offset voltage with bipolar transistor

mishima

I have a few questions about offset voltage for a bipolar transistor...

A. Why is this value not specified on datasheets? My EE book says it is usually between 0.6 and 0.7 for silicon, and provides its exact value in textbook problems. Besides experimental measurement, how could I know this from a datasheet?

B. Does this value change depending on operating region? Does this offset voltage even exist during cutoff?

Thanks.

skeptic2

The value varies with bias current and temperature and probably with the doping of each particular batch of transistors. The reason it isn't specified is that 1. the chances that your circuit will use precisely the same current and temperature as the test circuit are nil and 2. it's not really important anyway. It is easy to design around a small variation in that voltage. I'm sure the values used in textbooks are arbitrary values.

I measured a few small signal transistors and found that the voltage varied from about 0.55 V at 55 C to about 0.73 at -30 C with 0.65 C being fairly normal for room temperature.