A composite step-graded collector of InP/InGaAs/InP DHBT has been inve
stigated for minimised carrier blocking. The optimised collector has t
he following sub-layers: a 100 Angstrom n InGaAs layer; three 200 Angs
trom n InGaAsP layers; and a 100 Angstrom, n = 3 x 10(17) cm(-3) InP l
ayer, and the rest are n InP. The InGaAsP layers should be chosen to g
ive approximately equal band offset at the heterointerfaces.