Novel fabrication of C-doped base InGaAs/InP DHBT structures for high speed circuit applications

We have fabricated InGaAs/InP based DHBTs for high speed circuit applications. A process involving both wet chemical and ECR plasma etching was developed. Carbon was employed as the p-type dopant of the base layer for excellent device stability. Both the emitter-base and base-collector regions were graded using quaternary InGaAsP alloys. The extrinsic emitter-base junction is buried for junction passivation to improve device reliability. The use of an InP collector structure with the graded region results in high breakdown voltages of 8-10 V, with no current blocking. The entire structure is encapsulated with spin-on-glass. These devices show no degradation in d.c. characteristics after operation at an emitter current density of 90 kA cm^-2 and a collector bias, V_CE, of 2 V at room temperature for over 500 h. Typical common emitter current gain was 50. An f_t, of 80 and f_max of 155 GHz were achieved for 2 x 4 μm² emitter size devices.