Are GE diodes affeced by the temperature problem too?

[mr_doyle]

Hi,
could anybody please tell me if the temperature problem affects Germanium diodes too, or if it's only related to transistors?
TIA :-D
D.

[puretube]

they make nice thermometers...
(varying the forward-voltage with temperature)

but IMHO, it`s not that noticeable...

[R.G.]

The temperature problem affects all germanium junctions. It's a property of the electro-chemical physics of the atoms.

All forward biased junctions (that is, the base emitter of a transistor in particular) have a forward voltage drop that decreases with increasing temperature. Silicon and germanium both do this. Hence their use for temperature sensors.

All reverse biased junctions leak. Germanium is inherently about a thousand times more leaky than silicon. And it's this reverse-bias leakage that gets you into trouble with transistors. The collector-base junction of a transistor is a reverse biased diode that is "poisoned" by inserting charge carriers into the base-emitter. The charge carriers decrease the effectiveness of the reverse biased collector base in preventing current from flowing, so current flows.

However, the collector base diode region cannot tell the difference in charge carriers injected from outside through the base lead, and charge carriers that just leak through into the base from the collector. It's all charge carriers in the base region, and any charge carrier in the base region is amplified.

So the leakage of a germanium collector-base and a reverse biased germanium diode are quite similar, with the exception that the transistor's leakage is then amplified by the gain of the transistor. This makes any leakage Hfe times as bad.

So the temperature "problem" is really a problem of germanium transistors. The temperature effect is what happens on isolated junctions, and is less of a problem than a simple fact of life.

[mr_doyle]

Thank you guys,
really appreciated!
D.

[DDD]

An interesting fact: the Ge junction has approx. the same temperature drift as Si junction (2.4 mV per C ).

[analogmike]

Germanium diodes used for clipping sound TERRIBLE at over 90 degrees