low (sub 10 ohms). A colleague has suggested blasting the
whiskers by tying the ecb leads together and zapping them against the
screen with a 500nF cap charged to 500VDC. Now this seems a bit
counter-
intuitive to me and I'd have thought higher current lower voltage
would
be safer for these delicate germanium devices, but WTF do I know?
Is it feasible to remove the whiskers by this sort of method or any
other?
Thanks,
CD
Does anyone still make ge transistors? I can't think of any use for
them.
The only production ge devices I know of (excepting SiGe) are back
diodes, which I think are the only germanium parts fabricated using
lithography.
Ge makes good photodiodes for some uses.Â* They're very leaky, but if you
make the epi thin enough, they can cover 350-1800 nm, which otherwise
requires expensive stacked-die devices.Â* Garden-variety ones are more
like ordinary InGaAs, i.e. 800-1800 nm, which is much less interesting.
Cheers
Phil Hobbs
Our FLIR has a germanium lens.
https://www.dropbox.com/s/uda77g9w66...Lens.JPG?raw=1
Germanium makes really good IR lenses.Â* Besides the built-in filtering
action, it has a refractive index of 4.Â* The optical power of a given
surface is (n-1)/r, where n is the index and r is the radius.
Glass is generally around n = 1.5-1.8, so a Ge lens of a given power has
4-6 times the radius of curvature.Â* Aberrations are much reduced due to
the weaker curvature, so a simpler lens can have better performance.
Also of course the diffraction spot size goes like lambda, so in terms
of the diffraction limit a Ge lens at 10 um is like 100 times easier to
design than a glass lens at 500 nm.
Oh, and the dispersion of Ge out in the thermal IR is much less than
glass in the visible, so you don't even need to achromatize it. The
tempcos of index and of optical path length are quite large, so you do
need to athermalize in general, something that's rarely required in the
visible.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com