Design study of passivation ledge in AlGaAs/GaAs heterojunction bipolar transistors

A thin AlGaAs passivation ledge on the extrinsic base surfaces of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is widely used to reduce the extrinsic base surface recombination current and increase the common emitter current gain. However, the design of the ledge and its effectiveness on base current reduction need to be further investigated. In this paper, HBTs with different ledge structures are analyzed with 2-dimensional simulation. The results indicate that a thinner ledge provides better suppression of electron lateral (horizontal) diffusion in the base, and therefore reduces the electron component in base current. Optimization of the ledge design and GaAs surface passivation process is essential for device reliability, too. A new device structure with heavily doped extrinsic base layers is proposed to further improve the base current.