Hello,
I should preface this question by adding that I am a rather novice user of COMSOL.
In the Semiconductor module, under Semiconductor Material Model, in the Equation drop-down menu, the electron and hole current densities are written as follows:
In both these equations, the first two terms on the right hand side are the familiar drift and diffusion terms, respectively.
But what is the physical origin of the third term on the right hand side of either equation? Clearly it is some sort of thermal diffusion term. However, it's a bit strange given the factor 
, wherein the natural logarithm of 
is taken. It is strange because this is not a dimensionless quantity.
My presumption is that these thermal diffusion terms account for thermoelectric effects in semiconductor materials. However, it would be very helpful if someone could explain their origin to me.
Thanks in advance, Ara G.
P.S. I have checked the COMSOL documentation and failed to find any information regarding this question.