The video describes the germanium transistor across three temperature ranges: at room temperature, the current gain is 183 and the leakage current is 851 mA; at 120F, the current gain is 28 and the leakage current is 1569 mA, almost double the previous leakage current; and when cooled, the current gain is 159 and the leakage current is 38 mA. 

Why does heat affects germanium transistors in guitar pedals?

Germanium transistors have a much smaller bandgap 0.66 eV vs silicon 1.12 eV. This means less energy is needed to excite an electron from the valence band to the conduction band, making it more susceptible to thermal energy. Their use in audio applications is limited at higher temperatures because leakage current increases significantly, which can lead to thermal runaway.

During testing, we have found that bringing down the temperature of the germanium transistor causes the leakage current to drop to nearly zero. This puts it on the same playing field as the silicon transistor's stability. Watch the video to the left.

Bringing down the temperature and stabilizing the transistors also helps with biasing during assembly.

The FreezeZone pedal cools its Germanium transistors with a temperature-controlled refrigeration unit.