Influence of a Barrier Layer on the Formation of AuBe Ohmic Contact With the p-GaAs Bases of Heterojunction Bipolar Transistors

Three AuBe ohmic contact structures with Ni, Pt, or Pd as a diffusion barrier layer are investigated and compared. It is found that the barrier layer plays an important role at the formation of a high-quality AuBe-type ohmic contact on p-GaAs. This paper reveals that Ni cannot function as an efficient barrier and the ohmic contact with Ni as a barrier layer is characterized with high specific contact resistance and poor surface morphology. Both the AuBe/Pt/Au and AuBe/Pd/Au ohmic contact structures can achieve a minimum contact resistance of several 10^-7 Ω·cm² after an oven annealing at 390°C for 30 min. The AuBe/Pt/Au contact structure has the best overall performance in terms of low specific contact resistance, good geometrical integrity, and high thermal stability. A thin Ti layer is used to promote adhesion without causing a negative impact on ohmic contact resistance. The AlGaAs/GaAs and InGaP/GaAs heterojunction bipolar transistors with Ti/AuBe/Pt/Au as the base ohmic contacts exhibit good linearity with low offset and knee voltages.