A heterojunction bipolar transistor is provided that has a reduced turn-on voltage
threshold. A base spacer layer is provided and alternately an emitter layer is
provided that has a lowered energy gap. The lowered energy gap of the base spacer
or the emitter spacer allow the heterojunction bipolar transistor to realize a
lower turn-on voltage threshold. The thickness of the emitter layer if utilized
is kept to a minimum to reduce the associated space charge recombination current
in the heterojunction bipolar transistor.