Ultra-shallow emitter formation for germanium bipolar transistor by diffusion from polycrystalline germanium

Germanium, with a higher electron mobility than in silicon and with the highest hole mobility, has promise for high performance CMOS circuits. However, for many analogue circuits, bipolar transistors are preferred as they generate less noise than MOS devices. To exploit the high carrier mobility in Ge for RF applications it is therefore necessary to investigate the fabrication of bipolar transistors in germanium. The adoption of modern technology for Ge BJTs is a challenging technical task. The authors have previously described novel Ge BJT technology for successful device manufacture.[1] Typical output characteristics of a BJT manufactured with an implanted emitter are shown in Fig.1. RF BJTs require tight control of emitter junction depth and base width. It is therefore essential to develop polycrystalline emitter structures to achieve this. In this paper, a polycrystalline Ge process has been developed and applied for the first time in n⁺ emitter formation and contact realization. This has been investigated by manufacture of diode structures by an innovative method, which employs phosphorus out-diffusion a from doped poly-germanium layer. SIMS has been employed to profile the phosphorus diffusions undertaken at several temperatures and drive-in times. The Athena simulation programme has been adapted to model this process in Ge and phosphorus diffusivity parameters have been extracted.