D.c. and microwave characteristics of In 0.32Al 0.68As/In 0.33Ga 0.67As heterojunction bipolar transistors grown on GaAs
The d.c. and microwave characteristics of graded and abrupt junction In 0.32Al 0.68As/In 0.33Ga 0.67As heterojunction bipolar transistors (HBTs) grown on GaAs were investigated. A step-graded In xGa 1- xAs buffer was employed to effectively suppress the threading dislocations resulting from the lattice mismatch between In 0.33Ga 0.67As and GaAs. These devices exhibited a small turn-on voltage of collector current and a high collector-emitter breakdown voltage (BV CEO>9.5 V) for a 0.35 μm-thick collector, demonstrating excellent quality of the base-emitter and base-collector junctions. Less size-dependence on current gain was observed for these metamorphic HBTs even without the emitter ledge. The peak common-emitter current gain at a collector current density of 40 kA/cm 2 is 53 for the graded junction device with an emitter size of 2×4 μm 2 and a base doping of 2×10 19 cm -3. An Fmax of 56 GHz was measured for this device.