Help deriving expression of current density

lagger2

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

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

It says k[itex]_{x}[/itex] >0 underneath the intergrale. The current is flowing to the right (velocity of v[itex]_{x}[/itex] > 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[itex]_{0}[/itex]<x<x[itex]_{1}[/itex] (energy barrier). The height of the barrier is W.

Plz help me?!