## Help deriving expression of current density

Hey I got an assignment where I have to get an expression of the current density to be:

j = -e$\int$ $\frac{dk}{4\pi ^{3}}$ v$_{x}$f(k)

It says k$_{x}$ >0 underneath the intergrale. The current is flowing to the right (velocity of v$_{x}$ > 0)

Where f(k) is the fermi-dirac distribution function and the interval where I have to find the Current density is in the region x$_{0}$<x<x$_{1}$ (energy barrier). The height of the barrier is W.

Plz help me?!

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