Fano Factor Fact and Fallacy

Over the years Fano factor measurements have been published which demonstrate a steady decline in the values with time. In fact, the reduction in these values is so monotonic that an extrapolation to 1980 suggests that the Fano factor is completely negligible, and that all (non-avalanching) ionization detectors should show nearly perfect resolution (and that--to avoid being tarred and feathered by our colleagues--we should hasten to move on to another research field). Obviously, this is not the actual case in nature, but in many of the previous works the value has been published as "the" Fano factor, justifying a healthy skepticism on our part. In this paper we try and show what we think has been happening; we derive from our own theory for ? (the average amount of energy expended in producing an electron-hole pair) expressions which suggest that the Fano factor in both silicon and germanium may be very nearly equal to 0.05 ; and we show with our experimental data, as well as with a reanalysis of the data of other workers, that experiment is very nearly approaching this value. Finally, in trying to reveal what we think we understand about the Fano factor situation, we also reveal the areas in which our understanding is very poor. These observations support the value of further properly corrected experiments and the need for further very basic theoretical work. Isn't this all fun?