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It's somewhat common to find a schematic that calls for a germanium diode in amateur radio contexts. For example:

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This is a QSK circuit from the Fldigi manual.

And yet, purchasing a germanium diode is a relatively difficult task. For example, Mouser's offerings from 600 manufacturers doesn't seem to include a germanium diode:

enter image description here

(Although, they do have a few germanium BJTs)

Germanium diodes can be found elsewhere, mostly from surplus or specialty stores, for a cost orders of magnitude higher than most other discrete semiconductors.

Why is it so common for amateur designs which do not appear to have any particularly stringent performance requirements to call for a relatively expensive diode not available through commodity channels?

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  • $\begingroup$ It's been my experience that Mouser is quicker to stop stocking older parts compared to Newark (and Allied?) and other distributors. $\endgroup$ – Mike Waters♦ Sep 2 '17 at 22:53
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The lower forward voltage drop (0.3 volts or even less), compared to silicon diodes (0.7 volts or more).

Having said that, there are some schottky diodes with a low forward voltage drop, too.

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    $\begingroup$ A lot of designs that include germanium diodes are older, or based on older designs, when germanium diodes were commonly available. Wikipedia says, "From 1950 through the early 1970s, this area provided an increasing market for germanium, but then high-purity silicon began replacing germanium in transistors, diodes, and rectifiers". After that, there was lot of old stock that could be had relatively cheaply. $\endgroup$ – user3486184 Aug 29 '17 at 17:52
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    $\begingroup$ @user3486184 I think that comment would actually be worth being an answer. In fact, an answer that would be worth upvoting! So please, could you post it as an answer? $\endgroup$ – Marcus Müller Aug 29 '17 at 20:09
  • $\begingroup$ This is the right answer. Emptying stock idea is probably a joke. $\endgroup$ – Pedja YT9TP Aug 30 '17 at 7:09
  • $\begingroup$ Are there applications of germanium diodes where a Schottky wouldn't be an appropriate replacement? $\endgroup$ – Phil Frost - W8II Aug 31 '17 at 12:39
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A lot of designs that include germanium diodes are older, or based on older designs, when germanium diodes were commonly available.

On the page for Germanium, Wikipedia says, "From 1950 through the early 1970s, this area provided an increasing market for germanium, but then high-purity silicon began replacing germanium in transistors, diodes, and rectifiers".

When germanium diodes started getting replaced with silicon diodes, there was lot of old stock that could be had relatively cheaply. Hams took advantage of that in their designs.

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  • $\begingroup$ So now that germanium diodes are no longer cheap, why are they still specified in designs? $\endgroup$ – Phil Frost - W8II Aug 30 '17 at 20:02
  • $\begingroup$ I can think of a couple of reasons. 1. People are using old designs as a basis for their new designs. If you've been designing circuits since the 1980s, you're not likely to change. If you're using an existing circuit from an old book as part of your design, you might not want (or know how) to change it. 2. The designers have 600 diodes left from a box of 1000 germanium diodes that they bought from a surplus store in 1985, and they aren't aware of current pricing. $\endgroup$ – user3486184 Aug 30 '17 at 20:34
  • $\begingroup$ There is another difference: the voltage drop across a forward-biased silicon diode is around 0.7V, whereas for a germanium diode it's around 0.3V. Sometimes a designer would use a germanium component (diode, transistor) to take advantage of this lower voltage drop compared to a silicon part $\endgroup$ – Scott Earle♦ Aug 31 '17 at 4:19
  • $\begingroup$ True, but some silicon diodes (like the BAT46) have an equally small forward voltage drop. $\endgroup$ – user3486184 Aug 31 '17 at 5:42
  • $\begingroup$ People interested in this question may also be interested in electronics.stackexchange.com/questions/105597/… $\endgroup$ – user3486184 Sep 2 '17 at 22:42
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In this particular application, silicon diodes will almost certainly work just fine. Maybe you will have to increase the output level of the sond card to compensate for the higher forward voltage.

Germanium diodes are often found in older schematics of RF measurement instruments. These designs rely on the characteristics of germanium diodes and you have to consider the specific application to decide if and how you can substitute a different diode.

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