On germanium transistors

Germanium transistors seem not to be used anymore in any mass consumer, industry, or special development, they indeed favored by some electronics enthusiasts. It is not so easy to say why. Nostalgia? Wish to research the past in order to best know the present? The belief that every thing is true at the time of it's rise? I do not know for sure.

But they are still there and some are even cheap. I would like to promote their use in various unusual hacks, until they finally gone melt into recycling. N-P-N types are getting scarse already. So i would like to share some common former soviet union types but with better parameters, because there is always confusion with those grades and letters, and complants to unexpected low gain. Of course this is very small part of what is still obtainable. For example, i completely omitted power Ge transistors (very interesting too!) and >1GHz range ones.

Here is image attached. Typical gain column is from my experience (as i have them all, almost)

Whenever you find them marked with "ОС" (OC) it is for applications where the failure costs most, aerospace or similar. But in fact, for the hobbyst applications this special quality control means simply more shelf life, hence less "dead on arrival" items from NOS batch. Do not expect special parts to be higher gain or higher maximum ratings.

Note: do not take important so called "leakage", widely discussed meaning the collector-emitter current caused by reverse collector-base current amplified by transistor amplification factor. Amplification factor at collector currents, let's say, near 100uA varies terribly. And, are we going to set base floating unconnected? This is forbidden in fact.

Insted, measure and take into account reverse collector-base current itself. Note that it is doubled each time case temperature increases by 10C. Hence most important when transistor dissipates more power. Take schematic measures simply to sink that parasitic current so it is not fed into forward-biased base-emitter junction, i.e. calculate proper value of the resistor between base and emitter (or ground).