Computational study of strain-engineered III–V tunneling transistors

Strain has been widely used to engineer electronic devices by altering the material band structures. Recently it has also been employed to improve the performance of tunneling field-effect transistors (TFETs). A TFET is a steep subthreshold swing (SS) device that is very promising in building future low-power integrated circuits. But its drive current (I
ON
) is usually limited leading to pronounced switching delay (CV/I). It has been shown that for InAs nanowire TFETs, strain can reduce the band gap and/or effective masses leading to improved I
ON
. We show that for the experimentally more favorable ultra-thin-body (UTB) InAs TFETs, certain types of strain improve I
ON
when channel length is long (30 nm). When channel length is short (15 nm), however, the improvement is marginal due to degraded SS as a result of increased ambipolar leakage. To mitigate this detrimental effect, we propose to apply the strain locally in an area around the source-channel tunnel junction. Since the band structures of the channel and the source remain unaffected, the ambipolar leakage does not increase and meantime the source Fermi degeneracy is removed. In this way we obtain a significant boost of SS and I
ON
. The simulations are performed by solving Poisson equation and open-boundary Schroedinger equation self-consistently within NEMO5 tool. The band structure of III–V materials is described by strained eight-band k·p Hamiltonian. Since tunneling current is very sensitive to band structures, we extract the k·p band parameters and deformation potentials from the corresponding atomistic tight binding calculations, whose parameters are fit to first-principles density functional theory (DFT) calculations with excellent match. We compare the confined band structures as well as I–V curves obtained from both methods and show that the accuracy of the k·p method is guaranteed.