When I was attending an Analog Devices seminar in Breda last week, I ran into a guy - Peter Hoeben - who is having his own Hall effects sensors made. IIRR he gets them done at a GaAs foundry.
Hs web site is
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and here 's a data sheet for one of his sensors
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His aim is to fill the gap in the market left when Siemens / Infineon discontinued the KSY14, KSY44 and KSY10 Hall sensors.
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which seem to have been useful in high-precision magnetic field sensing - even for magnetic fields up to the multi-Tesla level.
I've felt the need for such a component only once in my career, but when I needed it, it would have done a much better job than the next best thing.
-- Bill Sloman, Nijmegen