Germanium transistors

[CGS2510.SEP.STUDENT.UCA at CUB.UCA.EDU]

> I'm looking for germanium transistors for fuzz boxes like the fuzz face, 
> etc.  I have some really old unmarked transistors with wire leads in a 
> round black metal case and some transistors marked GT34HV in oval metal 
> cases.  How can I tell if either of these are germanium devices?

All semiconductor diode junctions have a potential differential which 
is based on the work functions of the diode junction and the 
temperature of the junction.  For standard doped silicon diodes the 
differential is about 0.6 volts and for germanium it is about 0.3 
volts.  To measure it, forward bias the diode with a current and 
measure the voltage across it.  Many multimeters do this as the 2K 
ohm range or something like that.  Otherwise, try a resistor from a 5 
volt supply or so.  Use maybe a few milliamps and read the voltage 
off the diode.  If it is 0.3 it is germanium, if it is 0.6 it is 
silicon and if it is significantly different it may be copper/copper 
oxide or lead sulphide based or MOST PROBABLY the diode is not 
forward biased.  The 0.3 and 0.6 volt markers are not exact and may 
very fairly much, but at normal room temperatures it should be very 
easy to distinguish between the two.