Influence of impurity concentration in Ge sources on electrical properties of Ge/Si hetero-junction tunneling field-effect transistors

We have experimentally demonstrated that there is the optimum B doping concentration in the Ge source in terms of the electric performance of Ge/Si hetero-junction tunneling field-effect transistors (TFETs). The degradation in subthreshold swing (SS) is observed for TFETs with the source B concentration higher than 1 × 1020 cm−3, which can cause the degeneration in Ge. This source concentration dependence can be explained by the depression of the energy filtering effect due to the degeneracy of the Fermi level (EF). This interpretation is supported by the temperature dependence of SS in the Ge/Si TFETs with different source concentrations. Also, a low SS value of 60.6 mV/dec, an Ion value of 82.3 nA/μm, and a large Ion/Ioff ratio of 6.8 × 106 are obtained for the 1.1% tensile strain channel with the optimized B concentration in the Ge source. It is found that the influence of the source EF on the electrical characteristics of TFETs is more pronounced for the strained-Si channel TFETs with smaller Eg.eff.