Parabolically graded base-collector double heterojunction bipolar transistor

摘要

A double heterojunction bipolar transistor (DHBT) is provided with a parabolic grade in bandgap at the base-collector junction. The parabolic grade in bandgap is close to parabolic in composition. The parabolic grade in bandgap is achieved by employing a chirped superlattice to mimic the parabolically varying alloy composition. Each period of the superlattice consists of one low-bandgap layer and one high- bandgap layer. The average composition in each period is determined by the relative thicknesses of these two layers. By varying the thickness ratio of these two layers approximately parabolically with (i) the distance from the base for a concave parabola and (ii) the distance from the collector for a convex parabola, the intended parabolic grade is achieved. The exact values may be computed numerically.