Monday, 15 July 2013

Germanium Diodes - Introduction, not just for guitar freaks

In my limited knowledge I thought that Germanium diodes were only of interest to an elite of electronic hackers wanting to rebuild, or recreate the sound of sought after "boutique" 1960s guitar effects pedals.
But on researching the topic further it turns out there are actually quite a few niche uses for this type of semiconductor diode:
They are ideal for building crystal radio radio sets.
They are perfect for high efficiency rectifiers and voltage regulators used in wind or water powered generators.

Here's what Sound on sound had to say about the history of Germanium based electronics:
Active electronics relied exclusively upon the thermionic 'valve' or vacuum 'tube' since its invention by Lee de Forest in 1906. However, by the 1950s the science of semiconductor materials had advanced to the point where it was possible to make bipolar junction transistors — and compact solid-state electronics became a reality. The first devices employed Germanium as the semiconductor and these were widely used for more than 10 years. However, silicon was found to be better in virtually all aspects. It was also ultimately much cheaper, and so silicon transistors gradually replaced germanium through the 1960s, until silicon was ubiquitous in virtually all applications by the mid-1970s.
Germanium devices are now largely relegated to optical applications, because of their inherent sensitivity to the infrared end of the light spectrum. So you'll find them in things like fibre-optic interfaces and night-vision equipment. However, there remains a small market for germanium transistors and some are still being made.
Compared with silicon devices, germanium transistors generally have lower gain, with much less consistency between devices. The gain is also more dependent on collector current and ambient temperature than silicon devices. In practice, that means that a lot of germanium-based electronics had a nasty habit of not working when they got hot! However, germanium transistors were able to operate at higher speeds and slew-rates than early silicon devices — an important consideration in the 1960s but largely irrelevant now.
At a technical level, the base-emitter voltage (Vbe) — the control voltage, if you like — for germanium devices is half that of silicon (300mV instead of 600mV) and that means that you can't just substitute silicon for germanium (or vice versa) without also completely re-engineering the biasing arrangements.
Despite the universality of silicon these days, the 'germanium period' coincided largely with a rapid growth of the music industry, and many people still associate classic sounds with germanium-based products like EMI's first solid-state TG mixers, Neve's 1053 and contemporary modules, and some of the vintage Fairchild and Telefunken products. Similarly, many of the classic guitarist stomp boxes that date from the 1950s and 1960s also employed germanium devices.
Image of 2 diodes under a magnifying glass
Diodes - currently under examination


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