The flux method of McKelvey, Longini, and Brody [Phys. Rev.123, 51 (1961)] is used to analyze current blocking in double‐heterostructure bipolar transistors with composite collectors (CC). The effects of electron accumulation in the spacer layer are included; these are shown to impose an intrinsic limit on the simplest CC design. The analysis is extended to include compositional or band‐gap grading, which can substantially reduce the current blocking at high currents, though it has less effect at lower currents. ©1996 American Institute of Physics.