High-Speed and High-Reliability InP-Based HBTs With a Novel Emitter

This paper describes InP HBTs with a novel emitter simply consisting of a degenerately doped n⁺-InGaAs layer and an undoped InP thin layer. An n⁺-InP layer is not necessary because the quasi-Femi level in the n⁺-InGaAs layer is high enough to exceed the conduction band discontinuity between the n⁺ -InGaAs layer and the undoped InP layer. In the proposed structure, a thin ( ~ 10 nm) ledge structure can easily be fabricated by etching the n⁺-InGaAs layer. The fabricated HBTs with a 15-nm-thick ledge structure provide a high collector current density of over 6 mA/¿m² . There is almost no degradation of current gain, although the emitter width is reduced to as small as 0.5 ¿m. The HBTs also exhibit an ft of 324 GHz at a collector current density of 5.5 mA/¿m², which is comparable with that of HBTs with a conventional emitter consisting of an n⁺ -InGaAs layer, an n⁺-InP layer, and an n-InP layer. From the results of accelerated life tests, the activation energy of the degradation in HBTs is estimated to be around 1.8 eV, and the extrapolated mean time to failure is estimated to be over 10⁸ h at a junction temperature of 125°C.