Transport properties at a sigmoidal graded heterojunction M. Tchoffo, L. S. Yonya Tchapda, M. Vubangsi and L. C. Fai Eur. Phys. J. Plus, 135 1 (2020) 128 Published online: 23 January 2020 DOI: 10.1140/epjp/s13360-020-00191-4