Modelling of electrical characteristics of ultrashallow pure amorphous boron p+n junctions

Diodes fabricated by a pure amorphous boron (PureB) deposition technology show outstanding performance. Depositing a PureB-layer on Si at temperatures from 500 - 700°C creates an effective p⁺-layer at the interface and ultrashallow p⁺n-junctions less than 10 nm deep can be made. The PureB layer can also be used as an emitter region in pnp bipolar transistors having a high effective emitter Gummel number (G_E) that appears to be related to the properties of the amorphous boron layer. In this paper, we suggest a wide-bandgap model of the amorphous boron layer to explain how its properties can lead to a suppression of the electron injection from the base into the emitter region thus giving the high G_E. The presence of trap states in the bandgap of amorphous boron layer is also considered. They could reduce the majority carrier concentration and change the mobility in the layer contributing to a decrease of G_E. It is concluded that the wider bandgap together with the trap states in the bandgap of amorphous boron layer could account for the high emitter Gummel number.