Paul is experimenting with a Joule Thief using some germanium transistors I sent him. I thought about a few quirks of germanium transistors that I may not have (or may have) blogged earlier.
The vintage germanium transistors were made at a time when the solid state manufacturing technology was much less advanced than it is today. Back then, in the late1950s through late 1970s, the processes were much less automated; they were more like baking a cake, with more dependence on humans with their frailties when it came to carrying out precisely timed procedures and precisely measured ingredients. The chances of completing a procedure exactly as specified were much lower, and consequently greater variation in the transistor specifications had to be accommodated.
One specification is the current gain of the transistor. This may be greatly affected by the time and temperature of the process. Those in turn may have been controlled by analog controls that were typically seen on many appliances in the home. As a result the time and/or temperature may have varied a few percent either way.
The transistors of today benefit from the technology that is used in integrated circuits. Early germanium transistors didn’t have this benefit; the transistor was a small bar of N type germanium with two dots of P type germanium baked into each side. Much of the transistor’s performance was determined by the process controls.
Another factor may have been the precision of the process. The emitter is much more like the collector, so the transistor is more symmetrical; the two may be swapped and the transistor may still have enough current gain to amplify the signal. I have seen others accidently swap the emitter and collector leads of a silicon transistor and it still worked in a Joule Thief circuit, but not as good as the correct way.
Another parameter that germanium transistors have is the emitter to base breakdown voltage. Silicon transistors may be rated at 5 or 6 volts, and germanium transistors may be rated at double that amount or more. I have seen germanium transistors rated at the same max voltage for both emitter to collector and emitter to base, 25 volts, for example.
Early germanium transistors were packaged in a package that was not hermetically sealed. The package may have been plastic or epoxy, but the humidity from the air often got between the plastic and wire leads, and caused the performance to degrade. Since the government was still a major purchaser, the transistors had to be tested for reliability for use in computers, missiles and space vehicles. Germanium transistors cannot be passivated with an oxide coating as can silicon. The manufacturers changed the package to a hermetically sealed metal package, and the reliability was greatly improved. Today you will seldom, if ever, see a germanium transistor in a plastic package.
I connected a Joule Thief to a AAA cell that I found in the street, dented and smashed by cars running over it. It keeps on going and going, like the Energizer Bunny. One part – probably the largest part – is that the germanium transistors, as I said above, have much lower gain. Thus the typical 1k resistor used for a silicon transistor, is not supplying as much current, especially at low battery voltage, to give the Joule Thief as much base current, and therefore the collector current is much lower, giving a much longer runtime. The brightness is lower, but that is not noticed; the JT just keeps on running. At low battery voltages below 1/2 volt, the resistor may be zero ohms, but that will reduce the runtime a lot.
Thanks for that. Paul