jats:pThis paper presents a numerical study of single‐electron resonant tunnelling (RT) assisted by a few ionised donors in a laterally‐confined resonant tunnelling diode (LCRTD). The 3D multi‐mode S‐matrix simulation is performed newly introducing the scattering potential of discrete impurities. With a few ionised donors being placed, the calculated energy‐dependence of the total transmission rate shows new resonances which are donor‐configuration dependent. Visualised electron probability density reveals that these resonances originate in RT via single‐donor‐induced localised states. The I‐V characteristics show current steps of order 0.1 nA per donor before the main current peak, which is quantitatively in good agreement with the experimental results.</jats:p>