A theory of the base transport in an abrupt-junction heterostructure b
ipolar transistor (HBT) is developed in the diffusion limit. The theor
y is valid for a continuous range of emitter injection energies DELTA
and accounts for the Early effect (EE) both in the static and the high
-frequency limits. Small-signal network parameters strongly depend on
DELTA and differ from those in a graded emitter-base junction HBT.