an important aspect of hemts is that the band discontinuities across the conduction and valence bands can be modified separately. this allows the type of carriers in and out of the device to be controlled. as hemts require electrons to be the main carriers, a graded doping can be applied in one of the materials making the conduction band discontinuity smaller, and keeping the valence band discontinuity the same. this diffusion of carriers leads to the accumulation of electrons along the boundary of the two regions inside the narrow band gap material. the accumulation of electrons leads to a very high current in these devices. the accumulated electrons are also known as 2deg or two-dimensional electron gas.