InP/InGaAs-based metamorphic HBT material grown in multi-wafer production MBE system

The InP-based materials system with InGaAs is recognized as having one of the best potential for high speed HBTs. InP/InGaAs SHBTs have demonstrated good cut-off frequencies but suffer from low breakdown voltage because of the narrow InGaAs collector energy gap. The use of InP collector in DHBTs enhances breakdown voltage by reducing impact ionisation in the collector layer. The high cost of InP substrate and the lack of 6 inch diameter limit the large volume production of InP-based devices (especially for 6 inch fab users). Using GaAs substrates with a metamorphic buffer layer allows one to reduce the cost of the epi-wafer. Moreover, DHBT structures were grown on 4-inch GaAs substrates in a production environment on a multi-wafer MBE machine. The active layers are grown on 1 mm-thick InAlAs graded buffer that matches the lattice parameter constant to InP. Optimisation of the buffer was performed to reduce both the dislocation density and the RMS surface roughness. The device structure includes a 80 nm-thick Be-doped base layer with sheet resistance of 860 ohm/sq. Preliminary results on large area device shows common-emitter current gain β = 100-110 and high breakdown voltage BVceo > 9 V. These results are approaching those published on InP substrates attesting for the high quality of the metamorphic buffer layer.