Why VBE decrease with temperature ?

[MahmoudHassan]

Hello again
happy weekend
Please help me with this question

IF VBE = VT ln (IC/IS)
how can VBE decrease with temperature eventhough VT = KT/q and it is directly related to temperature ?
IS is also dependent on temperature but I don't know if its effect is more than VT on VBE and how ?

Thanks a lot
your help is really appreciated

 

[KlausST]

Hi,

For a silicon bjt the temp coefficient is about -2mV/K.
It is caused by the PN junction. It is just a physical effect.

Klaus

 

[FvM]

Model-wise, the effect is caused by the exponential Is temperature dependency. I notice that some sources mention Vt versus temperature behaviour but don't tell anything about Is versus temperature. Look out for a profound text book!

 

[crutschow]

It's related to the energy required for the carriers to cross the PN junction. The thermal motion of the carriers reduces the amount of added energy needed, thus as the temperature increases, the forward voltage drop goes down.