Germanium Transistors
Diagram from "Handbook of Semiconductor Electronics" Edited by Hunter, McGraw-Hill 1956
This book discusses transistor collector-base leakage, which is essential to understanding the quirks of germanium transistors in some fuzz circuits. This diagram is a schematic look inside the transistor.
Soft Curves
Germanium devices are used as rectification devices in that they can be biased forward to pass one polarity of current, and reverse biased to block the opposite polarity of current.
The ideal rectifier is the ultimate electron traffic light that has a sudden change from "stop" to "go" and can shuffle positive signals one way and negative signals another way.
Germanium makes a relatively poor rectifier because it does not have crisp changes from blocking current to passing current. Rather, it has gentler curves than its silicon cousin's curves, and it's difficult to predict what the curves will look like from one germanium device to the next.
These properties lead to germanium having a strong reputation for use as a "soft clipper" in distortion circuits.
The oldest of these applications is in 1960s fuzz effects. The 1960s makers used germanium because it was the available transistor of choice. Silicon devices were new and expensive. By the late 1960s, silicon was a reasonable choice and makers began to switch.
Later on, discriminating guitarists notes tonal differences between germanium based fuzz pedals and silicon based fuzz pedals.
In overdrive and distortion effects, clipping diodes can be considered a key element of the sound. The infamous Klon pedal uses germanium diodes that are widely held to be an essential ingredient - not just getting the same part number, but getting ones with the exact same properties, which is difficult to do with germanium. The maker of the Klon says you cannot make a Klon by the part numbers alone.
(Main article under "Fuzz Topologies")
Examples:
Maestro Fuzz Tone FZ-1, FZ-1A
Tone Bender Mk I, II, III, and IV
This could be called "collector leak" bias. It requires a leaky germanium device to be "on" without signal applied. Otherwise, the device is cutoff until driven by a guitar signal. The device will only be active when the guitar signal forward biases the base-emitter junction and cutoff when the guitar swings the other way.
Germanium devices are preferred for "no-bias" circuits because they need far less voltage to "turn on" than silicon. The forward bias voltage of germanium will be between 100 and 300mV, which is well within a guitars capability. The forward bias voltage of silicon is 500 to 700mV, which is much higher and more difficult for a guitar to activate.
Additionally, the germanium transistor can leak voltage to the base from the collector internally, which will either "self bias" itself or simply help lower whatever additional voltage is needed to pass signal. Silicon devices leak too little current to be noticeable or affect circuit operation, so this element of transistors is rarely taught or understood anymore, but it can be found in very old transistor manuals. See the diagram at the top of the page from a 1956 handbook.
If a device is leaky enough, the unit will stay on and the effect is more linear, like overdrive. The amount of leak is directly affected by ambient temperature and will noticeably change with each degree of temperature change. These variables account for the inconsistent behavior of "no-bias" type circuits.