Quaternary Graded-Base InP/GaInAsSb DHBTs With ${f}_{\text{T}}$ / ${f}_{\text{MAX}}$ = 547/784 GHz

We report a “Type-II” metal-organic chemical vapor deposition (MOCVD)-grown compositionally graded quaternary base InP/GaInAsSb/InP double heterojunction bipolar transistor (DHBT) with simultaneous cutoff frequencies f
_T
/f
_MAX
= 547/784 GHz. Compositional grading of the As/In mole fractions enables stronger quasi-electric fields in the base and favors better RF performance. Consideration of the higher bandstructure reveals new insights on the performance limits of “Type-II” DHBTs: Indium incorporation in a GaAsSb base appears to increase the efficiency of electron injection from the base into the InP collector. The present results correspond to the fastest MOCVD-grown quaternary base DHBTs reported to date for a device with f
_MAX
> f
_T
.