Why do point contact diodes use wire whiskers?

Question

I've recently been learning about the operating principles of various semiconductor devices, and I've run into difficulty regarding the point contact/metal-semiconductor diode. I can't find out why exactly diodes like these (and their precursors, crystal rectifiers) use a very narrow wire of tungsten/gold, and not just a 'chunk' of metal as in selenium rectifiers. I imagine it's something to do with the depth/size of the P-side and the depletion zone or the junction capacitance. But I've been unable to find any info. Any info/papers would be greatly appreciated.

Thanks!

Answer 1

In a point-contact diode, atoms from the "cat whisker" are diffused into the semiconductor material, and provide the acceptor impurities in the semiconductor crystal which make the region near the "cat whisker" P-type. Because the cat whisker is small, and the cat whisker's atoms do not diffuse very far into the semiconductor crystal, the P region is very small, and so the area of the PN junction is likewise small. The small area of the junction makes both the junction capacitance, and the reverse recovery charge of the diode small. Both of these factors contribute(d) to the usefulness of point contact diodes at high frequencies.

A point contact diode differs from a Schottky diode in that metal atoms from the contact actually diffuse into the semiconductor crystal in a point contact diode. It is a true PN diode. In contrast, in a Schottky diode the active junction is metal to semiconductor.

Answer 2

The breakdown voltage depends on the radial gap between conductors around 1~3V / um for air and more for creepage on silicon.

For this reason 1N4xxx series use a large gap.

For LED's it is a question of blocking the light so the smallest gold whisker blocks the least light.