Sze Eq. 38

mzh

Dear PF users
What is the rationale for Sze to write the exponential terms in ch. 1, Eq. 38 once as
[itex]\exp \left[ -\frac{E_C - E_F}{kT} \right] [/itex] and once as [itex]\exp \left[ \frac{E_F - E_D}{kT} \right] [/itex]?
In both cases [itex]E_F[/itex] is larger than either [itex]E_C[/itex] or [itex]E_D[/itex]. To me, for the sake of not having to worry about correctly placing minus signs as in the first term, it would seem most consistent to always subtract the lower from the larger term. Why is he not doing this here?

Thanks for comments.