Sorry for digging up this ancient topic, but this is the first google result for "transistor Uf", so I suppose that many others who are looking for the answer are also going to stumble upon this post.The correct answer seems to be:
Uf=(Vf)
forward voltage drop between the
base and the emitterFor silicon transistors this is usually just below 700mV, whilst some of the germanium transistors (I've tested with a similar 'electronic components tester') Uf goes somewhere between 100mV and 200mV (OC74=~120mV) (OC79=~176mV) (OC44=~146mV).
This makes me wonder, since, to my understanding, they should be around 300mV. However, it's also possible that this 'tester' just doesn't work well with such -nowadays- rather rare components.
reference:
https://www.darc.de/uploads/media/Transistortester_df1rn_rev_1.1_2013_03_11.pdf"Für das Beispiel von Bild 5 ist dies der Typ des Transistor NPN, die Zuordnung der Anschlüsse Basis, Kollektor und Emitter zu den Anschlusspunkten 1, 2 und 3, die Stromverstärkung hFE sowie die Basis-Emitter Flussspannung Uf in mV."
which roughly translates to:
"The example in image no. 5 shows an NPN-type transistor; the mapping of the pins 'base', 'collector' and 'emitter' to the tester connectors 1, 2 and 3; the amplification hFE as well as the base-emitter forward voltage Vf in mV."
(Since English isn't my native language and I'm still learning about Electronics, it may well happen that I mix up some of the terminologies.)