  We use the non-equilibrium Green function (NEGF) method in the ballistic
limit to provide a quantitative description of the conductance of graphene pn
junctions - an important building block for graphene electronics devices. In
this paper, recent experiments on graphene junctions are explained by a
ballistic transport model, but only if the finite junction transition width,
Dw, is accounted for. In particular, the experimentally observed anamolous
increase in the resistance asymmetry between nn and np junctions under low
source/drain charge density conditions is also quantitatively captured by our
model. In light of the requirement for sharp junctions in applications such as
electron focusing, we also examine the pn junction conductance in the regime
where Dw is small and find that wavefunction mismatch (so-called pseudo-spin)
plays a major role in sharp pn junctions.
