A numerical model for heterojunctions is presented in which thermionic emission current is formulated at the heterojunction interface, in series with drift-diffusion current in the bulk. It is shown that the thermionic emission current at the heterojunction interface is regarded as a boundary condition to give a relation between quasi-Fermi levels at both sides of the interface. GaAs/AIGaAs heterojunctions are simulated as an example, and the results are compared with those obtained by a conventional diffusion model in which current transport across the heterojunction interface is not considered explicitly. It is shown that the thermionic emission mechanism at the interface is important and should be considered, particularly in isotype heterojunctions. According to the model presented here, more general numerical analyses including thermionic emission, drift, and diffusion phenomena can be achieved.
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