Injection over and through heterojunction barriers is becoming increasingly more important in modern electronic devices. We consider the properties of graded AlxGa1−xAs heterojunction barriers using a self‐consistent ensemble Monte Carlo method. In this paper, we consider barriers with two doping levels, 1×1015cm−3and 1×1017cm−3, and two barrier heights, 100 and 265 meV. The 100‐meV barrier resulted in small rectification at room temperature whereas the higher barrier exhibited considerable rectification. In both cases the structure with the lower doped barrier has resulted in a smaller current in both forward and reverse regions due to space‐charge effects. The energy and momentum distribution functions deviate from a Maxwellian distribution inside the barrier region and in general show two peaks: one is comprised mainly of electrons near equilibrium and the second arises mainly from ballistic electrons. The higher doped structure resulted in a faster electron relaxation toward equilibrium as a function of position because the electric field decreases rapidly in the barrier region.