AN ANALYTICAL MODEL FOR CONDUCTION AND VALENCE-BAND EDGE PROFILES OF BANDGAP GRADED AND DISPLACED HETEROJUNCTIONS

An analytical model for conduction and valence band edge profiles appl icable for a wide variety of p-n heterojunction devices is presented b y analytically solving the Poisson equation, taking the displacement o f p-n junction relative to material junction as well as spatial variat ion of permittivity into account. The model is applicable for abrupt a nd compositionally (linearly and parabolically) graded systems. Unlike other analytical models, the present model considers the variation of the built-in potential depending on the positions of the depletion ed ges in the graded region. A displacement of the p-n junction into the wide bandgap semiconductor is shown to cause strong barriers in the co nduction band and the width and height of the barrier increase with th e amount of displacement. Displacement into narrow bandgap semiconduct ors is found to cause dips in the conduction band, the depths of which increase with the amount of displacement. The developed analytical ex pressions are particularly suitable for circuit simulation purposes as well as for the design of heterojunction devices like heterojunction bipolar transistors (HBT). (C) 1998 Elsevier Science Ltd. All rights r eserved.