Influence of emitter-base junction displacement on the band structure in heterojunction bipolar transistors

Analytical expressions for conduction band edges are presented for compositionally graded Heterojunction Bipolar Transistors (HBT's) taking the displacement of the P-N junction relative to material junction into account. In order to demonstrate the effect of P-N junction displacement on the band structure the expressions are applied to AlGaAs/GaAs material system. It is shown that displacement of P-N junction into the wide bandgap (referred to as EB+case) causes barriers whereas displacement into narrow bandgap semiconductor (referred to as EB- case) leads to dips in the conduction band. In EB+ case linear grading (LG) seems to offer a lower turnon voltage compared to parabolic grading (PG).