Current/voltage characteristics of p-n Ge-Si and Ge-GaAs heterojunctions

The current characteristics of solution-grown and vapour-grown p-n Ge-Si and Ge-GaAs heterojunctions have been studied as functions of voltage and temperature. All the junctions studied exhibit a range over which the forward-current characteristics are proportional to exp (AV)×exp (Br), where A and B are constants essentially independent of voltage and temperature. Although this is the only type of behaviour observed in nGe-pGaAs junctions, the other heterojunction pairs often exhibit this type of characteristic only above a certain temperature-dependent forward bias. Below this bias, the forward current is proportional to exp (eV/¿kT) The forward characteristics of nGe-pGaAs devices have been successfully explained by a multistep recombination-tunnelling model, in which tunnelling is the predominant current-transport mechanism across the entire junction. This model, however, cannot explain the thermal-current type of characteristic often observed at low forward bias in pGe-nGaAs, nGe-pSi and pGe-nSi heterojunctions. In these junctions, it appears that a tunnelling current flows in the germanium, while a diffusion current, which recombines at the interface, flows in the wide-gap material. These currents flow in series, and are related by interface-state parameters. A model based on this type of current flow is developed and is shown to agree with the observed characteristics.