Numerical Study of Carrier Transport in n+/n/n+GaAs/AlGaAs Heterostructure at High Current Densities

Carrier transport across isotype GaAs/Al
_0.4
Ga
_0.6
As graded heterojunction has been numerically studied at high injection levels in the framework of the energy balance model. It is shown that, under reverse bias, impact ionization in the domain formed at the heterojunction leads to accumulation of electron-hole plasma in GaAs, but does not cause a breakdown. The latter occurs when the domain reaches the N/N+ interface in AlGaAs and impact ionization is developed throughout the AlGaAs layer. By contrast, under forwarding bias, the initial domain at the heterojunction rapidly disappears with increasing voltage, and in its place, a peak of an electric field of the opposite sign arises since the carrier mobilities of GaAs and AlGaAs are different. Impact ionization in the peak gives rise to a new domain at the n+/n interface in GaAs, which leads to breakdown and the appearance of negative differential resistance. It is shown that a peak of electron temperature is formed near the heterojunction for both current directions. However, the reasons why it appears are different. Under reverse bias, the electrons are heated by the electric field in the domain, while under forwarding bias, it is caused by the energy gradient of the conduction-band edge in the graded-composition layer of the heterojunction. The surprising thing is that, under reverse bias, electrons in GaAs remain cold throughout the whole region extending to the junction at all currents, which results in the extremely great and physically unreasonable values of the velocity overshoot.